DIGESTION AND ABSORPTION
Although 95% of ingested fats are digested, it is a complex process. The chemi-cal digestion of fats occurs mainly in the small intestine. Fats are not digested in the mouth. They are digested only slightly in the stomach, where gastric lipase acts on emulsified fats such as those found in cream and egg yolk. Fats must be mixed well with the gastric juices before entering the small intestine.
In the small intestine, bile emulsifies the fats, and the enzyme pancreatic lipase reduces them to fatty acids and glycerol, which the body subsequently absorbs through villi (Figure 5-3).
Fats are insoluble in water, which is the main component of blood. There-fore, special carriers must be provided for the fats to be absorbed and transported by the blood to body cells. In the initial stages of absorption, bile joins with the products of fat digestion to carry fat. Later, protein com-bines with the final products of fat digestion to form special carriers called lipoproteins. The lipoproteins subsequently carry the fats to the body cellsby way of the blood.
Lipoproteins are classified as chylomicrons, very-low-density lipo-proteins (VLDLs), low-density lipoproteins (LDLs), and high-density lipoproteins (HDLs), according to their mobility and density. Chylomicronsare the first lipoprotein identified after eating. They are the largest lipoproteins and the lightest in weight. They are composed of 80% to 90% triglycerides. Lipoprotein lipase acts to break down the triglycerides into free fatty acids and glycerol. Without this enzyme, fat could not get into the cells.
Very-low-density lipoproteins are made primarily by the liver cells and are composed of 55% to 65% triglycerides. They carry triglycerides and other lipids to all cells. As the VLDLs lose triglycerides, they pick up cholesterol from other lipoproteins in the blood, and they then become LDLs. Low-density lipoproteins are approximately 45% cholesterol with few triglycerides. They carry most of the blood cholesterol from the liver to the cells. Elevated blood levels greater than 130 mg/dl of LDL are thought to be contributing factors in atherosclerosis. Low-density lipoprotein is sometimes termed bad cholesterol.
High-density lipoproteins carry cholesterol from the cells to the liver for eventual excretion. The level at which low HDL becomes a major risk factor for heart disease has been set at 40 mg/dl. Research indicates that an HDL level of 60 mg/dl or more is considered protective against heart disease. High-density lipoproteins are sometimes called good cholesterol. Exercising, maintaining a desirable weight, and giving up smoking are all ways to in-crease one’s HDL.