Hypocholesterolemic effect of dietary soy protein versus casein in rhesus monkeys (Macaca mulatta)

Ten mature female rhesus monkeys were alternately fed semipurified diets containing casein or soy protein for periods of 13 to 17 wk. Each diet was fed for two periods. When the animals were changed from the commercial diet to the semipurified diet containing soy protein, a significant elevation in serum cholesterol occurred. The serum cholesterol levels gradually increased further, when the soy protein in the diet was replaced by casein. However, when subsequently the casein in the diet was replaced by soy protein, a significant decrease in serum cholesterol levels was observed. Finally, when the animals were changed back to the casein diet, a significant increase in serum cholesterol again occurred. Changes in serum cholesterol due to modulations in the type of protein in the diet were reflected mainly by changes in low-density lipoprotein cholesterol. Thus, the results of this study clearly show that the type of protein in the diet markedly affects serum cholesterol levels not only in experimental animals such as the rabbit, as is well known, but also in the rhesus monkey, which is more akin to man.