Dietary fats modulate age-dependent effects of dietary proteins on cholesterol metabolism in rats.

Different aged male rats, 4 weeks or 9 months old, were fed diets containing either casein (CAS), milk whey protein (WHY) or soybean protein (SOY) with corn oil or sardine oil for 4 weeks. The hypocholesterolemic effect of SOY, compared to CAS, was more evident in rats fed corn oil than in those fed sardine oil, and in young rats than in adult rats, irrespective of dietary cholesterol. In contrast, the liver cholesterol-lowering effect of SOY was more marked in adult than in young rats in all experiments. WHY exerted an intermediate effect on the concentration of liver cholesterol. At both ages, the response of liver 3-hydroxy-3-methylglutaryl (HMG) CoA reductase activity was diverse and dependent upon the source of dietary fat and age of the animal. Fecal steroid excretion was significantly higher in rats fed SOY than in those fed either CAS or WHY, especially in adult rats. The results showed a diverse interaction of the protein type, fat and age in respect to lipid metabolism.