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Effects of Dietary Proteins and Amino Acid Mixtures on Plasma Cholesterol Levels in Rabbits¹

Department of Biochemistry, University of Western Ontario, London, Ontario, Canada N6A 5C1

Amino acid mixtures corresponding to either casein or egg yolk protein when fed to rabbits in a low fat, cholesterol-free semipurified diet, produced the same degree of hypercholesterolemia as the proteins themselves. Amino acid mixtures corresponding to soy protein isolate or sunflower protein gave low plasma cholesterol levels, but not as low as those obtained with the intact proteins. Experiments were designed to identify amino acids responsible for the differential in plasma cholesterol produced by casein or soy protein amino acid mixtures. Plasma cholesterol levels obtained with a casein amino acid mixture containing half the amounts of essential amino acids were significantly higher than the corresponding soy protein mixture. Feeding trials based on systematic modifications to the casein and soy protein mixtures indicated that interaction between essential and non-essential amino acids was important in determining plasma cholesterol levels. In further experiments, protein components were formulated by adding amino acids to casein to give a mixture corresponding to soy protein, or by adding amino acids to soy protein to give a mixture equivalent to casein. These diets failed to reverse plasma cholesterol levels which suggests differences in digestion and absorption of proteins relative to amino acid mixtures.

KEY WORDS: • casein • soy protein • amino acid mixtures • plasma cholesterol

¹ Supported by the Ontario Heart Foundation and the Medical Research Council of Canada.

² Present address: The Baker Medical Research Institute, Commerical Road, Prahran, Victoria, Australia, 3181.

³ Career Investigator of the Medical Research Council of Canada.

Manuscript received 3 January 1980.