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.
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.
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