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Figure 19. Triglyceride digestion.

fatty acids, and glycerides emulsify fat, thus facilitating its digestion by lipase.

In the small intestine, half of the ingested triglycerides are hydrolyzed by lipase to form free fatty acids and glycerols. The rest are changed to monoglycerides and a small amount of diglycerides. The monoglycerides are absorbed into the intestinal mucosa, where they are further hydrolyzed to glycerols and free fatty acids.

Free fatty acids in the intestine are absorbed in two ways. Fatty acids with less than 10-12 carbon atoms pass directly from the intestinal lumen, through mucosal cells, into the portal vein, and to the liver. Here some are released into circulation as free fatty acids, some are converted to triglycerides for deposition, and some are circulated in the blood as glycerides or as fatty acids that reside within the complex of lipoproteins. Fatty acids with more than 10-21 carbon atoms are absorbed into the mucosal cells, where they are regrouped with glycerols to form triglycerides. The triglycerides attach themselves to very low density lipoproteins to form chylomicrons, which enter the systemic circulation via the lymph and thoracic duct. Chylomicrons are fat globules 1 /¿m in diameter and visible under the microscope.

Other fatty substances are absorbed to varying degrees. For example, animal sterols are absorbed easier than plant sterols. Pancreatic secretion, fatty acids, and bile salts, which together emulsify and esterify cholesterol, are necessary for cholesterol absorption of cholesterol. It is currently believed that cholesterol is absorbed mainly in the ileum. Like the triglycerides, absorbed cholesterol is incorporated into the chylomicrons, which reach the systemic circulation.

Within two to three hours after the ingestion of food containing short-chain fatty acids, the blood level of chylomicrons remains unchanged, although it may rise sharply if the meal contains long-chain fatty acids. Normally, after a mixed meal, the plasma develops a milky appearance because of the presence of chylomicrons in the blood. This is sometimes known as lipemia. In the presence of the enzyme lipoprotein lipas, these plasma chylomicrons are cleared and their contents diverted to the liver and adipose tissue.

Blood plasma, therefore, contains fat in the following forms: fatty acids, glycerol, glycerides, cholesterol, choles terol esters, and phospholipids. These forms are bound to the albumin, a-globulin, and /?-globulin fractions of the plasma proteins. The resulting lipid-protein complexes have varying densities. The highest densities occur in those with the most protein and least lipid; the lowest densities occur in those with the least protein and most lipid. Consequently, the complexes are classified into high-density, low-density, and very low density lipoproteins. In general, very low density lipoproteins carry mainly triglycerides; low-density lipoproteins carry mainly cholesterol; and a-lipoproteins carry phospholipids, albumin, and free fatty acids. For a normal person, about 95% of ingested fat is absorbed, mainly in the duodenum and jejunum, with some absorption by the ileum. About 5% of fecal waste is fat, which comes from the diet, cell debris, and bacterial synthesis.

Although most of the fats are emptied into the lymphatic system after absorption and eventually reach the systemic circulation, the bile salts separate from the fats and travel through the portal vein into the liver. There they are reincorporated into the bile. Bile salts are thus cycled through the enterohepatic circulation (the liver, gallbladder, intestinal lumen, portal vein, and back to the liver). About 80-90% of bile salts in the intestinal lumen are reabsorbed in this way; the rest are lost in the stool.

Cellular Metabolism

The adult body distributes fats to two main locations: the membranes and other structural parts of cells (commonly called structural fats) and the fat cells (neutral fats), which are mainly white. Infants have some brown fat cells, which can regulate body temperature by producing heat to support the baby's higher metabolic rate. Neutral body fat contains mainly triglycerides, plus small amounts of diglycerides and monoglycerides, which are important metabolic intermediates. Consequently, triglycerides are the main form of stored energy.

Fat Degradation. Stored fat is degraded as needed to provide energy. Fat degradation occurs in two major stages: hydrolysis of glycerides and oxidation of fatty acids. In the adipose tissues, glycerides are hydrolyzed by a lipase to form fatty acids and glycerols. Both of these are released into the circulation for transport to the liver, where further hydrolysis may occur. When triglycerides are hydrolyzed, the released glycerols can be converted to phosphoglyceraldehyde in the liver (Fig. 6 and 20). This compound can in turn be converted to either carbon dioxide and water or glucose.

