Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
FAT METABOLISM A discussion of the changes that the various fat components under- go in the body is pertinent to a consideration of the role of fat in health and disease. Digestion and Absorption A small amount of fat-splitting may take place in the stomach, but most of the digestion of food fat is carried on in the intestine through the action of intestinal and pancreatic enzymes and of bile. The main path of fat digestion progresses from triglycerides to 1,2-diglycerides, to 2-monoglycerides, and finally to free fatty acids and glycerol, perhaps after isomerization to the 1-monoglyceride (159). During digestion, an exchange of free fatty acids with glyceride fatty acids occurs. Furthermore, some synthesis of triglycerides from the mono- and diglycerides takes place simultaneously with hydrolysis of the fat (2). Thus the earlier stages of fat digestion are reversible processes and modify the makeup of ingested food fat. There is some selectivity in reincorporation of liberated fatty acids into the glyceride molecule in the intestine. For example, butyric acid fed at the same time as long-chain glycer- ides is not incorporated into the glycerides, whereas longer chain acids are readily interchanged (24). Thus, in the intestinal lumen, the action of pancreatic lipase on ingested fat results in a complex mixture of tri-, di-, and monoglycerides and fatty acids. In addition, the entry of bile into the duodenum contributes important amounts of bile salts and lecithin, the latter quickly undergoing hydrolysis to lysolecithin. Both of these classes of compounds are essential to solubiliza- tion of the lipids in the intestinal contents, which form a two- phase systemâan oil phase containing almost all the tri- and diglycerides, and a water-clear micellar solution of monoglycer- ides, bile salts, lysolecithin, and soaps. All present evidence is consistent with the view that absorption takes place from the 11
Dietary Fat and Human Health micellar phase into the mucosal lining of the intestine. In this manner, extremely small aggregates of monoglycerides and fatty acids are transferred through the brush border of the intestinal mucosa and are then re-esterified to triglycerides within the mucosal cell. In turn, through further lypolysis, the micellar phase is continually replenished from the oil phase. Thus, it is doubtful that significant amounts of tri- and diglycer- ides are absorbed as such. The mechanism of absorption of sterols, lysolecithin, and the fat-soluble vitamins remains to be elucidated. Absorption of fatty acids takes place mainly in the upper small intestine, and the absorbed long-chain fatty acids (with more than 12 carbon atoms), after re-esterification to triglycer- ides, are transported through the intestinal lymphatics into the thoracic duct, thence into the great veins in the neck in the form of chylomicrons. These chylomicrons appear in the blood stream 3 to 6 hr after a fatty meal. Cholesterol is largely re-esterified in the mucosal cell and is incorporated into the chylomicron along with the triglycerides. By contrast, bile acids and fatty acids of chain length shorter than 12 carbon atoms pass into the portal circulation rather than the intestinal lymphatics; they are transported directly to the liver, probably bound to albumin and to the nonparticulate lipoproteins. Under normal circumstances, fat absorption is 95 to 100 per- cent for most food fats. More than 50 percent of dietary choles- terol is absorbed, but the percentage is reduced in the presence of plant sterols. In addition to absorption of dietary fat, reabsorption of the lipids contributed by the bile takes place. The bile acids re- circulate from the portal vein blood through the liver into the bile and intestinal tract and back to the liver several times each day. Cholesterol and ledithin in the bile are also largely reabsorbed. The enterohepatic circulation of bile acids and cholesterol regulates the endogenous synthesis of cholesterol and the conversion of cholesterol to bile acids, exemplifying a negative feedback control system. Plasma Lipids and Their Transport The transport of fatty acids through the extracellular fluid is in the range of 200 to 300 gm per day. The bulk of the fatty acids in net transit is in either glycerides or free fatty acids. 12
Fat Metabolism The transport of such large quantities of water-insoluble material in the plasma and lymph occurs in complexes stabilized by specific proteins as well as more polar lipids such as phospho- lipids. The transport systems may be divided into: the albumin- free fatty acid complex; high-density lipoproteins (HDL) or a-lipoproteins; low-density lipoproteins (LDL) or ft-lipoproteins; and glyceride-rich lipoproteins or larger aggregates called particles. Glyceride arising endogenously from synthesis in the liver is found in very LDL of a, or pre-0 electrophoretic mobility. When the glyceride is of exogenous origin it is mainly in particles called chylomicrons (46). Free Fatty Acids The free fatty acids make up only a small proportion of the total fatty acids in plasma. Their concentration normally ranges from 0.2 to 0.7 meq per liter. Tightly bound to their albumin carrier, they nevertheless join and leave this carrier easily at cell surfaces. Their plasma turnover is extremely rapid, per- mitting the equivalent of several thousand calories per day to be transported in this form (58). Lipoproteins In the fasting state, most of the plasma lipid is present