Effects of dietary protein and amino acids on the metabolism of cholesterol-carrying lipoproteins in rats.

The effect of various dietary proteins and amino acids on serum lipid metabolism was studied by using male Sprague-Dawley rats. A stock diet containing casein as a protein source was fed to control animals, whereas a vegetable protein diet (cottonseed based) was fed to one experimental group. Two other experimental diets were formulated to determine if the amino acid ratios in the protein played a role in the alteration of serum cholesterol levels. One of these diets contained casein plus enough additional arginine to make its arginine-to-lysine ratio similar to that found in cottonseed protein. The other diet contained cottonseed protein plus enough lysine to duplicate the arginine-to-lysine ratio of casein. Rats fed a diet containing protein from animal sources had greater serum and high-density lipoprotein (HDL)-cholesterol concentrations as well as increased lecithin: cholesterol acyltransferase (LCAT, EC 2.3.1.43) activities than those which had been fed a diet containing protein from plant sources. Animals fed arginine-supplemented casein diet showed a decrease in both serum and HDL-cholesterol when compared to the casein control group, whereas the addition of lysine to cottonseed protein diet caused an increase in the same two cholesterol fractions.