Soy Protein Isolate Reduces Liver Cholesterol and Lipids in Ovariectomized Rats

Soy is a rich source of isoflavones which are reported to have cardioprotective effects in humans and animal models. Isoflavones are structurally similar to 17β-estradiol (E2) and can therefore bind to estrogen receptors. Dietary intake of soy protein is often recommended for improvement of total cholesterol and low-density lipoprotein cholesterol, contributors to the development of coronary heart disease. The objective of this study was to investigate the degree to which consumption of soy protein isolate (SPI) for 30 days would modulate lipid metabolism in thirty-two 90-day-old female Sprague-Dawley rats. Rats were assigned to four groups: sham-operated (Sham); ovariectomized (Ovx); Ovx + soy; Ovx + E2. Serum samples were analyzed for total cholesterol, high-density lipoprotein cholesterol (HDL) and triglycerides. Liver samples were evaluated for total cholesterol and total lipids. Total liver lipids (mg/gram liver) in animals fed SPI (38.5 ± 0.86) were significantly lower than the Sham, Ovx and E2 groups (43.7 ± 1.28, 45.4 ± 1.54, 43.7 ± 1.47, respectively). Total liver cholesterol levels (µmol/gram liver) in animals fed SPI (13.4 ± 3.57) were significantly lower than Ovx animals (21.2 ± 6.21) but not those of Sham (16.4 ± 3.72) or E2 (17.7 ± 2.74) animals. SPI had no effect on serum total cholesterol or triglyceride levels, whereas E2 increased serum total cholesterol and triglyceride levels without affecting HDL or liver values. Our findings suggest that consumption of soy protein reduces the production of cholesterol and lipids in the liver; however, it may take longer to see similar effects on serum values.