Growth Depression and Pancreatic and Intestinal Changes in Rats Force-fed Amino Acid Diets Containing Soybean Trypsin Inhibitor

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Abstract
This investigation was conducted with rats force-fed amino acid diets, compounded to simulate 14% soybean, in order to investigate whether soybean trypsin inhibitor (SBTI) could induce growth depression when food intake and protein availability were not contributing factors. Two groups were fed the basal diet with and without SBTI for 21 days. Two other groups were fed the same diets supplemented with methionine, threonine and valine. Two hours after the last feeding, pancreases were removed and the contents of the small intestine collected, lyophilized, then analyzed for protease activity and trichloroacetic acid-precipitable nitrogen. Amino acids were determined on the trichloroacetic acid (TCA) precipitate. Although food intakes were controlled, rats fed SBTI grew less, had enlarged pancreases, more intestinal protease and more TCA-insoluble nitrogen than did controls. Analysis of the TCA-insoluble material showed it to contain more essential amino acids and about 17 times more cystine than similar material from controls. Methionine, threonine and valine prevented the growth depression induced by SBTI, but did not prevent the intestinal changes. Thus, SBTI caused rat growth depression unrelated to food intake or availability of dietary amino acids. The results indicate that the mechanism of this effect appears to be due to a loss of the critical amino acids, methionine (via cystine), threonine and valine, caused by the SBTI's ability to stimulate the pancreas to discharge excessive quantities of endogenous protein into the intestinal tract. Although much of this endogenous nitrogen may be reabsorbed, bacterial degradation, especially of those amino acids most limiting for growth, would prevent their normal re-utilization in structural protein.