Comparison of rat hepatic cholesterol biosynthesis during skim milk versus whey permeate ingestion.

Whey permeate is an ultrafiltrate of whey that is devoid of protein but contains lactose, salts, and other soluble low molecular weight compounds. These experiments compared cholesterol concentrations of blood plasma, hepatic lipids, and hepatic cholesterol biosynthesis of rats ingesting skim milk powder versus whey permeate powder. Groups of young male rats weighing 90 to 92 g were fed a casein-based diet into which skim milk powder or whey permeate powder was incorporated isocalorically. No effects of skim milk or whey permeate on plasma cholesterol concentrations were observed at any time during 5-wk of feeding. However, 3-hydroxy-3-methylglutaryl co-enzyme A reductase activity was increased by either skim milk or whey permeate feeding. Hepatic cholesterol, triglyceride, and phospholipid concentrations at wk 5 were unchanged. Plasma and hepatic cholesterol responses of rats to whey permeate ingestion are similar to those that occur with skim milk consumption, and plasma and hepatic cholesterol concentrations do not reflect necessarily an increase in hepatic cholesterol biosynthesis.

Variability in Cholesterolemic Response of Rats Consuming Skim Milk.

The hypocholesterolemic property of skim milk powder (SMP) was investigated using rats in a series of six separate experiments. SMP was incorporated at a level of 25% by weight into stock diets or semipurified diets with casein or soy as the protein source. Ingestion of SMP with casein-based diets for periods of 5 to 12 weeks produced a transient hypocholesterolemia only when diets were introduced within 1 week of weaning. In two different experiments, when weanling rats were fed stock diets with or without SMP for periods of 8 and 16 weeks, SMP ingestion led to a transient decrease in plasma cholesterol (CH) levels in only one experiment. Finally, SMP was not hypocholesterolemic when fed to weanling rats with a soy-based diet for 8 weeks. Together, these results reveal that consumption of SMP does not consistently elicit hypocholesterolemia in rats. We found that to observe significant differences in plasma CH levels between the control and SMP groups, semipurified diets containing SMP had to be fed to rats within the first week of weaning; in addition, the diets had to be free of, or reduced in other components with cholesterol-lowering properties. Finally, the SMP-induced hypocholesterolemia that we observed appeared to facilitate the normal rate of decline in plasma CH levels during the post-weaning period, producing only a transient difference between control and SMP groups.