Diarrhea

Diarrhea, frequent passage of abnormally loose, watery stool. Diarrhea usually develops suddenly and may last from several hours to a few days. It is often accompanied by abdominal pains, low fever, nausea, and vomiting. If the attacks are severe or increasingly frequent, exhaustion and dehydration can result. In normal digestion the large intestine absorbs excess water from liquid food residues produced by earlier phases of the digestive process before excreting semisolid stools. When the mucous membrane lining the large intestine is irritated or inflamed, food residues move through the large intestine too quickly and the resulting stool is watery because the large intestine cannot absorb the excess water.

Diarrhea is not a disease. It is a symptom of numerous disorders, such as food poisoning from contaminated foods or beverages, infections by viruses and bacteria, or anxiety. Chronic diarrhea, which lasts weeks or months, may be caused by amoebic dysentery (intestinal infection), tumors, and other serious intestinal disorders such as Crohn’s disease, ulcerative colitis, or irritable bowel syndrome. Except in the case of irritable bowel syndrome, the stool may contain blood or pus.

The usual treatment for diarrhea consists of bed rest, drinking liquids to replace fluids and salts lost from the body, and eating soft foods. Dehydration is a serious concern in infants and the elderly. If the condition lasts more than a few days, a physician should be consulted.

Myxedema

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Myxedema (Greek myxa, “slime”; oidema, “swelling”), deficiency disease caused by insufficient or lack of production of hormone by the thyroid gland. Patients with myxedema complain of fatigue, lethargy, sleepiness, poor tolerance to cold, mental sluggishness, a tendency to gain weight, and generalized aches and pains. Their faces often look puffy and waxy. Their skin is dry and coarse; their hair is coarse, dry, and brittle, and it tends to fall out easily. Often patients also lose the outer portion of their eyebrows. These and other symptoms are caused by a low metabolic rate resulting from a deficiency of the thyroid hormone that stimulates metabolism. Myxedema differs from cretinism in that it develops after birth and produces less severe cerebral inadequacy. The disease may occur in several members of a single family. Any condition that decreases the elaboration of thyroid gland hormone may bring on myxedema. The disease is treated by the administration of thyroxine, other thyroid extracts, or a synthetic preparation such as levothyroxine.

Goiter

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Goiter, disease of the thyroid gland, characterized by an enlargement of the gland, visible externally as a swelling on the front of the neck. In simple goiter the basal metabolic rate is somewhat lowered, and in toxic goiter it is elevated.

Iodine deficiency in the body can cause goiter. Treatments include the ingestion of small doses of iodine, or, in extreme cases, the removal of the thyroid gland.

SIMPLE GOITER

This condition is characterized by an enlargement of the entire gland, or of one of its two lobes, caused by a deficiency of iodine in the diet. The disease is especially apt to appear in adolescence. Prevention requires taking small doses of iodine for long periods. Ingestion of iodine during pregnancy prevents development of the disease in the infant as well as in the mother. Public health measures, including the addition of iodine to water supplies and to table salt, have helped to reduce the incidence of simple goiter in certain areas. Iodine is most effective when administered to children who have the disease. Thyroidectomy, or surgical removal of the gland, may be necessary in cases in which the gland has become greatly enlarged.

TOXIC GOITER

This disease, also called exophthalmic goiter, hyperthyroidism, thyrotoxicosis, or Graves' disease, for the Irish physician Robert James Graves, is caused by an excess of thyroxine secretion. The cause of the excessive secretion is obscure. In some cases it may result from excessive stimulation by the pituitary gland. The symptoms of toxic goiter may include a rapid heartbeat, tremor, increased sweating, increased appetite, weight loss, weakness, and fatigue. Some patients have eye problems, such as staring or protrusion. Thiouracil and iodine are sometimes used in the treatment of toxic goiter, as is irradiation of the gland by radioactive iodine.

See also Myxedema.

Disorders of Human Teeth

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The three main diseases of human teeth are tooth decay, also called dental caries; gum disease, or periodontal disease; and problems with tooth alignment, called malocclusions. Human teeth problems are treated or prevented by dentists, professionals who are specially trained to practice dentistry.

Tooth decay affects approximately 90 percent of all children by the time they are 14 years old. Tooth decay begins when bacteria are passed from mothers or caregivers to children between their first and second birthdays. When these bacteria are exposed to sugars commonly found in foods, the bacteria produce harmful acids that attack tooth enamel. Left unchecked, the acid eats holes in the enamel and forms cavities of tooth decay. Most tooth decay forms in the deep grooves on the chewing surfaces of the molars, called pits and fissures. Daily tooth brushing and proper dental care help prevent and reduce tooth decay. Dentists use preventive treatments to reduce the risk of tooth decay; clear plastic coatings painted on the teeth, called dental sealants, and applications of the mineral fluoride, which fortifies tooth enamel, are two such treatments.

Gum disease, or periodontal disease, is a progressive condition that worsens with age. Gum disease occurs when bacteria eat away at gum tissue, causing it to pull away from the teeth. This space between the tooth and gum, called a periodontal pocket, traps even more bacteria. Gum disease develops in two stages. Gingivitis, the early stage, causes red, swollen gums that bleed easily. Gingivitis can be eliminated through good oral hygiene and dental care. If not treated, gingivitis can progress to periodontitis, when bacteria attack the bone supporting the teeth. To treat periodontitis, dentists may have to surgically cut out the infected portion of the gum so the bacteria can be removed.

Malocclusions—teeth that are crowded, crooked, or out of alignment—make it more difficult to clean teeth, which can lead to other oral health problems such as tooth decay and gum disease. Many of these disorders start to appear between the ages of 6 and 12, when permanent teeth begin to erupt. Generally, malocclusions result when the jaw is too small to hold all of the teeth. Malocclusions are often genetic, tending to run in families. In other cases, dental injury or chronic thumb sucking may lead to poorly aligned teeth. Malocclusions are treated by dentists specially trained to correct them, called orthodontists.

Disorders of the Urinary System

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Urologists are physicians who specialize in treating urinary system disorders. One of the most serious disorders urologists treat is renal failure, which slows or stops the filtration of blood, causing toxic waste products to build up in the blood. Acute renal failure, which occurs suddenly, may be caused by bacterial infection, injury, shock, congestive heart failure, drug poisoning, or severe bleeding following surgery. Treatment may include drugs to address the underlying cause or to stimulate proper kidney function, blood transfusions, surgery, or, in some cases, kidney dialysis, in which the blood is mechanically filtered.

Chronic renal failure is a progressive deterioration of kidney function over a long period of time. It can be caused by diseases such as hypertension, diabetes, lupus erythematosus, acquired immunodeficiency syndrome (AIDS), and a form of cancer called myeloma. If diagnosed early the degenerative process can be slowed, but not reversed, by interventions that can range from simply restricting fluid intake and protein consumption (proteins are the main source of waste products) to taking drugs to address the underlying disease that is damaging the kidneys. Some patients may go on to develop end-stage renal failure, a life-threatening condition that requires long-term dialysis or a kidney transplant.

Urinary calculi, commonly known as kidney stones, result from the gradual buildup of crystallized salts and minerals in the urine. Kidney stones can cause intense pain if they obstruct a passageway that carries urine. Usually, the stones pass through and out of the urinary tract on their own. If they fail to pass out of the body, they can be removed surgically or broken up nonsurgically by an ultrasound technique called lithotripsy.

Bacterial infections, most frequently caused by the common intestinal bacterium Escherichia coli, can occur in any part of the urinary system. Antibiotics usually conquer the infection, although recurrent and chronic infections are not uncommon.

Among the many inherited and congenital disorders of the urinary system are polycystic renal diseases, in which numerous cysts form in the kidney, reducing the amount of functioning renal tissue. Kidney dialysis or transplantation usually is necessary to avoid kidney failure and death. Hypospadias is a birth defect in which the male urinary opening is misplaced on the penis; it may be under the head of the penis or as far away as the scrotum. Surgery before the child reaches 24 months can correct the defect, permitting normal urination and, later, sexual intercourse.

Human Nutrition

Human Nutrition is learning how food affects the health and survival of the human body. Human beings require food to grow, reproduce, and maintain good health. Without food, our bodies could not stay warm, build or repair tissue, or maintain a heartbeat. Eating the right foods can help us avoid certain diseases or recover faster when illness occurs. These and other important functions are fueled by chemical substances in our food called nutrients. Nutrients are classified as carbohydrates, proteins, fats, vitamins, minerals, and water.

When we eat a meal, nutrients are released from food through digestion. Digestion begins in the mouth by the action of chewing and the chemical activity of saliva, a watery fluid that contains enzymes, certain proteins that help break down food. Further digestion occurs as food travels through the stomach and the small intestine, where digestive enzymes and acids liquefy food and muscle contractions push it along the digestive tract. Nutrients are absorbed from the inside of the small intestine into the bloodstream and carried to the sites in the body where they are needed. At these sites, several chemical reactions occur that ensure the growth and function of body tissues. The parts of foods that are not absorbed continue to move down the intestinal tract and are eliminated from the body as feces.

