Effect of saturated and unsaturated fat diets on lipid profiles of plasma lipoproteins

Four to five healthy normolipidemic men aged 21-23 years were maintained for 5 weeks on controlled diets containing 40% of calories from either unsaturated (unsaturated /saturated fatty acid ratio 4) or saturated (unsaturated/saturated fatty acid ratio 0.25) fat, with a 5-week period of ad libitum diet in between. The effect of the diets on the total lipid profiles of the very low (VLDL), low (LDL) and high (HDL₃) density lipoproteins was determined by high temperature gas-liquid chromatography of the intact fatty esters and free cholesterol. When compared with the saturated, the unsaturated fat died caused a significant decrease (25%) in the protein content in HDL₃ and to a lesser extent (maximum 10%) in LDL, which were compensated for by a proportional increase in all lipid classes, resulting in essentially similar lipid class proportions on both high fat diets. Furthermore, there were no significant alterations induced by the diet in the neutral lipid/polar lipid ratios, so that the radii of the particles calculated on the basis of the surface and core component content gave comparable values for corresponding lipoprotein classes on both diets. There was a minor relative increase in the proportion of triacylglycerols and a decrease in the proportion of cholesteryl esters in the LDL fractions from the unsaturated fat diet. The opposite effect was observed for VLDL, while the HDL₃ showed no change on these diets. The extreme dietary variations resulted in significant changes in the molecular weight or carbon number profiles of the cholesteryl esters, phosphatidylcholines and triacylglycerols. There was a decrease in the lower molecular weight species and an increase in the higher molecular weight species on the unsaturated fat diet and vica versa on the saturated fat diet for all lipoprotein classes. In comparison to the two controlled high fat diets, the ad libitum diet produced a significantly lower free cholesterol/total phospholipid ratio in VLDL and of sphingomyelin/phosphatidylcholine ratio in LDL, with the alterations in the composition of the molecular species of the lipid classes falling between the two extremes observed on the saturated and unsaturated fat diets. The present results suggest that the decreases in plasma cholesterol and triacylglycerols commonly observed on unsaturated fat diets are due to a decrease in number of plasma lipoprotein particles rather than to a change in the particle size or composition as suggested previously.