EFFECTS OF CASEIN VERSUS SOYPROTEIN DIETS ON BODY-COMPOSITION AND SERUM-LIPID LEVELS IN ADULT-RATS

The effect on body composition and serum lipids of diets providing protein as 36 percent of total calories from either casein or soy protein was investigated in fifteen male Sprague-Dawley rats weighing 212 +/- 5 g (mean +/- SEM). Eight received the casein diet and 7 the soy protein diet. During the last 3 days of the 7-week ad libitum feeding period, fecal nitrogen and fat were measured. Then, after a 12-hour fast, the rats were killed and blood samples collected for analysis of serum triglycerides, total and HDL cholesterol, insulin and glucose. Rate of weight gain was not statistically different between the two groups. However, carcass analysis showed that total dissectible fat, percent body fat, and adipocyte size, but not adipocyte number, were significantly greater in the casein-fed than in the soy-protein fed rats. Serum insulin levels were higher and fecal fat was lower in the casein-fed rats. Casein-fed rats also maintained significantly higher serum triglycerides and total and HDL cholesterol levels than their soy protein-fed counterparts. Higher insulin levels and increased fat absorption may have been responsible for the increase in adiposity and serum lipids in the casein-fed animals.