Once digested, carbohydrates, proteins, and fats provide the body with the energy it needs to maintain its many functions. Scientists measure this energy in kilocalories, the amount of energy needed to raise 1 kilogram of water 1 degree Celsius. In nutrition discussions, scientists use the term calorie instead of kilocalorie as the standard unit of measure in nutrition.

See: ESSENTIAL NUTRIENTS

Skin Cancer

Skin Cancer, malignancy in the skin, and the most common of all cancers. There are three main types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and melanoma. Basal cell carcinoma, the most common type, develops in the basal, or bottom, layer of the epidermis, the top layer of skin. Squamous cell carcinoma, the second most common type, develops in the upper layers of the epidermis.

The third type of skin cancer is melanoma. It develops in the melanocytes, or pigment cells, which are found throughout the basal layer. Melanoma is the most dangerous type of skin cancer and is responsible for about three-quarters of all skin cancer deaths.

Skin cancers can grow in any part of the skin, but approximately 90 percent develop in areas exposed to the sun, especially the face, neck, backs of the hands, and the scalps of bald men. Exposure to direct sunlight, especially during childhood, is the chief risk factor for skin cancer, and the greater the number of blistering sunburns a person has had, the greater the risk of developing skin cancer later in life. People with pale skin are at greatest risk for skin cancer, particularly if their skin burns or freckles easily.

SYMPTOMS AND DIAGNOSIS

Early detection and treatment are the keys to survival, especially for melanoma. Any of several skin changes may signal skin cancer and should be reported to a physician. These include any change in the size, shape, color, or texture of a mole or other darkly pigmented area; any mole that begins to itch or becomes tender; the development of a new mole or other growth, particularly if it feels hard; any mole or other growth that spontaneously or persistently bleeds; a skin ulcer that does not heal; and a black spot under a toenail or fingernail that extends beneath the cuticle. To confirm a diagnosis a physician may perform a biopsy, removing a small piece of the skin to examine under a microscope for the presence of cancerous cells.

TREATMENT

One of several surgical methods is used to remove the cancerous tissue. In some cases, the tissue is destroyed through the application of intense cold (cryosurgery). In advanced cases, surgical removal of the tissue coupled with radiation, chemotherapy, or both may be required.

A class of vitamin-based drugs called retinoids have been shown to reduce the number of basal cell and squamous cell carcinomas. However, the drugs can cause liver abnormalities, bone problems, fetal malformation, and other undesirable side effects.

Potential skin cancer treatment studies now underway include the use of the drugs interleukin-2 and interferon. A vaccine to reduce the recurrence of melanoma has shown promise in early tests.

Appendicitis

Appendicitis, acute inflammation of the vermiform (wormlike) appendix, a blind tube projecting from the cecum,—the beginning of the large intestine. The appendix, located in the lower right side of the abdomen, is an organ with no known function in humans. If the appendix wall ruptures, infection may spread to the abdominal cavity, causing peritonitis. Symptoms of appendicitis include pain and cramps in the area between the right hip bone and the navel, fever, nausea and vomiting, constipation, and diarrhea. The treatment is surgical removal of the appendix (appendectomy). Although the frequency of appendicitis is highest among young adults, the ailment can affect persons of any age.

Peritonitis

Peritonitis, inflammation of the membrane that lines the abdominal cavity and the organs contained therein. Peritonitis is most often an acute illness, caused by introduction of infection from a perforation of the bowel such as a ruptured appendix or diverticulum. The disease is also caused by introduction of chemically irritating material such as stomach acid from a perforated ulcer or bile from a rupture of the gall bladder or laceration of the liver. Localized peritonitis most often occurs in the pelvis from an infected fallopian tube or a ruptured ovarian cyst. Occasionally peritonitis may occur when the rupture of an abscess causes pus to spill into the abdominal cavity. This sometimes follows surgery inside the bowel with leaking from the site of closure, or from a stab wound through the abdominal wall. Occasionally peritonitis may occur from inflammation of the peritoneal membrane such as in kidney failure, rheumatic fever, and lupus erythematosus. The principal symptom is severe abdominal pain much aggravated by any motion. The patient often has nausea and vomiting and usually is feverish. Untreated cases of acute peritonitis are often fatal. Treatment is directed at the underlying cause. In addition, intravenous fluids and antibiotics are given.

Eye Disorders and Diseases

Eye disorders may be classified according to the part of the eye in which the disorders occur.

The most common disease of the eyelids is hordeolum, known commonly as a sty, which is an infection of the follicles of the eyelashes, usually caused by infection by staphylococci. Internal sties that occur inside the eyelid and not on its edge are similar infections of the lubricating Meibomian glands. Abscesses of the eyelids are sometimes the result of penetrating wounds. Several congenital defects of the eyelids occasionally occur, including coloboma, or cleft eyelid, and ptosis, a drooping of the upper lid. Among acquired defects are symblepharon, an adhesion of the inner surface of the eyelid to the eyeball, which is most frequently the result of burns. Entropion, the turning of the eyelid inward toward the cornea, and ectropion, the turning of the eyelid outward, can be caused by scars or by spasmodic muscular contractions resulting from chronic irritation. The eyelids also are subject to several diseases of the skin such as eczema and acne, and to both benign and malignant tumors. Another eye disease is infection of the conjunctiva, the mucous membranes covering the inside of the eyelids and the outside of the eyeball.

See Conjunctivitis; Trachoma; Glaucoma; Cataract; Color Blindness.

Color Blindness

Color Blindness, defect of vision affecting the ability to distinguish colors, occurring mostly in males. Color blindness is caused by a defect in the retina or in other nerve portions of the eye. The first detailed report on this condition was written by the British chemist John Dalton, who was himself afflicted with it. Total color blindness, in which all hues are perceived as variations of gray, is known as achromatopsia or monochromatism. This condition is congenital, extremely rare, and affects men and women almost equally. Partial color blindness, called dichromatism, consists generally of the inability to differentiate between the reds and the greens or to perceive either reds or greens; infrequently, the confusion may involve the blues or the yellows. Dichromatism is the most common form of color blindness, affecting about 7 percent of men and less than 1 percent of women. Dichromatism is identified as a sex-linked hereditary characteristic. Color blindness also may occur as a temporary condition following a serious illness.

Trachoma

Trachoma, contagious infection of the eye caused by Chlamydia trachomatis, a bacteria related to the organism that causes psittacosis (disease of birds). Infection is spread by the bite of a host fly. It is a major cause of blindness characterized by hard pustules or granular excrescences on the inner surface of the eyelids, inflammation of the membrane, and subsequent involvement of the cornea. It most commonly occurs among populations living under poor sanitary conditions.

The disease begins after an incubation period of five to seven days with inflammation of the eye. Ensuing symptoms are considerable discharge of pus, swelling of the lids, tearing, and increased sensitivity to light. It goes on in a few weeks to chronic swelling, formation of blisters in the eye, and destruction and scarring of the cornea, which eventually causes blindness. In its early stages trachoma responds readily to the topical, and sometimes oral, administration of broad-spectrum antibiotics

Hernia

Hernia, the protrusion of an organ or part of an organ or other structure through the wall of the cavity that normally contains it. Hernias (incorrectly called ruptures) are qualified by the name of the part that protrudes or the area through which protrusion occurs. Thus, an inguinal hernia, perhaps the most common form, is one that passes through the abdominal wall in the groin area. A diaphragmatic or hiatus hernia is one in which part of the stomach or a loop of the intestine herniates upward through an opening in the diaphragm into the chest cavity. An umbilical hernia is one in which the bowel or the membranous apron overlying it, known as the omentum, protrudes through the abdominal wall under the skin at the navel.

Hernias often develop in an area of weakness following unusual stress or strain. An inguinal hernia, for example, may follow the abrupt lifting of a heavy object or a severe bout of coughing. Inguinal hernias are more likely to occur in men than in women because the spermatic cord passes through the abdominal wall in the inguinal region, leaving a site of natural weakness prone to hernia formation.

An inguinal hernia usually causes no distress, but hernias within the abdominal cavity can cause intense pain. Many hernias will slide back, or reduce, either spontaneously or when mild pressure is exerted on them. A reducible hernia is often held in place with an external support called a truss. Incarcerated (trapped) or strangulated hernias are often painful and must be treated with surgery because the blood supply to the organ trapped within the hernia can be interrupted, leading to gangrene within the hernial sac. Surgical repair involves reinforcing the site of the hernia with a sheet of synthetic mesh or with sutures.

Peritoneum

Peritoneum, serous membrane in the abdominal cavity. It consists of two layers, a parietal one lining the walls of the cavity and a visceral one closely investing the majority of the abdominal organs, and mooring them firmly in position. Between the two layers there is provision for a space, the peritoneal cavity; in the male this is closed, while in the female there is a direct communication between the sac and the fallopian tubes to enable the ova to reach the cavity of the uterus

Poliomyelitis

Poliomyelitis, infectious viral disease that sometimes results in paralysis. The infection chiefly affects children and young adults and is caused by any one of three related viruses called polioviruses.

