Soy protein concentrate lowers serum high-density lipoprotein cholesterol concentrations compared with casein in ovariectomized rats fed a low-fat, cholesterol-free diet

The cholesterol-lowering properties of soy protein are commonly attributed to the presence of isoflavones in the product and hypercholesterolemia. The present study was designed to investigate the effects of either replacing casein with soy protein concentrate containing isoflavones or adding purified soy isoflavones to casein on plasma lipids and lipoprotein cholesterol concentrations in ovariectomized (OVX) rats fed a low-fat, cholesterol-free diet. Thirty-two female rats, 8 weeks of age, were fed 1 of 4 semipurified diets containing 21% protein by energy for 8 weeks. Group 1 was fed casein and was sham operated (CAS-S). Group 2 was fed casein and was OVX (CAS-O). Group 3 was fed soy protein concentrate containing isoflavones and was OVX (SPC-O). Group 4 was fed casein + soy isoflavones and was OVX (CAS-I-O). After 8 weeks of dietary treatment, the SPC-O rats had significantly lower serum total cholesterol (TC) concentrations compared with the CAS-S, CAS-O, and CAS-I-O rats. Furthermore, the SPC-O rats had significantly lower serum high-density lipoprotein–cholesterol (HDL-C) concentrations compared with the CAS-S, CAS-O, and CAS-I-O rats. The serum TC/HDL-C and non–HDL-C/HDL-C ratios were significantly increased in the SPC-O rats compared with those in the CAS-S and CAS-I-O rats. The primary conclusion of the present study is that dietary replacement of casein with soy protein concentrate containing isoflavones in OVX rats fed a low-fat, cholesterol-free diet significantly lowers the serum HDL-C concentration and raises the serum TC/HDL-C ratio greater than OVX alone, thereby possibly increasing the risk of developing atherosclerosis.