Effects of dietary monounsaturated fatty acids on plasma lipids in humans

In order to test whether monounsaturated fatty acids (MUFA, M) would lower plasma cholesterol and/or triglycerides (E), 14 young men were studied in a metabolism ward. They were given two experimental diets with the same P/S ratio (P: polyunsaturated; S:saturated fatty acids) but different P + MIS ratios in composition of dietary fatty acids. Six men were in experiment I. In two groups of three, each group was given either the high or the low P + M/S ratio diet for 6 weeks. In experiment 11, two groups of four men were subjected to a 3-week cross-over feeding trial (total of 6 weeks). We found in the crossover study that when the dietary P/S ratio was fixed at 1.0, those subjects given a diet with the higher P + M/S ratio of 4.5 showed significantly higher plasma total TG, very low density lipoprotein (VLDL)-E, low density lipoprotein (LDL)-TG, and LDLcholesterol than when given a diet with a lower P + M/S ratio of 1.5. Plasma total cholesterol, apoprotein A-I, and apoprotein B100 were not significantly different between the two dietary periods. Plasma fatty acid patterns showed a significantly higher plasma MUFA level during the period of a higher P + M / S diet. 1111 In contrast to some recent reports, our studies indicate that a large amount of dietary MUFA may raise some fractions of plasma lipids in humans. Chang, N. W., and P. C. Huang. Effects of dietary monounsaturated fatty acids on plasma lipids in humans. J Lipid Res. 1990. 31: 2141-2147. Supplementary key words saturated and saturated fatty acids triglycendes apoA-I apB-100 very low density lipoprotein polyunEpidemiological studies show that diets rich in saturated fatty acids (SFA) and cholesterol seem to increase the risk of coronary heart disease (CHD) (1, 2). A high plasma LDL-C level is now widely recognized as a major risk factor of CHD, whereas a high level of HDL is considered a negative risk factor of C H D (3-5). During the last three decades many investigators have reported that polyunsaturated fatty acids (PUFA) will decrease and SFA will increase plasma total cholesterol and LDL-C (6-10). But monounsaturated fatty acids (MUFA), such as oleic acid, are considered to have no effect on plasma lipids (11). Therefore, for the purpose of predicting the cholesterolemic effect of dietary fat, fatty acid composition is frequently expressed in terms of a PUFA to SFA (P/S) ratio. Diets with higher P/S ratios decrease the plasma LDL-C (10, 12, 13), HDL-C (6, 7, 10, 13-16), and VLDL-C (6, 10, 12, 15, 17) when compared to the diets with lower P/S ratios. Very recently, MUFA-rich diets were reported to decrease plasma total cholesterol and LDL-C without decreasing HDL-C in humans (14, 18, 19), and thus the role of MUFA in lipid metabolism has received renewed attention. However, Peifer et al. (20) and Beynen (21) report that MUFA might increase plasma and liver total cholesterol levels in hypercholesterolemic rats and rabbits. Huang and Chang (22) also observed that, when the dietary P/S ratio was fixed at 1.0 and the diet was supplemented with 1% cholesterol, those rats given a diet with a PUFA + MUFA/SFA (P + MIS) ratio of 5.7 showed significantly higher plasma total cholesterol, triglyceride, LDL-C, and liver total cholesterol than those given a diet with a P + M/S ratio of 1.4. They also observed that, when either PUFA or SFA was fixed at a constant level, rats given a diet of a higher M/S ratio showed significantly higher plasma cholesterol levels (N. W. Chang and P. C. Huang, unpublished data). It seems that the cholesterolemic effect of MUFA on plasma lipid levels is still uncertain. In the present study, apparently healthy young men were enrolled in a study to determine the effects of dietary MUFA on plasma lipids. The experimental diets contained fat with a P/S ratio of 1.0 but different P + M/S ratios of 1.5 or 4.5. MATERIALS AND METHODS