Doctors diagnose polio by isolating the virus from an infected person using throat cultures, stool samples, or samples of fluids from the brain and spinal cord. Blood tests that indicate the presence of antibodies specific for the virus will also confirm a poliovirus infection.

As yet there is no cure for polio—no drug or other medical treatment can halt the destruction of poliovirus in the body. However, several medical treatments can lessen the severity of the disease. Mild cases of polio do not require specific treatment. For the more serious cases of paralytic polio, keeping patients still and quiet can, in some cases, minimize the severity of paralysis. For these patients initial treatment consists of immediate hospitalization and strict bed rest. Simple treatments, including moist heat applied to affected muscles, can ease pain. Antispasmodic drugs can help patients who suffer involuntary muscle contractions as a consequence of nerve damage.

Vaccines work by exposing the body’s immune system to a microbial infection that is strong enough to provoke an immune response but not severe enough to result in full-blown illness. Once the body has overcome the vaccine-induced challenge of a given infection, the resulting antibodies can recognize and quickly handle any subsequent invasion by the same agent.

Psychosis

Psychosis, mental illness in which a person loses contact with reality and has difficulty functioning in daily life. Psychotic symptoms can indicate severe mental illnesses, such as schizophrenia and bipolar disorder. Unlike people with less severe psychological problems, psychotic individuals do not usually recognize that their mental functioning is disturbed.

Peritonitis

Peritonitis, inflammation of the membrane that lines the abdominal cavity and the organs contained therein. Peritonitis is most often an acute illness, caused by introduction of infection from a perforation of the bowel such as a ruptured appendix or diverticulum. The disease is also caused by introduction of chemically irritating material such as stomach acid from a perforated ulcer or bile from a rupture of the gall bladder or laceration of the liver. Localized peritonitis most often occurs in the pelvis from an infected fallopian tube or a ruptured ovarian cyst. Occasionally peritonitis may occur when the rupture of an abscess causes pus to spill into the abdominal cavity. This sometimes follows surgery inside the bowel with leaking from the site of closure, or from a stab wound through the abdominal wall. Occasionally peritonitis may occur from inflammation of the peritoneal membrane such as in kidney failure, rheumatic fever, and lupus erythematosus. The principal symptom is severe abdominal pain much aggravated by any motion. The patient often has nausea and vomiting and usually is feverish. Untreated cases of acute peritonitis are often fatal. Treatment is directed at the underlying cause. In addition, intravenous fluids and antibiotics are given.

Pus

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Pus, thick white or yellowish fluid, found at sites of bacterial infection of the body. Certain white blood cells, called phagocytes, migrate to the area of infection and engulf the invaders. They then kill the ingested bacteria by releasing toxic substances inside the cell. In the process the white blood cells also die, and these cells become constituents of pus. See Abscess; Carbuncle.

Yogurt, Cheese, and Other Fermented Milk Products

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Fermented milk products, including yogurt and cheese, are formed when bacteria break down lactose to produce lactic acid, which sours the milk. Yogurt is usually made from milk that has been fortified by the addition of nonfat powdered milk to improve its texture and taste. The fortified milk is pasteurized at a very high temperature and homogenized. A culture containing the bacteria Streptococcus thermophilus and Lactobacillus bulgaricus, and sometimes Lactobacillus acidophilus, is then added. The two most popular types of yogurt sold are Swiss-style, with added fruit mixed throughout, and sundae-style, with fruit on the top or bottom.

Cultured buttermilk and sour cream, like yogurt, are produced by the breakdown of lactose by bacteria. Cultured buttermilk used to be made from buttermilk, but is now more commonly made from skim milk. Sour cream is made from cream. Both have a tangy, sour taste imparted by lactic acid.

Cheese making is a complex process, with each of the more than 400 varieties of cheeses differing in the details of the process. However, most cheese making follows the same basic steps. Bacteria are introduced into milk that consume lactose and produce lactic acid. Cheeses are allowed to ferment longer than yogurt, buttermilk, and sour cream, which gives the bacteria time to also digest some of the fat and protein present. The waste products produced by bacteria from digesting fats and proteins provide cheeses with their distinctive and stronger flavors. Acidification of milk or the addition of the enzyme rennin precipitates moist lumps of protein called curds. The curds contain all of the milk’s protein and most of the fat, minerals, and vitamins. The residual thin liquid is known as whey. The curds are salted (for taste) and pressed into shape, which eliminates more of the whey. The pressed curd is then cured under controlled temperature and humidity for varying lengths of time to produce cheese.

Cream, Butter, and Ice Cream

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In order for a product to be labeled as cream, it must contain 18 percent milk fat or more. Light whipping cream must contain at least 30 percent fat, and 36 percent fat is the minimum fat content for heavy whipping cream. Half-and-half, an equal mixture of both milk and cream, must have at least 10.5 percent fat. The various grades of cream are valued for their smooth, thick texture. They are often used in cooking instead of milk, because they are less likely to form a skin when heated and can be whipped into a stiff foam.

When chilled cream is churned gently, the fat globules gather together to form butter, leaving buttermilk as the by-product. Butter contains at least 82.5 percent fat. In the past, before the advent of widespread refrigeration, salt was added to butter to retard bacterial spoilage; now salt is added mainly for flavor.

Ice cream, a popular frozen dessert, is made of milk, cream, sugar, and flavoring. The mixture is slowly beaten while chilled until it is partially frozen, then packed into containers, and chilled until firm. Variants of ice cream include ice milk, which contains less fat; sherbet, which has even less fat; and French ice cream, which is enriched with egg yolks.

Milk Products

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Milk in its natural form, directly from a cow, is called raw milk. It is an extremely versatile product from which a myriad of commercial products are derived.

A. Whole Milk, Low-fat Milk, and Skim Milk

Since the fat in raw milk is lighter in weight than the rest of the milk, it will naturally rise to form a layer of cream if allowed to stand. Spinning the milk in a large machine, called a centrifuge, accelerates the formation of a cream layer, or the separation of fat, from raw milk. Varying amounts of fat are removed from the raw milk, resulting in different kinds of fresh milk. If the fat content is lowered to 3.25 percent, the milk is sold as whole milk. Low-fat milk typically has 1 percent or 2 percent fat. Skim milk, or nonfat milk, is the liquid that remains after removing all the cream; it contains about half a percent milk fat. About half of the milk produced in the United States is consumed as one of these types of fresh milk; the rest is processed into other products such as cream, butter, cheese, yogurt, and ice cream.

Once the fat level has been reduced to the desired level, most fresh milk is homogenized to prevent the further separation of a cream layer. Homogenization is accomplished by forcing hot milk under high pressure through small nozzles. The fat globules become so small that they remain evenly dispersed throughout the milk. In order to insure its safety for human use, almost all milk undergoes pasteurization, in which milk is heated to a high temperature for a specified length of time to destroy pathogenic bacteria. Pasteurized milk can be stored in a refrigerator for a week or longer. Ultrapasteurized milk, common in Europe and Canada, is heated to an even higher temperature and can be stored at room temperature for several weeks.

Condensed, evaporated, and powdered milk are produced by evaporating some or all of the water in milk. Sweetened condensed milk is sweetened with the addition of sugar. These products have very long shelf lives.

B. Cream, Butter, and Ice Cream

C. Yogurt, Cheese, and Other Fermented Milk Products

Human Breast Milk

Human milk provides distinct advantages over formula or cow milk for human infants. Not only is breast milk’s nutritional composition uniquely designed for the needs of human babies, but it also contains antibodies from the mother’s immune system that help the infant fight off infections and diseases. During the first few days after giving birth, a mother releases colostrum, a yellowish liquid that contains less fat and lactose and more protein and antibodies than regular breast milk. After about three or four days, colostrum is replaced by a bluish-white milk that is higher in fats and carbohydrates, reflecting the energy needs of a growing baby (see Breast-feeding).

Nutritional Value of Milk

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Most milk is composed of 80 to 90 percent water. The remaining 10 percent consists of an abundance of the major nutrients needed by the body for good health, including fats, carbohydrates, proteins, minerals, and vitamins.

Cow milk typically contains about 3.5 to 5 percent fat, which is dispersed throughout the milk in globules. In addition to providing milk’s characteristic taste and texture, fat supplies vitamins A, D, E, and K, as well as certain fatty acids that the body cannot produce on its own.

Lactose, a kind of sugar found only in milk, gives milk its sweet taste. Making up about 5 percent of milk’s content, lactose is a carbohydrate that is broken down by the body to supply energy. Infants digest lactose easily, but many adults, especially those of Asian and African ancestry, have lost some of their ability to digest this sugar. When these adults drink milk, they often suffer gastric distress and diarrhea.

The most important protein in milk is casein, accounting for 80 percent of milk protein. Casein is a complete protein, meaning that it contains all of the essential amino acids, which the body cannot manufacture on its own. Casein molecules and globules of fat deflect light rays passing through milk, giving milk its opalescent appearance. Other proteins present in milk include albumin and globulin.

