Effect of dietary proteins on hepatic and plasma lipids, and some properties of major plasma lipoprotein fractions during dietary-induced hypercholesterolemia in male rats.

In order to characterize further the cholesterolemic effect of casein compared with soybean protein isolate, each of these proteins were fed to male rats at different levels of dietary cholesterol administration. An increase in the dietary cholesterol level from zero to 0.25% led to a several-fold increase in plasma triacylglycerols with a comparatively small rise in plasma cholesterol. Further increase in the cholesterol content of diet to 0.5, 1 and 2% resulted in a return of plasma triacylglycerols to normal or even subnormal values, whereas the plasma cholesterol values rose progressively. The triacylglycerolemic and cholesterolemic effects were 2-3-fold higher with casein diet than with soybean protein diet, respectively. A major part of the increased plasma cholesterol value was accounted for by cholesterol in the very-low-density lipoprotein (VLDL) fraction. Even at moderate levels (0.05-0.5%), the dietary cholesterol administration produced a series of changes in the electrophoretic pattern of plasma lipoproteins. These changes were more prominent in the rats fed casein diet, which often showed an enhanced VLDL band even on a cholesterol-free diet. An additional lipoprotein band, localized between VLDL and the LDL/HDL1 band, was observed in several rats fed casein diet with 0.05 or 0.1% cholesterol, and in rats fed soybean protein diet containing 0.25 or 0.5% cholesterol. In contrast, at the 0.25 and 0.5% levels of dietary cholesterol, plasma of casein-fed rats contained only one broad band within the VLDL-LDL/HDL1 region. All these findings indicate that casein diet promotes the appearance of one or more specific type(s) of cholesterol-induced plasma lipoprotein particles even at a comparatively low level of dietary cholesterol.