Fats provide 9 calories of energy per gram and 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. Animal fats are high in saturated fats and cholesterol, a chemical substance found in all animal fat. Vegetable fats are rich in monounsaturated and polyunsaturated fat. High intake of saturated fats can be unhealthy.
To understand the problem with eating too much saturated fat, we must first 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. Our bodies need cholesterol 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 the small intestine, manufacture all the cholesterol we require. When we eat saturated fatty acids we increase the level of a cholesterol carrying substance in our blood.
Cholesterol in addition to fat, is a lipid-an organic compound that is not soluble in water. Cholesterol must be transported through the body in special carriers, called lipoproteins found in blood. High-density lipoproteins (HDLs) remove cholesterol from the walls of arteries returning it to the liver. The liver excretes it as bile. 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 clogging the artery walls. Because almost 70 percent of the cholesterol in our bodies is carried by LDLs and VLDLs, we need to consume dietary fats that increase our HDLs and decrease our LDL and VLDL levels.
Saturated fatty acids 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. If consumed in excess (over 10 percent) they can decrease the blood levels of HDLs.
Health experts consider diets with more than 30 percent of calories from fat to be unsafe and can increase the risk of heart disease. High-fat diets contribute to obesity, which is linked to high blood pressure and diabetes mellitus. A diet high in both saturated and unsaturated fats has also been associated with greater risk of developing cancers of the colon, prostate, breast, and uterus. Choosing a diet that is low in fat and cholesterol is critical to maintaining health and reducing the risk of life-threatening disease.