Antinutritional factors
Soybean seeds contain several antinutritional factors. The most important ones are trypsin inhibitors, hemagglutinins, lectins and saponins. Soybean was indeed used to demonstrate the existence and the role of trypsin (Rackis et al., 1986).
Trypsin inhibitors: raw soybeans contain trypsin inhibitors that bind with trypsin (a digestive enzyme) in the small intestine and form an inactive complex that prevents trypsin from degrading feed proteins (van Eys et al., 2004). Trypsin inhibitors also induce pancreatic enlargement, increased trypsin secretion and, therefore, lower N retention, growth and feed conversion (van Eys et al., 2004; Rackis et al., 1986). Leaves and stems do not contain trypsin inhibitors and there is no problem in using soybeans as roughage or silage (Koivisto, 2006). Due to these antinutritional factors, raw soybeans are unable to support growth requirements in monogastrics. Ruminants are not as sensitive because trypsin inhibitors are deactivated and degraded in the rumen (Hoffmann et al., 2003). However, trypsin inhibitors are heat labile and largely destroyed by heating (Rackis et al., 1986) so heat treatment is required before feeding soybean products to animals, particularly to monogastrics.
Lectins are glycoproteins that bind with intestinal villi and reduce nutrient uptake. Lectins are also heat-labile.
Saponins are glycosides that diminish uptake of other nutrients.
Others anti-nutritional factors: soybean may contain antivitamins, urease and plant sterols that may interfere with the absorption of carotenoids and vitamin E (Brown et al., 2008).
Genetically-modified soybeans
The potential health issues of genetically-modified soybeans and other GM foods is a matter of considerable debate. While most studies have failed to show deleterious side-effects to the use of GM soybean (EFSA GMO Panel, 2008), these varieties remain controversial and subject to legal authorisations in some countries.