Local (intestinal) and systemic responses of animals to ingested soyabean Glycine max proteins, antinutritional effects of lectin and trypsin inhibitors
The poor growth of young rats fed fully supplemented diets containing raw soyabean appeared to be due to interference with local (intestinal) metabolism, resulting in apparently poor digestion and absorption of dietary nitrogen, coupled with changes in systemic intermediary metabolism, leaning to low overall retention of absorbed nitrogen and slightly incresed catabolism of body lipid. Low serum insulin concentrations and pancreas and small intestine enlargement were also evident. These changes were due to a number of anti-nutritional factors in soyabean: 1. Trypsin inhibitors (Kunitz + Bowman-Birk) depressed growth rate by reducing digestion and absorption of dietary nitrogen and interfering with retention of absorbed nitrogen. They also induced considerable enlargement of the pancreas. 2. The lectin inhibited growth primarily as a result of interference with retention of absorbed nitrogen. It also caused enlargement of the pancreas and of the small intestine. 3. Anti-nutritional factor/s, devoid of haemagglutinating or trypsin inhibitory activity, caused a loss of muscle and also possibly increased production and secretion of mucus in the small intestine. The poor growth of young soyabean-fed animals was thus due to the combined effects of these anti-nutritional factors. As the rats matured, the inhibitory effects of soyabean upon growth diminished and, after 16-24 weeks on the diet, were negligible. On the other hand, pancreas enlargement persisted upon long-term (up to 96 weeks) feeding with raw soyabean. Enlargement of the whole gastrointestinal tract was also evident upon prolonged soyabean feeding. With rats kept for more than 1 year on soyabean diet, there was apparently an increased incidence (approximately 15%) of pre-cancerous or cancerous changes in the pancreas. This amy have been due, in part, to a synergism between the lectin and trypsin inhibitors and unsaturated lipids. Aqueous heat-treatment greatly reduced but did not eliminate the anti-nutritional effects of soyabean. Pre-treatment of meal with hot aqueous ethanol was more effective.