Dietary protein level and origin (casein and highly purified soybean protein) affect hepatic storage, plasma lipid transport, and antioxidative defense status in the rat

The effects of different proportions (10, 20, and 30%) of dietary casein or highly purified soybean protein on lipid metabolism were studied in growin g Wistar rats. Hepatic, plasma and lipoprotein lipid, and protein concentra tions, plasma thiobarbituric acid-reactive substance (TBARS) levels, and re sistance of red blood cells against free-radical attack were determined aft er a 4-wk dietary regimen. Compared with the 20% casein diet, the 20% soybe an protein diet exhibited similar cholesterolemia but lower plasma triacylg lycerol concentrations and very-low-density lipoprotein (VLDL) particle num ber, as measured by diminished contents of VLDL-triacylglycerol, VLDL-prote in, and VLDL-apolipoprotein (Apo)B (B-100 and B-48). The soybean protein di et raised high-density lipoprotein (HDL)(2-3) particle number, as measured by enhanced concentrations of HDL2-3 cholesterol, HDL-phospholipid, and HDL -ApoA-I. Increasing casein or soybean protein level (from 10 to 30%) in the diet involved higher VLDL-ApoB (B-100 and B-48), indicating an increase in the number of VLDL particles. Feeding the 30% casein or 30% soybean protei n diet enhanced LDL-HDL, cholesterol contents. Despite similar HDL2-3-ApoA- I levels, the 30% casein diet enhanced the HDL2-3 mass and its cholesterol concentrations. In contrast, feeding either the 10 or 30% soybean protein d iet significantly lowered HDL2-3 cholesterol and ApoA-I levels. These effec ts on cholesterol distribution in lipoprotein fractions occurred despite un changed total cholesterol concentrations in plasma. Feeding 20% soybean pro tein versus 20% casein involved lower plasma TEARS concentrations. Decreasi ng casein or soybean protein levels in the diet were associated with higher plasma TEARS concentrations and had a lower resistance of red blood cells against free-radical attack. The present study shows that dietary protein l evel and origin play an important role in lipoprotein metabolism and the an tioxidative defense status but do not affect total cholesterol concentratio ns in plasma. (C) Elsevier Science Inc. 2000.