One liter of milk supplies as much calcium as 21 eggs, 12 kg of lean beef, or 2.2 kg of whole wheat bread.

Milk contains many minerals, the most abundant of which are calcium and phosphorus, as well as smaller amounts of potassium, sodium, sulfur, aluminum, copper, iodine, manganese, and zinc. Milk is perhaps the best dietary source of calcium—one liter (about 1 qt) of milk supplies as much calcium as 21 eggs, 12 kg (26 lb) of lean beef, or 2.2 kg (5 lb) of whole wheat bread. Milk is an excellent source of vitamins A and B2 (see riboflavin). All other vitamins are present also, but in lower doses. Vitamin D is typically added to commercially sold milk. Vitamin A, which is found in the globules of fat, is removed when fat is skimmed away to make low-fat or skim milk. Generally, vitamin A is replaced during the production of commercially sold low-fat milk.

See also Human Breast Milk; Milk Products.

Milk

Milk, highly nutritious, versatile food. People enjoy drinking milk in its natural form and also use it to make a wide range of food products, including cream, butter, yogurt, cheese, and ice cream.

Female mammals produce milk to feed their newly born young. Milk is produced in the mother’s mammary glands, which are found, for example, in the breasts of humans or the udders of cows, sheep, or goats. Each species of mammal produces milk with a unique composition designed to meet the specific needs of its infants. For instance, the milk of animals that need to develop a thick layer of insulating fat, such as seals, has a high fat content. The milk of animals that grow rapidly, such as cows, which double their birth weight in 50 days, is rich in protein and minerals.

Humans drink the milk produced from a variety of domesticated mammals, including cows, goats, sheep, camels, reindeer, buffaloes, and llama. By far the vast majority of milk used for commercial production and consumption is from cows. This article focuses on the nutritional value and production of cow milk. For information on the branch of agriculture concerned with the raising of cows (or other domesticated mammals) for the purpose of collecting their milk, see also Dairy Farming.

See also:

• Nutritional Value of Milk
• Human Breast Milk
• Milk Products

Disorders of the Human Skull

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Possibly the commonest skull disease in humans is sinusitis. Sinuses are cavities in the frontal bones, the sphenoid bones, the maxillary bone, and the mastoid processes of the temporal bone that are lined with mucous membrane and that open into the nasal cavity. Sinuses are often infected by bacteria, causing inflammation and producing severe pain. Acute sinusitis is treated with antibiotics; chronic sinusitis is extremely difficult to eradicate.

Skull deformity occasionally occurs in the womb or during delivery and is often accompanied by brain damage. The skull is also subject to fracture and to tumors. For skull measurement, see Cephalic Index.

Constipation

Constipation, difficulty in eliminating bowel movements from the body. Constipation is usually accompanied by the passage of hard, dry stools that may result in a tear in the lining of the lower rectum or anus or, more often, in painfully swollen and itchy veins in the lower rectum or anus, known as hemorrhoids. Tenseness and straining to achieve evacuation tend to worsen such conditions. Constipation is sometimes associated with feelings of sluggishness, headache, and distension of the abdomen.

Chronic constipation may be caused by a diet insufficient in roughage or fiber, irregular eating hours, improper use of laxatives, or the voluntary avoidance of bowel movements. Many cases are believed to be caused by emotional disturbances. Constipation can usually be relieved by drinking adequate fluids; eating healthy foods such as fruits, vegetables, and cereals; and establishing routine evacuation habits. A high-fiber diet is particularly important for people who are taking medications that tend to harden stools and produce irregular bowel habits.

Some people become stressed or depressed when their bowel habits are irregular. Neglecting dietary measures, they may become addicted to the use of laxatives, suppositories, and enemas. These individuals may eventually lose muscle strength in the bowel wall, making it difficult for them to return to normal bowel habits. They also face the possibility that the digestive system will be unable to absorb all the nutrients in their food supply.

Constipation that develops suddenly in people who have previously had normal bowel function may be a symptom of a serious disorder, such as excessive production of thyroid hormones or kidney failure. Disturbing changes in bowel habits should be reported to a physician.

Hemorrhoid

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Hemorrhoid, small swelling, occurring in or around the rectum or at the anus, caused by enlargement of the superficial veins. A hemorrhoid occasionally prolapses, or protrudes from the anus, causing discomfort by strangulation; to relieve the pain, the prolapsed hemorrhoid must be replaced within the anal sphincter muscle.

Hemorrhoids, or piles, sometimes itch or cause pain in the rectum. They often bleed. Symptoms usually occur after defecation, particularly after passing a hard stool. Continued bleeding, even if slight, over a long period may cause anemia.

A common ailment among adults, hemorrhoids often are associated with habitual constipation and with pregnancy. Prolonged sitting and mental stress may be contributory factors. The treatment includes palliative measures, such as the use of suppositories, astringent ointments, diet, or medication to keep the stool soft, and hot sitz baths. Pain from thrombosed hemorrhoids may be relieved by spraying with a local anesthetic, opening the vessel, and removing the clot. In advanced cases, hemorrhoids may be destroyed by freezing with liquid nitrogen, by injection of sclerosing agents, by rubber band ligation, or by surgery.

Disorders of the Circulatory System

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Disorders of the circulatory system include any injury or disease that damages the heart, the blood, or the blood vessels. The three most important circulatory diseases are hypertension, arteriosclerosis, and atherosclerosis.

Hypertension, or elevated blood pressure, develops when the body’s blood vessels narrow, causing the heart to pump harder than normal to push blood through the narrowed openings. Hypertension that remains untreated may cause heart enlargement and thickening of the heart muscle. Eventually the heart needs more oxygen to function, which can lead to heart failure, brain stroke, or kidney impairment. Some cases of hypertension can be treated by lifestyle changes such as a low-salt diet, maintenance of ideal weight, aerobic exercise, and a diet rich in fruits, vegetables, plant fiber, and the mineral potassium. If blood pressure remains high despite these lifestyle adjustments, medications may be effective in lowering the pressure by relaxing blood vessels and reducing the output of blood.

In arteriosclerosis, commonly known as hardening of the arteries, the walls of the arteries thicken, harden, and lose their elasticity. The heart must work harder than normal to deliver blood, and in advanced cases, it becomes impossible for the heart to supply sufficient blood to all parts of the body. Nobody knows what causes arteriosclerosis, but heredity, obesity, smoking, and a high-fat diet all appear to play roles.

Atherosclerosis, a form of arteriosclerosis, is the reduction in blood flow through the arteries caused by greasy deposits called plaque that form on the insides of arteries and partially restrict the flow of blood. Plaque deposits are associated with high concentrations of cholesterol in the blood. Blood flow is often further reduced by the formation of blood clots (see Thrombosis), which are most likely to form where the artery walls have been roughened by plaque. These blood clots can also break free and travel through the circulatory system until they become lodged somewhere else and reduce blood flow there (see Embolism). Reduction in blood flow can cause organ damage. When brain arteries become blocked and brain function is impaired, the result is a stroke. A heart attack occurs when a coronary artery becomes blocked and heart muscle is destroyed.

Risk factors that contribute to atherosclerosis include physical inactivity, smoking, a diet high in fat, high blood pressure, and diabetes. Some cases of atherosclerosis can be corrected with healthy lifestyle changes, aspirin to reduce blood clotting, or drugs to lower the blood cholesterol concentration. For more serious cases, surgery to dilate narrowed blood vessels with a balloon, known as angioplasty, or to remove plaque with a high-speed cutting drill, known as atherectomy, may be effective. Surgical bypass, in which spare arteries are used to construct a new path for blood flow, is also an option.

Diseases and Disorders of Respiratory System

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The diseases and disorders of the respiratory system can affect any part of the respiratory tract and range from trivial to life-threatening. The nasal passages and pharynx, for example, are targets for the viruses that cause colds. These viruses infiltrate and destroy the cells of the nasal passage membranes. The immune system fights back by increasing blood flow to the area, bringing numerous virus-attacking white blood cells to the scene; this causes the membranes to swell, resulting in the stuffy nose associated with colds. Mucous secretions increase in response to the viral attack, creating the runny nose typical of colds. The infection can spread to the sinuses, the membrane-lined cavities in the head, as well as the lower respiratory tract and the middle ear.

The respiratory system is also subject to allergic reactions such as hay fever and asthma, brought about when the immune system is stimulated by pollen, dust, or other irritants. Hay fever is characterized by a runny nose, watery eyes, and sneezing. It usually occurs seasonally in response to abundant pollen in the air. In asthma, a person has difficulty breathing because the bronchi and bronchioles are temporarily constricted and inflamed. An asthma attack is typically mild, but can be severe enough to be life threatening.

