Serum and liver lipids in growing rats fed casein with L-lysine

Liver and serum lipid conentrations were studied in rats fed 7.5, 15 and 30% casein supplemented with 0, 2.5 and 5% lysine. Increasing dietary casein from 7.5 to 15% increased serum total lipids, triglycerides and cholesterol. When casein was increased from 15 to 30%, there was a further increase in plasma cholesterol and a decrease in triglycerides. Rats fed 7.5% casein alone developed fatty livers. Adding 5% lysine to 15% casein caused fatty livers but prevented them with casein at 7.5%. Lysine had no effect on total liver lipids with casein at 30%. The lipids of the fatty livers with 7.5% casein or with lysine were similar in composition. In both cases liver triglycerides increased fourfold and cholesterol twofold. Phospholipids remained unchanged and liver glycogen decreased. Excess dietary lysine did not change serum lipids ang glucose significantly. The data show that excess lysine caused changes in lipid metabolism which varied with the quantity of casein fed. The hypercholesterolemia with casein feeding seen by others appears unrelated to the high lysine content of this protein.