in the a- and £ -lipoproteins. In apparently healthy American adults, this comprises total lipids varying as widely as from 400 to 1,000 mg per 100 ml of plasma that may include 120 to 350 mg of cholesterol, 150 to 380 mg of phospholipid, and 25 to 150 mg of triglyceride (59). Consideration of age and sex is required in evaluating the significance of plasma lipid concentrations, and what is usual cannot be considered necessarily normal. Approxi- mately 45 percent of the fasting plasma fatty acid content is in phospholipids, 35 percent in glycerides, 15 percent, relatively more unsaturated, in cholesterol esters, and 5 percent as free fatty acids. a-Lipoproteins In the postabsorptive state, roughly one fourth of the plasma cholesterol by weight and one half of the phospholipids are com- bined with a specific globulin designated A or a-apolipoprotein. The composition of these resultant macromolecules is fairly constant, 13
Dietary Fat and Human Health and they have hydrated densities between 1.06 and 1.2. When isolated between these density limits in the ultracentrifuge, they are called HDL. When they are identified or isolated by electro- phoresis, these lipoproteins are called a-lipoproteins from their characteristic mobility. HDL concentrations do not change greatly with age. They are significantly lower in men than in women (29, 67, 100, 130, 157, 173) and in men can be elevated by giving estrogens (51). HDL concentrations have been reported to be low in patients with coronary artery disease (15, 100, 173), but neither predictive value nor an etiologic role in atherosclerosis has been assigned them. It has recently been reported (128) that a-lipoproteins are not only present as HDL but are also part of the very low-density complexes involved in the transport of endogenous glyceride. It is possible, then, that a - lipoproteins take part in the transport of glyceride but this is not proved. From patients with Tangier disease (61) it is known that severe deficiency of the A apopro- tein is associated with marked tissue storage of cholesterol esters, and this protein may have some special function in transporting cholesterol. /3 - and Pre- /3 - lipoproteins Although the protein in LDL is considered to be homogeneously the B apoprotein, some minor antigenic variations among humans, which are genetically determined, have been suggested (23). From observations of patients with abetalipoproteinemia (174), it ap- pears that when significant amounts of glyceride are synthesized by the liver, there is probably a requirement for the /3 - lipoprotein (or at least the B apolipoprotein) to mobilize the glyceride from the cells. As already noted, the triglyceride plus B lipoproteins is joined by a-lipoproteins to form "very low density" lipoproteins that have an electrophoretic mobility between the 0- and a-lipo- proteins (hence, the terms pre-0 on paper (127) and a2-lipoproteins on starch) (120). When the amounts of endogenous glyceride become large enough, there is aggregation into bigger molecules or particles, which scatter light. On paper electrophoresis in a buffer containing albumin (127), these endogenous particles trail from the pre-^ region to the origin (126). The origin of the component lipids and special proteins and the sites of assembly of the lipoprotein molecules remain to be precise- ly defined. It seems most likely that the liver synthesizes much of 14
Fat Metabolism the cholesterol (63, 76) and most of the phospholipid and triglycer- ide found in plasma lipoproteins. The apoproteins A and B are also probably synthesized mainly in the liver (139, 168), although the intestines (171) and other tissues may also have this capacity. The ability of the liver for lipoprotein synthesis will obviously be dependent on the flow to it of precursors such as amino acids, carbohydrates, and adipose tissue fatty acids. Thus many pro- cesses, remote from the liver, can grossly affect the plasma lipoproteins. For example, with malabsorption there is severe decrease in plasma concentrations of LDL and HDL. In the American population, a- and ft - lipoprotein concentrations nearly double in the first few weeks of age, then rise very slowly up to about age 20-25 years. Then the /3-lipoproteins almost invariably rise more sharply with age during at least the third, fourth, and fifth decades in both sexes (67). The pre-/3-lipopro- teins also tend to be more commonly detectable after the third decade; this latter group of lipoproteins is very common in men who have had a myocardial infarction (18), although no prospective study of such an association has been made. From the foregoing, it can be seen that the disordering of metabolism or transport of the several lipids found in plasma involves generally increase in concentrations of LDL and decrease in those of HDL. It is also apparent that in hyperlipidemia, with increased cholesterol or glycerides or both, there will also be an increase in one or more classes of LDL. The apparent value of measurements of both lipids and these lipoproteins in attempts to predict vulnerability to atherosclerosis arises from these associations. Particles (Chylomicrons) As already noted, when glycerides are present in high concentra- tion they tend to form particulate emulsions (46). These are stabilized by phospholipids in combination with cholesterol and perhaps small amounts of protein. They scatter visible light and produce lactescence in blood. Such particles isolated from extracellular fluid may be as large as 1,500 mji in diameter, although it is uncertain whether they achieve such size in_ vivo. The physiological particle is the chylomicron formed during fat absorption. This package of glyceride is carried up the thoracic duct into the blood stream and is rapidly removed by many tissues, including muscle and adipose tissue. The role of the liver in this process is still uncertain. Within the tissues, 15