Laryngitis, an inflammation of the larynx, is caused by a viral infection, irritants such as cigarette smoke, or by overuse of the voice. Laryngitis may cause hoarseness, or the voice may be reduced to a whisper until the inflammation subsides. Bronchitis is an inflammation of the membranes that line the bronchi or bronchioles. Bronchitis results from viral or bacterial infection or from irritating chemicals. Infections caused by bacteria or viruses can lead to pneumonia, a potentially serious condition of the lungs in which fluid and inflammation builds up in the alveoli, impeding the flow of oxygen and carbon dioxide between the capillaries and the alveoli.

Tuberculosis is caused by a bacterium that attacks the lungs and sometimes other body tissues as well. If infections in the lungs are left untreated, the disease destroys lung tissue. In the past, antibiotics have controlled tuberculosis, but recently, new antibiotic-resistant strains of the tuberculosis bacterium have evolved. These new strains now pose a significant public health problem.

In emphysema the alveolar tissue is partially destroyed and the remaining alveoli are weakened and enlarge. The bronchioles collapse on exhalation, trapping air in the alveoli. Over time this process impairs the ability to exchange oxygen and carbon dioxide with the circulatory system, leading to breathing difficulties. A noncontagious disease, emphysema results from multiple factors, including a genetic predisposition to the condition, smog, cigarette smoke, and infection.

Lung cancer develops in individuals with a genetic predisposition to the disease who are exposed to cancer-causing agents, such as tobacco smoke, asbestos, and uranium. Cancerous tumors may start in the bronchi, bronchioles, or in the alveolar lung tissue. If lung cancer is detected before the cancer has spread to other parts of the body, treatments are more effective, and the prognosis for full recovery is good. Unfortunately, 85 percent of lung cancer cases are diagnosed after the cancer has spread, and for these cases, the prognosis is very poor.

Respiratory Distress Syndrome (RDS) is the name for a cluster of symptoms that indicate severe malfunctioning of the lungs. In infants, RDS is termed Infant Respiratory Distress Syndrome (IRDS). Commonly found in premature infants, IRDS results when the alveoli fail to fully expand during inhalation. Expansion of the alveoli requires a chemical called surfactant, but in many premature infants, the alveoli are not developed enough to produce this vital substance. IRDS is treated by administering air and surfactant through a breathing tube until the alveoli begin producing surfactant on their own. Adult Respiratory Distress Syndrome (ARDS) results when lungs are severely injured, for example, in an automobile accident, by poisonous gases, or as a response to inflammation in the lungs. ARDS is a life-threatening condition with a survival rate of about 50 percent.

Diseases of the Endocrine System

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Endocrine disorders are classified in two ways: disturbances in the production of hormones, and the inability of tissues to respond to hormones. The first type, called production disorders, are divided into hypofunction (insufficient activity) and hyperfunction (excess activity). Hypofunction disorders can have a variety of causes, including malformations in the gland itself. Sometimes one of the enzymes essential for hormone production is missing, or the hormone produced is abnormal. More commonly, hypofunction is caused by disease or injury. Tuberculosis can appear in the adrenal glands, autoimmune diseases can affect the thyroid, and treatments for cancer—such as radiation therapy and chemotherapy—can damage any of the endocrine organs. Hypofunction can also result when target tissue is unable to respond to hormones. In many cases, the cause of a hypofunction disorder is unknown.

Hyperfunction can be caused by glandular tumors that secrete hormone without responding to feedback controls. In addition, some autoimmune conditions create antibodies that have the side effect of stimulating hormone production. Infection of an endocrine gland can have the same result.

One of the most common diseases of the endocrine systems is diabetes mellitus, which occurs in two forms. The first, called diabetes mellitus Type 1, is caused by inadequate secretion of insulin by the pancreas. Diabetes mellitus Type 2 is caused by the body's inability to respond to insulin. Both types have similar symptoms, including excessive thirst, hunger, and urination as well as weight loss. Laboratory tests that detect glucose in the urine and elevated levels of glucose in the blood usually confirm the diagnosis. Treatment of diabetes mellitus Type 1 requires regular injections of insulin; some patients with Type 2 can be treated with diet, exercise, or oral medication. Diabetes can cause a variety of complications, including kidney problems, pain due to nerve damage, blindness, and coronary heart disease. Recent studies have shown that controlling blood sugar levels reduces the risk of developing diabetes complications considerably.

Diabetes insipidus is caused by a deficiency of vasopressin, one of the antidiuretic hormones (ADH) secreted by the posterior lobe of the pituitary gland. Patients often experience increased thirst and urination. Treatment is with drugs, such as synthetic vasopressin, that help the body maintain water and electrolyte balance.

Hypothyroidism is caused by an underactive thyroid gland, which results in a deficiency of thyroid hormone. Hypothyroidism disorders cause myxedema and cretinism, more properly known as congenital hypothyroidism. Myxedema develops in older adults, usually after age 40, and causes lethargy, fatigue, and mental sluggishness. Congenital hypothyroidism, which is present at birth, can cause more serious complications including mental retardation if left untreated. Screening programs exist in most countries to test newborns for this disorder. By providing the body with replacement thyroid hormones, almost all of the complications are completely avoidable.

Addison's disease is caused by decreased function of the adrenal cortex. Weakness, fatigue, abdominal pains, nausea, dehydration, fever, and hyperpigmentation (tanning without sun exposure) are among the many possible symptoms. Treatment involves providing the body with replacement corticosteroid hormones as well as dietary salt.

Cushing's syndrome is caused by excessive secretion of glucocorticoids, the subgroup of corticosteroid hormones that includes hydrocortisone, by the adrenal glands. Symptoms may develop over many years prior to diagnosis and may include obesity, physical weakness, easily bruised skin, acne, hypertension, and psychological changes. Treatment may include surgery, radiation therapy, chemotherapy, or blockage of hormone production with drugs.

Thyrotoxicosis is due to excess production of thyroid hormones. The most common cause for it is Graves' disease, an autoimmune disorder in which specific antibodies are produced, stimulating the thyroid gland. Thyrotoxicosis is eight to ten times more common in women than in men. Symptoms include nervousness, sensitivity to heat, heart palpitations, and weight loss. Many patients experience protruding eyes and tremors. Drugs that inhibit thyroid activity, surgery to remove the thyroid gland, and radioactive iodine that destroys the gland are common treatments.

Acromegaly and gigantism both are caused by a pituitary tumor that stimulates production of excessive growth hormone, causing abnormal growth in particular parts of the body. Acromegaly is rare and usually develops over many years in adult subjects. Gigantism occurs when the excess of growth hormone begins in childhood.

Autoimmune Diseases

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Autoimmunity is the immune response of the body turned against its own cells and tissues. Autoimmune diseases may involve either cell-mediated responses, humoral responses, or both. For example, in Type 1 diabetes, the body makes an immune response against its insulin-producing cells and destroys them, with the result that the body cannot use sugars. In myasthenia gravis, the immune system makes antibodies against the normal molecules that control neuromuscular activity, causing weakness and paralysis. In rheumatic fever, the immune system makes antibodies that bind to the heart’s valves, leading to permanent heart damage. In systemic lupus erythematosus, commonly known as lupus, the body makes antibodies against many different body tissues, resulting in widespread symptoms.

The mechanisms of autoimmune diseases are poorly understood, and thus the basis for autoimmunity is unclear. Much research focuses on trying to understand these mechanisms and should eventually result in cures.

Immune Deficiency

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Deficiencies in immune function may be either inherited or acquired. Inherited immune deficiencies usually reflect the failure of a gene important to the generation or function of immune system components.

Some inherited diseases damage a person’s innate immunity by making macrophages incapable of ingesting or breaking down invading organisms. Individuals affected by these diseases are especially susceptible to opportunistic infections—that is, infections by normally harmless organisms that can flourish in a person whose immune system has been weakened.

DiGeorge syndrome is an inherited immune disorder in which a person has no thymus and, therefore, cannot produce mature T lymphocytes. People with this disorder can mount only limited humoral immune responses, and their cell-mediated immune responses are severely limited.

The most extreme example of a hereditary immune deficiency is severe combined immunodeficiency (SCID). Individuals with this disease completely lack both T and B lymphocytes and thus have no adaptive immune responses. People with SCID must live in a completely sterile environment, or else they will quickly die from infections.

Acquired immune deficiencies can be caused by infections and also other agents. For example, radiation therapy (see Radiology) and some kinds of drugs used in treating disease reduce lymphocyte production, resulting in damaged immune function. People undergoing such therapies must be carefully monitored for lowered immune function and susceptibility to infections. Environmental and lifestyle factors, such as poor nutrition or stress, can also affect the immune system’s general status.

An infectious agent resulting in fatal immune deficiency is the human immunodeficiency virus (HIV). This virus causes acquired immunodeficiency syndrome (AIDS) by infecting and eventually destroying helper T cells. Because helper T cells regulate all immune responses, their loss results in an inability to make adaptive immune responses. This complete lack of immune function makes individuals with AIDS highly susceptible to all infectious agents.

Transplant Rejection

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The immune system recognizes and attacks anything different from the substances normally present within an individual, even substances that are only slightly different, such as transplanted tissues and organs (see Medical Transplantation).

