Cholesterol-lowering effects of soybean, potato and rice proteins depend on their low methionine contents in rats fed a cholesterol-free purified diet

Rats were fed cholesterol-free purified diets containing casein, rice (RP), potato (PP) or soybean (SP) proteins having different amounts of methionine (25.9, 21.3, 16.2 and 10.9 g methionine/kg, respectively). Each protein was fed at 250 g/kg diet for 14 d. Growth rates of rats were the same in all groups. Serum total cholesterol concentrations were lower in rats fed SP, PP and RP than in those fed casein. Fecal bile acid plus neutral steroid excretion was significantly higher in rats fed the RP, PP and SP diets compared with those fed casein. There was a significant negative correlation between serum cholesterol concentration and fecal total steroid excretion (r = -0.490, P = 0.01). However, a stronger positive correlation was observed between serum cholesterol concentration and dietary methionine concentration (r = 0.674, P = 0.0003) or methionine:glycine ratios (r = 0.656, P = 0.0005). In a separate experiment in rats fed diets containing amino acid mixtures simulating the RP, PP and SP diets, serum total cholesterol concentrations were lower than in rats fed simulated casein. Fecal total steroid excretion was the same in all groups. A strong correlation was found between serum cholesterol concentration and dietary methionine concentration (r = 0.743, P = 0.0002) or the methionine:glycine ratio (r = 0.685, P = 0.0009) in rats fed the amino acid mixtures. Finally, we examined the hypocholesterolemic effects of 250 g SP or casein/kg diet with or without supplementation with 0.3 g/100 g sodium taurocholate (TC). Supplementation with TC did not alter the hypocholesterolemic effect of SP. These results support the view that RP, PP and SP lower serum cholesterol concentration in a similar manner.