Effects of dietary soybean meal on the digestive physiology of red seabream Pagrus major

Abstract

To clarify the effects of dietary soybean meal (SBM) on the digestive physiology of red seabream Pagrus major, two isonitrogenous and isolipidic diets; a fishmeal-based diet (diet FM) and a SBM-based diet (diet SBM), were prepared and the acute and chronic effects of the diets on the digestive physiology of the fish were examined. In the acute effects trial (<48 h), the only difference observed was a lower level of trypsin activity in the intestinal content of the SBM group than the FM group, indicating that SBM administration reduces trypsin secretion on a short-term basis. In the chronic effects trial (six weeks), there were clear differences in several parameters assessed. There was a clear retardation of growth in fish fed the SBM diet, in addition to a faster gastric transit rate, stacked intestinal content, lower gallbladder weight, lower activities/gene expression levels of hepatopancreatic enzymes (trypsin, chymotrypsin, amylase and/or lipase) and reduced protease activity (trypsin and chymotrypsin) in the intestinal content compared to fish fed the FM diet. Taken together, these results indicate that feeding SBM over a long period (chronic exposure) leads to reduced storage, production and secretion of bile and pancreatic digestive enzymes. SBM protein does not appear to fully activate or stimulate the digestive system of red seabream in either the short or long term. This may be due to a lack of stimulation or to an inhibitory effect that becomes more pronounced on a long-term basis. Such inhibition of the digestive process may be one of the main reasons for the low growth performance observed in fish fed SBM-based diets.