The process of oxidizing the fatty acids to carbon dioxide, water, and energy is called /^-oxidation, or alternate oxidation. It occurs mainly in the mitochondria of liver cells. The carbon chain is broken down by the successive removal of two-carbon fragments from the carboxyl end to form acetic acids. These can combine with CoA to form acetyl-CoA, which can enter the citric acid cycle to be oxidized (Figs. 8, 17, and 20). When the fatty acids are reduced to acetyl-CoA, hydrogen atoms are also released, which can be passed on to the respiratory chain. When fatty acids are completely oxidized, they generate more ATP than the molecular equivalent of carbohydrate because less oxygen is present. This explains why fat has a higher caloric value. However, unsaturated fatty acids generate less energy than the molecular equivalent of saturated fatty acids because less hydrogen is present in the former.

Most of the naturally occurring fatty acids are even numbered, and thus their oxidation always produces acetyl-CoA at the end. However, if the fatty acids happen to be odd chained, propionyl-CoA is formed instead.

Propionyl-CoA can also enter the citric acid cycle if the coenzyme with vitamin B12 is available.

Fat Synthesis. Fat synthesis takes place in two major stages: the formation of fatty acids and the formation of triglycerides. Fatty acids synthesis is achieved in two places: the mitochondria and the cytoplasm. Within the mitochondria, ^-oxidation is reversed and two-carbon units are added until the appropriate fatty acids are formed. Outside the micochondria, in the cytoplasm, another form of fatty acid synthesis occurs. Here the starting compound is acetyl-CoA which serves as the end of the fatty acid molecule. The remaining carbons are incorporated as two-carbon units derived from the malonyl group. The incorporation is accompanied by simultaneous recarboxylation. The fatty acids formed are mainly 12-14 carbons long and rarely more than 16. The body can synthesize unsaturated fatty acids from the saturated ones by removing hydrogen, although it is unable to synthesize the essential ones.

In the adipose tissues, fatty acids combine with glycerol to form triglycerides, or neutral fats. This reaction occurs in the mitochondria. Figure 20 summarizes the information on fat synthesis.

As indicated earlier, glycerol can be converted to glucose (gluconeogenesis). However, acetyl-CoA cannot be converted to pyruvic acid (Fig. 10). Although keto acids can enter the citric acid cycle, there is very little net conversion

Triglycerides

Triglycerides

Fatty acids, glycerols

Fatty acids, glycerols

Liver

Triglycerides — Fat

Glucose

Glycerols— Phosphoglyceraldehyde

Fatty acids

Keto acids

Acetyl-CoA

Ketone bodies

Citric acid cycle

Muscle Fat

Citric acid cycle

I Triglycerides Acetyl-CoA

Ketone bodies

Fatty acids, giycerots

Ketone bodies

Fatty acids, glycerols

Blood

Figure 20. An overview of fat metabolism.

of fat to carbohydrate in the body, with the exception of the small amount of phosphogyceraldehyde formed from glycerol.

Ketone Bodies. During the normal process of ^-oxidation of fatty acids, the liver has the appropriate enzyme to remove the CoA from acetoacetyl-CoA to form acetoacetic acid. Acetones and /i-hydroxybutyric acids can be formed from acetoacetic acids. The last three compounds are collectively called ketone bodies. The small amount of ketone bodies normally made by the liver is transported by the circulation to the muscle for conversion to acetyl-CoA, which is put through the citric acid cycle (Fig. 20). Acetone is eliminated via urination and respiration. Because under normal circumstances the ketone bodies are metabolized as soon as they are formed, a person rarely excretes more than 1 mg of ketone each day, and blood levels are usually less than 1 mg/100 mL.

However, the ketones can accumulate under certain conditions, and the resulting clinical condition is known as ketosis. The main cause of ketosis is the accumulation of acetyl-CoA because the citric acid cycle in the liver is not operating at its normal or optimal efficiency. The most common cause is a sequence of events called intracellular carbohydrate starvation. First, decreased supply of glucose leads to a reduction in pyruvic acid, acetyl-CoA, and cellular energy supply. Second, for compensation, fatty acid oxidation is increased to provide energy with an accumulation of acetyl-CoA. Third, the oversupply of acetyl-CoA leads to the formation of ketone bodies.

Glucose supply to cells is reduced in people with diabetes mellitus and people who undergo dietary alterations

I ngested nucleic acids

Products of digestion

Figure 21. Digestion of nucleic acids.

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