When an organ is transplanted, the MHC of the donor organ is recognized as foreign and attacked by the recipient’s immune system. To minimize the chances of transplant rejection, physicians seek transplant donors who share as many MHC genes as possible with the transplant recipient. Even then, most transplant recipients are given drugs to suppress their immune response and prevent rejection of the transplant.

If the transplanted tissue contains T lymphocytes from the donor, as in bone marrow transplants, these donor T lymphocytes may recognize the recipient’s tissues as foreign and attack them. Physicians can reduce or prevent this potentially fatal graft-versus-host (GVH) reaction by removing all mature T lymphocytes from the organ or tissue before performing the transplant.

Allergy

Allergy, sometimes called hypersensitivity, is caused by immune responses to some antigens. Antigens that provoke an allergic response are known as allergens. The two major categories of allergic reaction, rapid and delayed, correspond to the two major types of immune responses.

Rapid allergic reactions, such as those to bee venom, pollen or pets, are caused by humoral immune mechanisms. These immediate hypersensitivity reactions result from the production of IgE antibodies when a person is first exposed to an allergen. The IgE antibodies become attached to mast cells—white blood cells containing histamine, the chemical that causes the familiar allergic symptoms of runny nose, watery eyes, and sneezing. Mast cells are particularly abundant in the lungs and intestine. If the antigen-binding sites of mast cells become filled with an allergen, the mast cells release histamine.

Allergic reactions that are slow in onset (known as delayed-type hypersensitivity, or DTH), such as those to poison ivy or poison oak, are cell mediated. Extreme examples of DTH occur when macrophages cannot easily destroy invading substances. As a result, T cells are activated, leading to inflammation of the body tissue. This inflammation continues for as long as the T cells are activated. The bacterium that causes tuberculosis also falls into this category because this bacterium is covered with a waxy coat that macrophages cannot destroy. The resulting DTH leads to the lung and liver damage associated with tuberculosis.

Immune System Disorders

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Disorders of the immune system can range from the less serious, such as mild allergy, to the life threatening, such as more serious allergy, transplant rejection, immune deficiencies, and autoimmune diseases.

A. Allergy
B. Transplant Rejection
C. Immune Deficiency
D. Autoimmune Diseases

Vitamin and Mineral Chart

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Vitamins and Minerals

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Both vitamins and minerals are needed by the body in very small amounts to trigger the thousands of chemical reactions necessary to maintain good health. Many of these chemical reactions are linked, with one triggering another. If there is a missing or deficient vitamin or mineral—or link—anywhere in this chain, this process may break down, with potentially devastating health effects. Although similar in supporting critical functions in the human body, vitamins and minerals have key differences.

Among their many functions, vitamins enhance the body’s use of carbohydrates, proteins, and fats. They are critical in the formation of blood cells, hormones, nervous system chemicals known as neurotransmitters, and the genetic material deoxyribonucleic acid (DNA). Vitamins are classified into two groups: fat soluble and water soluble. Fat-soluble vitamins, which include vitamins A, D, E, and K, are usually absorbed with the help of foods that contain fat. Fat containing these vitamins is broken down by bile, a liquid released by the liver, and the body then absorbs the breakdown products and vitamins. Excess amounts of fat-soluble vitamins are stored in the body’s fat, liver, and kidneys. Because these vitamins can be stored in the body, they do not need to be consumed every day to meet the body’s needs.

Water-soluble vitamins, which include vitamins C (also known as ascorbic acid), B1 (thiamine), B2 (riboflavin), B3 (niacin), B6, B12, and folic acid, cannot be stored and rapidly leave the body in urine if taken in greater quantities than the body can use. Foods that contain water-soluble vitamins need to be eaten daily to replenish the body’s needs.

In addition to the roles noted in the VITAMIN AND MINERAL CHART, vitamins A (in the form of beta-carotene), C, and E function as antioxidants, which are vital in countering the potential harm of chemicals known as free radicals. If these chemicals remain unchecked they can make cells more vulnerable to cancer-causing substances. Free radicals can also transform chemicals in the body into cancer-causing agents. Environmental pollutants, such as cigarette smoke, are sources of free radicals.

Minerals are minute amounts of metallic elements that are vital for the healthy growth of teeth and bones. They also help in such cellular activity as enzyme action, muscle contraction, nerve reaction, and blood clotting. Mineral nutrients are classified as major elements (calcium, chlorine, magnesium, phosphorus, potassium, sodium, and sulfur) and trace elements (chromium, copper, fluoride, iodine, iron, selenium, and zinc).

Vitamins and minerals not only help the body perform its various functions, but also prevent the onset of many disorders. For example, vitamin C is important in maintaining our bones and teeth; scurvy, a disorder that attacks the gums, skin, and muscles, occurs in its absence. Diets lacking vitamin B1, which supports neuromuscular function, can result in beriberi, a disease characterized by mental confusion, muscle weakness, and inflammation of the heart. Adequate intake of folic acid by pregnant women is critical to avoid nervous system defects in the developing fetus. The mineral calcium plays a critical role in building and maintaining strong bones; without it, children develop weak bones and adults experience the progressive loss of bone mass known as osteoporosis, which increases their risk of bone fractures.

Vitamins and minerals are found in a wide variety of foods, but some foods are better sources of specific vitamins and minerals than others. For example, oranges contain large amounts of vitamin C and folic acid but very little of the other vitamins. Milk contains large amounts of calcium but no vitamin C. Sweet potatoes are rich in vitamin A, but white potatoes contain almost none of this vitamin. Because of these differences in vitamin and mineral content, it is wise to eat a wide variety of foods.

Fats

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Fats, which provide 9 calories of energy per gram, are the most concentrated of the energy-producing nutrients, so our bodies need only very small amounts. Fats play an important role in building the membranes that surround our cells and in helping blood to clot. Once digested and absorbed, fats help the body absorb certain vitamins. Fat stored in the body cushions vital organs and protects us from extreme cold and heat.

Fat consists of fatty acids attached to a substance called glycerol. Dietary fats are classified as saturated, monounsaturated, and polyunsaturated according to the structure of their fatty acids (see Fats and Oils). Animal fats—from eggs, dairy products, and meats—are high in saturated fats and cholesterol, a chemical substance found in all animal fat. Vegetable fats—found, for example, in avocados, olives, some nuts, and certain vegetable oils—are rich in monounsaturated and polyunsaturated fat. As we will see, high intake of saturated fats can be unhealthy.

To understand the problem with eating too much saturated fat, we must examine its relationship to cholesterol. High levels of cholesterol in the blood have been linked to the development of heart disease, strokes, and other health problems. Despite its bad reputation, our bodies need cholesterol, which is used to build cell membranes, to protect nerve fibers, and to produce vitamin D and some hormones, chemical messengers that help coordinate the body’s functions. We just do not need cholesterol in our diet. The liver, and to a lesser extent the small intestine, manufacture all the cholesterol we require. When we eat cholesterol from foods that contain saturated fatty acids, we increase the level of a cholesterol-carrying substance in our blood that harms our health.

Cholesterol, like fat, is a lipid—an organic compound that is not soluble in water. In order to travel through blood, cholesterol therefore must be transported through the body in special carriers, called lipoproteins. High-density lipoproteins (HDLs) remove cholesterol from the walls of arteries, return it to the liver, and help the liver excrete it as bile, a liquid acid essential to fat digestion. For this reason, HDL is called “good” cholesterol.

Low-density lipoproteins (LDLs) and very-low-density lipoproteins (VLDLs) are considered “bad” cholesterol. Both LDLs and VLDLs transport cholesterol from the liver to the cells. As they work, LDLs and VLDLs leave plaque-forming cholesterol in the walls of the arteries, clogging the artery walls and setting the stage for heart disease. Almost 70 percent of the cholesterol in our bodies is carried by LDLs and VLDLs, and the remainder is transported by HDLs. For this reason, we need to consume dietary fats that increase our HDLs and decrease our LDL and VLDL levels.

Saturated fatty acids—found in foods ranging from beef to ice cream, to mozzarella cheese to doughnuts—should make up no more than 10 percent of a person’s total calorie intake each day. Saturated fats are considered harmful to the heart and blood vessels because they are thought to increase the level of LDLs and VLDLs and decrease the levels of HDLs.

Monounsaturated fats—found in olive, canola, and peanut oils—appear to have the best effect on blood cholesterol, decreasing the level of LDLs and VLDLs and increasing the level of HDLs. Polyunsaturated fats—found in margarine and sunflower, soybean, corn, and safflower oils—are considered more healthful than saturated fats. However, if consumed in excess (more than 10 percent of daily calories), they can decrease the blood levels of HDLs.

Proteins

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Dietary proteins are powerful compounds that build and repair body tissues, from hair and fingernails to muscles. In addition to maintaining the body’s structure, proteins speed up chemical reactions in the body, serve as chemical messengers, fight infection, and transport oxygen from the lungs to the body’s tissues. Although protein provides 4 calories of energy per gram, the body uses protein for energy only if carbohydrate and fat intake is insufficient. When tapped as an energy source, protein is diverted from the many critical functions it performs for our bodies.