Dietary Fat and Human Health the glycerides are hydrolyzed and the fatty acids appear in phospholipids and other glycerides. Some of the fatty acids re- appear quickly in plasma, both as esterified lipids containing lipoproteins and as free fatty acids. In heart and adipose tissue, although not in liver tissue, the enzymes responsible for hydrolysis of particulate glycerides include lipoprotein lipase. This enzyme catalyzes hydrolysis of glyceride and is activated in plasma by heparin or other sulfated polysaccharides. The particles of glyceride arising endogenously (pre-/3- lipoproteins) may also be handled by the same mechanisms utilized for clearing exogenous glycerides. This is not yet known, however, it appears that most examples of endogenous hyperlipemia (hyperglyceridemia with lactescent plasma) are not associated with low activity of postheparin lipases (mainly lipo- protein lipase) (60). Lipid Synthesis and Utilization The metabolism of lipids cannot be considered independently of other metabolic processes. Phospholipids, cholesterol, and tri- glycerides are intricately linked in a highly complex network of reactions. Both cholesterol and the fatty acids are synthesized enzymatically in the body from the same fundamental unit, acetylcoenzyme A. Enzymatic syntheses of phospholipids and triglycerides also progress through common intermediates. Because of the close interlinking of these metabolic reactions, it is necessary to consider lipid metabolism in the broad sense in connection with nutrition or with pathological conditions. Fatty Acids and Glycerides Most tissues in the body not only participate in the synthesis of fat but also have the ability to oxidize fats completely and rapidly. A high percentage of the energy used by heart muscle is derived from free fatty acids. The biosynthesis of fatty acids from carbo- hydrate and amino acids and their oxidation are accomplished through reversible reactions involving acetate and coenzyme A. Fatty acids may also be interconverted in the body through stepwise alteration of the chain length by addition or removal of two-carbon fragments and by introduction or removal of double bonds. Most mammals, including man, are able to 16
Fat Metabolism introduce one double bond into stearic acid to make oleic acid, but cannot continue the desaturation to yield linoleic acid. Given a dietary source of linoleic acid, the body can synthesize arachi- donic acid, but cannot do so from a saturated fat, oleic acid, or the basic reactions of fat synthesis. Thus, fat derived largely by synthesis from carbohydrate is more saturated and more firm than that derived from dietary fat, due to the low rate of conver- sion of the newly formed monoenoic acids to more unsaturated acids. Arachidonic acid is the principal polyunsaturated fatty acid that the animal synthesizes from linoleic acid. When linolenic acid is fed, even more highly unsaturated and longer chain acids are formed (194, 202). It is possible that vitamin Bg is required for conversion of linoleic acid into arachidonic acid (206). Mammary tissue is able to synthesize both fatty acids and cholesterol from nonfat precursors. Thus, milk fat can arise through biosynthesis from acetate in the mammary gland as well as in other tissues, such as liver. On low fat or fat-free regi- mens, milk fat is synthesized by the mammary gland. On a high- fat diet, however, a large portion of the fatty acids in human milk may come from dietary fat via the plasma lipids, and the fatty acid composition of human milk can be highly variable (99). Phospholipids Human plasma phospholipids are nearly all choline- containing lipids. Lecithin (phosphatidyl choline) greatly predominates, with sphingomyelin accounting for an appreciable fraction (164). Phos- phatidyl serine and phosphatidyl ethanolamine comprise only a small fraction of the total phospholipids in plasma (164), but are much more prominent among tissue phospholipids. The phospholipids, like the glycerides, contain a variety of fatty acids, but they have a proportionately higher content of polyunsaturated fatty acids. They are important structural con- stituents of the membranes of cells and their organelles and are essential components of some enzyme systems. They probably perform an essential function in transport of lipids, because their emulsifying properties serve to solubilize other fats and to stabilize both the lipoprotein and particulate systems in extra- cellular fluid. The liver appears to be the chief organ for synthesis as well as degradation of the phospholipids of plasma. Pathways for bio- synthesis of phospholipids have been clarified by work with isolated 17
