Significance for humans of biologically active factors in soybeans and other food legumes

Abstract

Among the many biologically active factors present in the soybean, only protease inhibitors (PI) have been shown to exert significant adverse effects on animals consuming diets containing soybean protein. Evidence is presented to suggest that (a) PI are only partially responsible for the poor nutritive value of inadequately processed soybeans, (b) low levels of PI are relatively harmless to animals, (c) human trypsin is only weakly inhibited by PI, and (d) the human pancreas is probably insensitive to the hypertrophic effects of PI. Parelleling the wide spread distribution of PI in the plant kingdom are the so called phytohemagglutinins or lectins. Unlike the lectin present in soybeans which appears to have only a marginal effect on the nutritional quality of the protein, the lectin of the common bean (Phaseolus vulgaris) is quite toxic. Moreover, the major storage protein of such beans is quite resistant to digestion unless denatured by heat, thus emphasizing the importance of adequate processing of those legumes when used in the human diet. Although goiter-inducing compounds are present in most cruciferous plants and cyanide-producing substances may be found in cassava and lima beans, traditional methods of preparation and present technology have served to minimize any harmful effects that may accompany the ingestion of these foods by man. Brief mention will also be made of two human diseases, lathyrism and favism, associated with the consumption ofLathyrus sativus andVicia faba, respectively, their causative agents and mechanism of action. Although there are numerous examples of so called toxic constituents in legumes, they nevertheless have provided a valuable source of protein to man over the centuries. This can be attributed, in part, to the fact that man has learned how to detoxify them by suitable preparative measures. The varied nature of our diet also minimizes the contribution of a toxicant from any one foodstuff. Nevertheless, there is the ever present possibility that the prolonged consumption of a particular legume that may be improperly processed could bring to the surface toxic effects that otherwise would not be apparent. As the shortage of protein becomes more acute, it is not unlikely that much of the population of the world will be faced, in the future, with a more limited selection of protein-foods, most of which will be of plant origin and, hence, potential carriers of toxic constituents. The food scientist should at least be cognizant of such a possibility and be prepared to apply his knowledge and skill to meeting this challenge.