Proteins are made of smaller units called amino acids. Of the more than 20 amino acids our bodies require, eight (nine in some older adults and young children) cannot be made by the body in sufficient quantities to maintain health. These amino acids are considered essential and must be obtained from food. When we eat food high in proteins, the digestive tract breaks this dietary protein into amino acids. Absorbed into the bloodstream and sent to the cells that need them, amino acids then recombine into the functional proteins our bodies need.

Animal proteins, found in such food as eggs, milk, meat, fish, and poultry, are considered complete proteins because they contain all of the essential amino acids our bodies need. Plant proteins, found in vegetables, grains, and beans, lack one or more of the essential amino acids. However, plant proteins can be combined in the diet to provide all of the essential amino acids. A good example is rice and beans. Each of these foods lacks one or more essential amino acids, but the amino acids missing in rice are found in the beans, and vice versa. So when eaten together, these foods provide a complete source of protein. Thus, people who do not eat animal products can meet their protein needs with diets rich in grains, dried peas and beans, rice, nuts, and tofu, a soybean product.

Some health conditions, such as illness, stress, and pregnancy and breast-feeding in women, place an enormous demand on the body as it builds tissue or fights infection, and these conditions require an increase in protein consumption. For example, a healthy woman normally needs 45 grams of protein each day. Experts recommend that a pregnant woman consume 55 grams of protein per day, and that a breast-feeding mother consume 65 grams to maintain health.

A man of average size should eat 57 grams of protein daily. To support their rapid development, infants and young children require relatively more protein than do adults. A three-month-old infant requires about 13 grams of protein daily, and a four-year-old child requires about 22 grams. Once in adolescence, sex hormone differences cause boys to develop more muscle and bone than girls; as a result, the protein needs of adolescent boys are higher than those of girls.

Carbohydrates

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Carbohydrates are the human body’s key source of energy, providing 4 calories of energy per gram. When carbohydrates are broken down by the body, the sugar glucose is produced; glucose is critical to help maintain tissue protein, metabolize fat, and fuel the central nervous system.

Glucose is absorbed into the bloodstream through the intestinal wall. Some of this glucose goes straight to work in our brain cells and red blood cells, while the rest makes its way to the liver and muscles, where it is stored as glycogen (animal starch), and to fat cells, where it is stored as fat. Glycogen is the body’s auxiliary energy source, tapped and converted back into glucose when we need more energy. Although stored fat can also serve as a backup source of energy, it is never converted into glucose. Fructose and galactose, other sugar products resulting from the breakdown of carbohydrates, go straight to the liver, where they are converted into glucose.

Starches and sugars are the major carbohydrates. Common starch foods include whole-grain breads and cereals, pasta, corn, beans, peas, and potatoes. Naturally occurring sugars are found in fruits and many vegetables; milk products; and honey, maple sugar, and sugar cane. Foods that contain starches and naturally occurring sugars are referred to as complex carbohydrates, because their molecular complexity requires our bodies to break them down into a simpler form to obtain the much-needed fuel, glucose. Our bodies digest and absorb complex carbohydrates at a rate that helps maintain the healthful levels of glucose already in the blood.

In contrast, simple sugars, refined from naturally occurring sugars and added to processed foods, require little digestion and are quickly absorbed by the body, triggering an unhealthy chain of events. The body’s rapid absorption of simple sugars elevates the levels of glucose in the blood, which triggers the release of the hormone insulin. Insulin reins in the body’s rising glucose levels, but at a price: Glucose levels may fall so low within one to two hours after eating foods high in simple sugars, such as candy, that the body responds by releasing chemicals known as anti-insulin hormones. This surge in chemicals, the aftermath of eating a candy bar, can leave a person feeling irritable and nervous.

Many processed foods not only contain high levels of added simple sugars, they also tend to be high in fat and lacking in the vitamins and minerals found naturally in complex carbohydrates. Nutritionists often refer to such processed foods as junk foods and say that they provide only empty calories, meaning they are loaded with calories from sugars and fats but lack the essential nutrients our bodies need.

In addition to starches and sugars, complex carbohydrates contain indigestible dietary fibers. Although such fibers provide no energy or building materials, they play a vital role in our health. Found only in plants, dietary fiber is classified as soluble or insoluble. Soluble fiber, found in such foods as oats, barley, beans, peas, apples, strawberries, and citrus fruits, mixes with food in the stomach and prevents or reduces the absorption by the small intestine of potentially dangerous substances from food. Soluble fiber also binds dietary cholesterol and carries it out of the body, thus preventing it from entering the bloodstream where it can accumulate in the inner walls of arteries and set the stage for high blood pressure, heart disease, and strokes. Insoluble fiber, found in vegetables, whole-grain products, and bran, provides roughage that speeds the elimination of feces, which decreases the time that the body is exposed to harmful substances, possibly reducing the risk of colon cancer.

Water

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If the importance of a nutrient is judged by how long we can do without it, water ranks as the most important. A person can survive only eight to ten days without water, whereas it takes weeks or even months to die from a lack of food. Water circulates through our blood and lymphatic system, transporting oxygen and nutrients to cells and removing wastes through urine and sweat. Water also maintains the natural balance between dissolved salts and water inside and outside of cells. Our joints and soft tissues depend on the cushioning that water provides for them. While water has no caloric value and therefore is not an energy source, without it in our diets we could not digest or absorb the foods we eat or eliminate the body’s digestive waste.

The human body is 65 percent water, and it takes an average of eight to ten cups to replenish the water our bodies lose each day. How much water a person needs depends largely on the volume of urine and sweat lost daily, and water needs are increased if a person suffers from diarrhea or vomiting or undergoes heavy physical exercise. Water is replenished by drinking liquids, preferably those without caffeine or alcohol, both of which increase the output of urine and thus dehydrate the body. Many foods are also a good source of water—fruits and vegetables, for instance, are 80 to 95 percent water; meats are made up of 50 percent water; and grains, such as oats and rice, can have as much as 35 percent water.

Essential Nutrients

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Nutrients are classified as essential or nonessential. Nonessential nutrients are manufactured in the body and do not need to be obtained from food. Examples include cholesterol, a fatlike substance present in all animal cells. Essential nutrients must be obtained from food sources, because the body either does not produce them or produces them in amounts too small to maintain growth and health. Essential nutrients include water, carbohydrates, proteins, fats, vitamins, and minerals.

An individual needs varying amounts of each essential nutrient, depending upon such factors as gender and age. Specific health conditions, such as pregnancy, breast-feeding, illness, or drug use, make unusual demands on the body and increase its need for nutrients. Dietary guidelines, which take many of these factors into account, provide general guidance in meeting daily nutritional needs.

Cretinism

Cretinism, deficiency disease caused by congenital absence of thyroxine, a hormone secreted by the thyroid gland, and characterized by defective mental and physical development. Cretins have dwarfed bodies, with curvature of the spine and pendulous abdomen. Their limbs are distorted, their features are coarse, and their hair is harsh and scanty. Mental development is retarded throughout life. An adult cretin may reach the intelligence of only a four-year-old child. Research has revealed that when an animal or human infant is born with a deficiency of thyroxine, the neurons (nerve cells) in the brain do not develop the multiple branches that normally form the brain's complex network. Treatment of adult cretins with thyroxine or thyroid extract results in some improvement; early treatment in infancy results in cure of the disease and normal development of the individual, provided the treatment is continued throughout life. Treatment is sometimes begun prenatally, for instance, when a mother suffers from severe goiter, a disease of the thyroid.

Hallucinogen

Hallucinogen, any one of a large number of natural or synthetic psychoactive drugs that produce marked distortions of the senses and changes in perception. Hallucinogens generally alter the way time is perceived, making it appear to slow down. As the name suggests, hallucinogens may produce hallucinations, which are shape- and color-shifts in the appearance of the outside world or, in extreme cases, the replacement of external reality with imaginary beings and landscapes. Hallucinogens may also lead to bizarre and antisocial thoughts as well as to disorientation and confusion. The physiological basis of such experiences is not clear, but evidence suggests that hallucinogens work by inhibiting the availability of serotonin, an important neurotransmitter in the brain.

The best-known and most potent hallucinogen is lysergic acid diethylamide (LSD). Other hallucinogens include psilocybin, which comes from certain species of mushrooms, and mescaline, which is derived from a cactus called peyote. These drugs are of medical interest because they produce effects similar to the symptoms of schizophrenia, a severe mental illness accompanied by distortions of reality. Some researchers believe that a hallucinogen-induced state can be used as a model to study schizophrenia.

Hallucinogens may be taken orally, injected, or, in the case of marijuana (a mild hallucinogen), smoked and inhaled. They usually take effect within an hour and cause increases in blood pressure, body temperature, and pulse rate as well as dilation, or enlargement, of the pupils of the eyes. These drugs may also cause nausea and numbness.