Dietary Fat and Human Health enzyme systems (107, 108, 119, 198). Cytidine coenzymes have been shown to be essential in this synthesis. In the enzymatic synthesis of lecithin, for example, the coenzyme cytidine diphos- phate choline is an important intermediate in converting 1,2-di- glyceride to phospholipid. Fat Storage Most of the energy stored in the body is in the form of triglycer- ide. This caloric reserve is maintained mainly in adipose tissue cells and may derive directly from the fatty acids in food or it may arise from the conversion of glucose or certain amino acids into fatty acids. Adipose tissue is capable of removing fatty acids from circulation for triglyceride synthesis, and can also manu- facture fat from blood glucose. Thus, the adipose cell can store, synthesize, and release fatty acids, depending on the requirements of the body. The rate at which triglyceride is manufactured within the fat cell depends primarily on the state of energy balance of the organism at the time. Excess dietary calories, whether from fat, carbohydrate, or protein, are stored as fat; when energy expenditure exceeds intake the fat stores help make up the deficit. Under normal circumstances, the process of lipogenesis from carbohydrate continues even though the individual is in energy equilibrium. This simply means that an appreciable pro- portion of dietary carbohydrate and, indirectly, some protein are converted to fat prior to utilization. The rate at which the fat cells release free fatty acids into the circulation is usually inversely related to the rate of carbohydrate utilization. In the fasting state, in uncontrolled diabetes, and in response to such hormones as somatotrophin and epinephrine, the adipose tissue releases fatty acids at increased rates. The fat depots not only are the major supply of storage energy but also provide a large proportion of the fuel used by the body under normal metabolic conditions. Cholesterol Metabolism Cholesterol is manufactured in the body, principally in the liver (50, 51), and circulates in the plasma as a component of lipo- proteins. 18
Fat Metabolism In healthy individuals, the body tends to maintain plasma cholesterol concentration by compensating for dietary intake through adjustment of synthesis and also through degradation and excretion of cholesterol and its products (71, 129, 161). Isotope studies have demonstrated that cholesterol can be synthesized from acetylcoenzyme A by a series of reactions in- volving the successive formation of such intermediates as mevalonic acid, squalene, and lanosterol (166). The rate of endogenous cholesterol synthesis is variable and has been estimated to range between 0.5 and 2 gm per day (74, 135). The ring structure of cholesterol is not readily degraded in the body. The principal catabolic pathway for cholesterol is conversion to bile acids by the liver. Some intact cholesterol leaves the body by excretion into the bile and by direct loss through the intestinal wall and feces. The quantities of cholesterol and bile acids that enter the small intestine in bile are variable. The quantity of cholesterol per se secreted with bile into the intestine may reach 50 percent of that synthesized each day in the body; however, much of this amount may be reabsorbed in the presence of fat and bile acids (180). The bile acids are continually reabsorbed through an enterohepatic cycle with only a small fraction of the circulating pool being lost each day in the feces. The quantity excreted daily approximates the amount of bile acids produced from cholesterol in the liver (17). In man, the bile acids produced in the liver are cholic acid and chenodeoxycholic acid. These acids are converted to other forms prior to excretion in the feces. Cholesterol is excreted in the feces as such or is changed in the intestine to products such as coprosterol and coprostanone. The chemical similarity of cholesterol and steroid hormones suggests a metabolic interrelationship of these compounds. Whether biosynthesis of the steroid hormones of the adrenals, testes, ovaries, and corpus luteum proceeds by way of cholesterol still is not certain. The endocrine tissues that produce these hormones do synthesize cholesterol from acetate and, at least in the case of the adrenal cortex, can convert cholesterol into steroid hormones (175). Proof that cholesterol is an obligatory intermediate is still lacking. 19
Dietary Fat and Human Health Lipotropic Factors Lipotropic factors are nutrients whose absence from the diet leads to excess deposition of liver fat. The most important of these factors are: the base choline, a constituent of some phospholipids (vide); the essential amino acid, methionine, which enables the body to synthesize choline; and vitamin BIZ, perhaps the most important of the three. On a diet deficient in the lipotropic factors, rats accumulate excessive amounts of liver fat. Choline deficiency results in increased hepatic-fat synthesis (207), decreased release of lipo- proteins from the liver (162, 182), and possibly decreased hepatic- fat oxidation (13). It is not known which mechanism is primarily responsible for the hepatic-fat accumulation. K the accumulation of hepatic fat becomes sufficiently great, the cells rupture and coalesce into fatty cysts. Eventually cirrhosis may occur. In the kidney, also, abnormal accumulation of lipid induces cellular destruction. Here, however, the sequence of events leads to interference with blood supply of the fatty swollen tubules and culminates in the hemorrhagic renal syndrome of choline deficiency. Inositol also has some lipotropic activity. Fatty livers may develop, however, in animals on low-protein diets even when dietary fat is at a minimum and adequate choline is present (179). Such fatty livers can be prevented by certain amino acids, notably threonine. Fatty livers also have been observed when lysine in the diet is low. It is not yet clear to what degree these results in animals are related to human liver disease. 20
