Nutritional and health benefits of soy proteins

Soy protein is a major component of the diet of food-producing animals and is increasingly important in the human diet. However, soy protein is not an ideal protein because it is deficient in the essential amino acid methioni ne. Methionine supplementation benefits soy infant formulas, but apparently not food intended for adults with an adequate nitrogen intake. Soy protein content of another essential amino acid, lysine, although higher than that of wheat proteins, is still lower than that of the milk protein casein. Ad verse nutritional and other effects following consumption of raw soybean me al have been attributed to the presence of endogenous inhibitors of digesti ve enzymes and lectins and to poor digestibility. To improve the nutritiona l quality of soy foods, inhibitors and lectins are generally inactivated by heat treatment or eliminated by fractionation during food processing. Alth ough lectins are heat-labile, the inhibitors are more heat-stable than the lectins. Most commercially heated meals retain up to 20% of the Bowman-Birk (BBI) inhibitor of chymotrypsin and trypsin and the Kunitz inhibitor of tr ypsin (KTI). To enhance the value of soybeans in human nutrition and health , a better understanding is needed of the factors that impact the nutrition and health-promoting aspects of soy proteins. This paper discusses the com position in relation to properties of soy proteins. Also described are poss ible beneficial and adverse effects of soy-containing diets. The former inc lude soy-induced lowering of cholesterol, anticarcinogenic effects of:BBI, and protective effects against obesity, diabetes, irritants of the digestiv e tract, bone, and kidney diseases, whereas the latter include poor digesti bility and allergy to soy proteins. Approaches to reduce the concentration of soybean inhibitors by rearrangement of:protein disulfide bonds, immunoas says of inhibitors in processed soy foods and soybean germplasm, the roles of phytoestrogenic isoflavones and lectins, and research needs in all of th ese areas are also discussed. This integrated overview of the widely scatte red literature emphasizes general concepts based on our own studies as well as recent studies by others. It is intended to stimulate interest in furth er research to optimize beneficial effects of soy proteins. The payoff will be healthier humans and improved animal feeds.