Individual reactions to hallucinogens are unpredictable, especially when these drugs are used recreationally—that is, for the pleasurable effects they produce and not for medical purposes. The experience of the drug may be pleasurable one day and highly disturbing the next, depending on the setting and circumstances in which the drug is taken and the individual’s personality and mood at the time. The effects of hallucinogenic drugs may last from a few hours to several days, and may recur months later in what are referred to as flashbacks.

Most hallucinogens do not cause physical dependence with chronic use, although tolerance of behavioral effects can develop, in which case more of the drug is needed to create the same mental states (see Drug Dependence).

Gonorrhea

Gonorrhea, infectious sexually transmitted infection of humans, which primarily involves the mucous membranes of the urogenital tract. Characterized by a discharge of pus, it is caused by the gonococcus bacterium, Neisseria gonorrhoeae. The incubation period is two to seven days.

SYMPTOMS AND DIAGNOSIS

Gonorrhea is much more obvious in males, who develop an acute discharge of pus from the urethra. Scant at the start, it becomes progressively thicker and heavier and causes frequent urination, often with a burning sensation. Should the prostate become infected, the passage of urine is partly obstructed. In females the infection occurs in the urethra, the vagina, or the cervix. Although discharge and irritation of the vaginal mucous membranes may be severe, more often few or no early symptoms appear.

Gonorrhea can be diagnosed by examining discharge from the penis or vagina for the presence of bacteria. A urine test can also detect the presence of bacteria and it is a noninvasive way to check people who are not exhibiting symptoms. Treatment in the early stages is usually effective. If the disease is untreated in the male, the early symptoms may subside but the infection may spread to the testicles, causing sterility. In the untreated female the infection usually spreads from the cervix into the uterus and fallopian tubes, causing pelvic inflammatory disease (see Gynecology). Severe pain may occur, or the infection may linger with few or no symptoms, gradually damaging the tubes and rendering the woman sterile. In both sexes the gonococcus may enter the bloodstream, resulting in arthritis, heart inflammation, or other diseases. Gonorrhea in pregnant women may be transmitted to the infant during birth and may, if untreated, cause a serious eye infection.

TREATMENT

Antibiotics are commonly used against gonorrhea, although over the years an increasing number of antibiotic-resistant strains of the gonorrhea bacteria have developed, causing a rise in the incidence of the disease in the 1990s. Still effective are antibiotics that are administered in a single dose, including ceftriaxone, cefixime, ciprofloxacin, and ofloxacin.

Glaucoma

Glaucoma, family of eye diseases characterized by increased pressure within the eye and resulting loss of vision. Although the vision loss caused by glaucoma is irreversible, medication or surgery can usually control the pressure within the eye and slow or halt the progression of the disease. If glaucoma is diagnosed in its early stages, it can be treated effectively, and vision can usually be preserved.

Glaucoma occurs in two main varieties, known as chronic simple and acute glaucoma. In chronic simple glaucoma, pressure within the eye rises gradually, and vision loss progresses over a period of years. In acute glaucoma, the pressure inside the eye rises suddenly and immediate medical treatment is necessary to preserve vision. Acute glaucoma is also known as narrow-angle or angle-closure glaucoma, because the angle between the cornea (the transparent layer of tissue at the front of the eye) and the iris (the colored part of the eye) becomes smaller than normal. Chronic simple glaucoma is also known as open-angle glaucoma because the angle between the cornea and iris remains normal. This article focuses on chronic simple glaucoma, which accounts for 95 percent of all cases of glaucoma.

SYMPTOMS AND DIAGNOSIS

In its early stages, glaucoma is symptomless. The gradual increase of pressure inside the eye does not cause any pain or discomfort. As the disease progresses, however, vision begins to deteriorate. The deterioration usually begins with the peripheral vision—sight at the outer edges of the visual field. If glaucoma is left untreated, the field of vision continues to shrink until a person becomes blind.

Glaucoma can be detected before vision loss occurs by a tonometry test, which is a simple, painless part of a routine eye exam. An instrument called a tonometer blows a puff of air into the eye to measure the pressure inside the eye. Some tonometers measure pressure by means of a small plastic prism that is pressed lightly against the surface of the eye. Tests to measure peripheral vision help detect vision loss due to glaucoma. Finally, an instrument called an ophthalmoscope permits examination of the inside of the eye to detect damage to the optic nerve (see Ophthalmology).

TREATMENT

Most cases of glaucoma can be controlled with medication in the form of eyedrops or pills. These medications reduce the pressure inside the eye either by decreasing the amount of aqueous humor produced or by increasing the rate at which the fluid drains from the eye.

When the pressure inside the eye cannot be controlled by medication, surgery may be necessary. In laser surgery, a high-energy laser beam is aimed at the eye to help stretch the trabecular meshwork and make it easier for fluid to flow out of the eye. Sometimes patients who already have had laser surgery develop increased pressure inside the eye again. At that point, conventional surgery is often performed to create a new channel through which fluid can leave the eye. A valve may be surgically implanted in the eye to help control the pressure.

Gene Therapy

Gene Therapy, experimental medical treatment that manipulates a gene or genes within cells in order to produce proteins that change the function of those cells. Gene therapy originated in efforts to treat and cure some of the more than 9,000 known genetic disorders, most of which lack an effective therapy.

The original goal of gene therapy was to substitute a healthy gene for a defective one, or to repair a faulty gene, thereby eliminating symptoms of disease. But researchers have moved beyond inherited genetic disorders to treat other kinds of diseases. Cancer begins in genes and may be caused by an inherited defect or a mutation (permanent alteration to a gene) that causes a cell to malfunction. AIDS is caused by a virus that disrupts the genetic material of immune cells. Other new gene therapy projects are targeted at conditions such as heart disease, diabetes mellitus, arthritis, and Alzheimer's disease, all of which involve genetic susceptibility to illness. Gene therapists hope to reduce or eliminate this susceptibility. Eventually, gene therapy might help older people to regain strength in withered muscles and density in thinned bones, and to increase pumping power in their aging hearts. Some researchers predict that in the distant future the technology could be used to eliminate genetic defects from families or even to produce “designer babies” with more muscle strength, higher intelligence, sweeter dispositions, or whatever traits parents desire.

Gastritis

Gastritis, acute or chronic inflammation of the mucosal lining of the stomach. The ailment was first described in 1833 by the American military surgeon William Beaumont, who was able to study the mucosa of a man who had suffered a gunshot wound in the abdomen. The wound healed with an opening in the stomach wall, through which Beaumont was able to observe the patient's digestive process. In gastritis the inflammation may be marked by the erosion of surface cells of the mucosa, formation of granular nodules, and hemorrhage. In chronic gastritis, there is a growth of fibrous tissue on the lining. Weight loss and delayed emptying of the contents of the stomach may accompany the disease. Gastritis may be caused by excessive consumption of alcohol, abnormal secretion of hydrochloric acid in the gastric juices, and various infections ranging from syphilis and tuberculosis to fungus. Psychological stress may also be involved in the development of gastritis.

Gangrene

Gangrene, mass death or necrosis of individual cells or tissues of a living organism. The immediate physiological cause of gangrene is blockage of arterial blood supply. Among the symptoms of gangrene are loss of sensation and function. The affected part, usually a limb, becomes cold, turns progressively darker in color, and finally undergoes decomposition. Gangrene may take two forms, dry gangrene or moist gangrene.

Dry, or chronic, gangrene occurs when the arteries are blocked gradually and the tissues are bloodless. It may be a result of arteriosclerosis (see Artery). It may also follow frostbite, injury, or the vascular collapse that often accompanies diabetes.

If the dead, bloodless tissue becomes infected by the anaerobic bacterium Clostridium, the condition is called moist, or acute, gangrene. It is also known as gas gangrene because of its putrid-smelling gaseous discharge. The victim's temperature rises sharply, and he or she suffers great pain, acute anemia, and prostration. Moist gangrene may follow contamination of a wound with dirt or other infected matter. It once occurred frequently with war wounds and still occurs among diabetics and after abortions performed under unsanitary conditions. Moist gangrene is fatal unless treated with antibiotics. In severe cases amputation is necessary.

Blood Transfusion

Blood Transfusion, in medicine, the procedure of introducing the blood of a donor or blood predonated by the recipient (autologous transfusion) into the bloodstream. It is a highly effective form of therapy and has saved the lives of incalculable numbers of people suffering from shock, hemorrhage, or blood diseases. Blood transfusion is employed routinely in cases of surgery, trauma, gastrointestinal bleeding, and in childbirths that involve great loss of blood.

Transfusions still tend to cause the development of sensitivity and increase the possibility that the recipient will react to any later transfusions. Transmission of viral hepatitis was a major risk until a method of screening blood for infectivity was developed in the 1960s; some other forms of hepatitis, however, are not detected by this test. In 1985 a test was introduced that screens donated blood for an antigen associated with AIDS, or acquired immunodeficiency syndrome.