No abstract

We studied the metabolic effects of stearic acid (18:0) on plasma lipoprotein levels in 11 subjects during three dietary periods of three weeks each. The three liquid-formula diets, which were used in random order, were high in palmitic acid (16:0), stearic acid, and oleic acid (18:1), respectively. Caloric intakes were the same during the three periods. As compared with the values observed when the subjects were on the high-palmitic-acid diet, plasma total cholesterol decreased by an average of 14 percent during consumption of the high-stearic-acid diet (P less than 0.005) and by 10 percent during consumption of the high-oleic-acid diet (P less than 0.02). Low-density lipoprotein cholesterol levels fell by 21 percent in subjects on the high-stearic-acid diet (P less than 0.005) and by 15 percent in subjects on the high-oleic-acid diet (P less than 0.005). No significant differences were observed in the plasma levels of triglycerides or high-density lipoprotein cholesterol among the three diets. Measurements of the intestinal absorption of palmitic, stearic, and oleic acids revealed essentially complete absorption of each during the three dietary periods. The oleic acid content of plasma triglycerides and cholesteryl esters increased significantly during the high-stearic-acid period, suggesting that stearic acid is rapidly converted to oleic acid. We conclude that stearic acid appears to be as effective as oleic acid in lowering plasma cholesterol levels when either replaces palmitic acid in the diet.

We compared a high-carbohydrate diet with a high-fat diet (specifically, a diet high in monounsaturated fatty acids) for effects on glycemic control and plasma lipoproteins in 10 patients with non-insulin-dependent diabetes mellitus (NIDDM) receiving insulin therapy. The patients were randomly assigned to receive first one diet and then the other, each for 28 days, in a metabolic ward. In the high-carbohydrate diet, 25 percent of the energy was in the form of fat and 60 percent in the form of carbohydrates (47 percent of the total energy was in the form of complex carbohydrates); the high-monounsaturated-fat diet was 50 percent fat (33 percent of the total energy in the form of monounsaturated fatty acids) and 35 percent carbohydrates. The two diets had the same amounts of simple carbohydrates and fiber. As compared with the high-carbohydrate diet, the high-monounsaturated-fat diet resulted in lower mean plasma glucose levels and reduced insulin requirements, lower levels of plasma triglycerides and very-low-density lipoprotein cholesterol (lower by 25 and 35 percent, respectively; P less than 0.01), and higher levels of high-density lipoprotein (HDL) cholesterol (higher by 13 percent; P less than 0.005). Levels of total cholesterol and low-density lipoprotein (LDL) cholesterol did not differ significantly in patients on the two diets. These preliminary results suggest that partial replacement of complex carbohydrates with monounsaturated fatty acids in the diets of patients with NIDDM does not increase the level of LDL cholesterol and may improve glycemic control and the levels of plasma triglycerides and HDL cholesterol.

Oxidized low density lipoproteins (LDLs) are thought to play an important role in atherogenesis. Nutritional and biochemical studies suggest that diet can modulate the susceptibility of plasma LDL to undergo oxidative degradation by affecting the concentration of polyunsaturated fatty acids and antioxidants in the lipoprotein particle. In the present study 11 healthy male volunteers underwent two diet phases. In one phase the diet was enriched in oleic acid (mono), while in the other it was high in linoleic acid (poly). Both diets lowered plasma total and LDL cholesterol contents. The sensitivity of plasma LDL to oxidation was estimated by challenging these lipoproteins with 2,2'-azobis(2-amidinopropane)dihydrochloride, a free-radical initiator. Although neither diet affected the antioxidant content of plasma LDL, the resistance to lipid peroxidation, measured after the consumption of antioxidants present in the lipoprotein, was higher during the mono phase. Indeed, the peroxidation rate of plasma LDL was inversely correlated with the oleic acid to linoleic acid ratio in the LDL particle. These results support the thesis that diets rich in monounsaturated fatty acids increase the resistance of plasma LDL to oxidative modification, independent of their content of antioxidants. This effect could lower the atherogenicity of these lipoproteins.

Canola oil is a newly marketed vegetable oil for use in salads and for cooking that contains 55% of the monounsaturated fatty acid; oleic acid, 25% linoleic acid and 10% alpha-linolenate [polyunsaturated fatty acid (PUFA)], and only 4% of the saturated fatty acids (SFAs) that have been implicated as factors in hypercholesterolemia. It is expressed from a cultivar of rapeseed that was selectively bred from old varieties in Canada to be very low in erucic acid--a fatty acid suspected to have pathogenic potential in diets high in the original rapeseed oil in experimental animals. Canola oil is free of those problems. It is the most widely consumed food oil in Canada, and has been approved for Generally Recognized as Safe (GRAS) status by the Food and Drug Administration (FDA) of the United States Department of Health and Human Services. The fatty acid composition of canola oil is consistent with its use as a substitute for SFAs, in meeting the dietary goals recommended by many health associations: an average diet containing about 30% of calories as fat made up of less than 10% SFAs, 8-10% PUFAs in a ratio of linoleic to linolenic acids between 4:1 and 10:1, the remainder being monounsaturated fatty acids. No single oil meets these current recommendations for ratios of PUFA/monounsaturated/polyunsaturated fatty acid ratios as the sole source of cooking and salad oil.