Plasma and hepatic cholesterol and hepatic HMG CoA reductase levels in rats fed fluid or powdered skim milk

The effects of skim milk powder (SMP) and fluid skim milk (FSM) on plasma cholesterol (CH) and hepatic lipid concentrations, and 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG CoA) reductase activity of rats of different ages were compared. Groups of young (23 days old) and older (45 days old) rats were fed a casein-based diet and provided tap water; the casein-based diet and FSM as fluid source; or tap water and the casein diet into which SMP (25% by wt.) had been isocalorically incorporated. Plasma CH concentrations were determined at 0, 1.5, 3 and 5 wk, hepatic total lipid, triglyceride and CH at 5 wk. Half of each group were killed at wk 3 and the other half at wk 5 for determinations of HMG CoA reductase activity. Both FSM and SMP decreased plasma CH levels at 1.5 and 3 wk of feeding in the young rats; plasma CH concentrations of the older rats were not altered by either FSM or SMP. Both milk derivatives increased HMG CoA reductase activity at wk 3 and wk 5 in both ages of rat, whereas hepatic lipid levels were unchanged. In these experiments the effects of feeding FSM of SMP along with a casein-based diet were comparable and included an increase in HMG CoA reductase activity, no change in hepatic lipid levels, and a decrease in plasma CH; the latter response depended on the initial age of the rat.