Regulation of lipid metabolism by soy protein and its implication in diseases mediated by lipid disorders

Soybeans have a high-quality protein that has been consumed for approximately 5000 years in Oriental countries. The awareness that soy products are healthy has increased their consumption in Western countries. Substantial data from epidemiological surveys and nutritional interventions in humans and animals indicate that soy protein reduces serum total and low-density lipoprotein (LDL) cholesterol and triglycerides as well as hepatic cholesterol and triglycerides. This review examines the evidence on the possible mechanisms for which soy protein has beneficial effects in diabetes, obesity and some forms of chronic renal disease. Consumption of soy protein due to low methionine content reduces serum homocysteine concentration, decreasing the risk of acquiring a cardiovascular disease. On the other hand, soy protein reduces the insulin/glucagon ratio, which in turn down-regulates the expression of the hepatic transcription factor sterol regulatory element binding protein (SREBP)-1. The reduction of this factor decreases the expression of several lipogenic enzymes, decreasing in this way serum and hepatic triglycerides as well as LDL cholesterol and very LDL triglycerides in diabetes and obesity, reducing lipotoxicity in the liver. Soy protein intake also reduces hepatic lipotoxicity by maintaining the number of functional adipocytes, preventing the transfer of fatty acids to extra adipose tissues. Furthermore, soy protein isoflavones stimulate the transcription factor SREBP-2, increasing serum cholesterol clearance. The reduction of serum cholesterol and triglyceride concentrations by soy protein intake produces beneficial effects in the kidney preventing the inflammatory response, increasing the renal flow by releasing endothelial nitric oxide (NO) synthase from the caveolae, facilitating the synthesis of NO. Thus, soy protein consumption may reduce the clinical and biochemical abnormalities in diseases mediated by lipid disorders.