Plasma cholesterol levels in rabbits fed low fat, cholesterol-free, semipurified diets: effects of dietary proteins, protein hydrolysates and amino acid mixtures

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Rabbits become hypercholesterolemic when fed a low fat, cholesterol-free, semisynthetic diet containing casein as the dietary protein. This did not occur when the casein was replaced by soy protein isolate or any one of seven other plant protein preparations. Doubling the amounts of either the casein or soy protein isolate from 25 to 50% by weight of the diet, made no significant difference to their effects on plasma cholesterol. Soy protein isolate was effective in counteracting the hypercholesterolemic response to casein when mixtures of the two proteins were fed. There appeared to be no relationship between body weight gains and plasma cholesterol levels in rabbits fed the different diets. Animals fed the higher level of casein failed to gain weight, whereas growth was not significantly impaired by doubling the level of soy protein isolate in the diet. Better growth was obtained with mixtures of casein and soy protein isolate than with either protein alone. An enzymatic hydrolysate of casein or a mixture of L-amino acids equivalent to casein gave elevated plasma cholesterol levels similar to those obtained with the intact protein. Plasma cholesterol levels remained low in rabbits fed an enzymatic digest of soy protein. A moderate, but not significant, increase in plasma cholesterol was observed when a mixture of L-amino acids equivalent to soy protein isolate was fed. The results of these experiments indicate that the level of plasma cholesterol can be influenced by the amino acids supplied in the diet.