Abstract
High levels of dietary soybean meal can result in soy-induced enteritis and decreased performance in rainbow trout (Oncorhynchus mykiss) and other aquaculture target species. Various mechanisms have been employed to combat this issue, including the use of dietary additives. In this study, we assessed the use of a feed additive, trimethylamine oxide (TMAO). TMAO is a small intracellular molecule and its accumulation has been shown to enhance growth performance, protein stability and membrane integrity across aquatic taxa. Here, we conducted a 12-week feeding trial to examine the effects of chemical TMAO on growth performance, digestibility, soy enteritis, and gene expression in soy-based diets fed to rainbow trout. Fish were stocked into triplicate tanks (320L) and fed one of five treatment diets: SOY0, fishmeal control; SOY40, 40 % soybean meal; TMAO3, 40 % soybean meal +3 g kg−1 TMAO; TMAO10, 40 % soybean meal +10 g kg−1 TMAO; and TMAO30, 40 % soybean meal +30 g kg−1 TMAO. TMAO was stable through extrusion and accumulated in the distal intestine in a dose-dependent manner and fish muscle. Dietary soybean meal and TMAO had no effect on growth, feed conversion ratio, or feed intake. Replacing fish meal with soybean meal decreased fish condition factor and hepatosomatic index, increased protein and phosphorus digestibility, and decreased energy digestibility. Further, TMAO30 increased fat and energy digestibility compared to SBM40. This is in line with previous studies that showed dietary TMAO's ability to decrease fat belching in saltwater reared rainbow trout and increase fat digestibility in swine. Soy-based diets also increased morphological signs of soy-enteritis but this was suppressed in fish fed TMAO10. Dietary addition of TMAO affected various genes involved in oxidative stress, inflammation, and the immune response, including HSP90 and TNF-α in red blood cells and MTA, MX1, and TXNIPa in distal intestine. This study provides evidence that TMAO can be used as a feed additive to enhance fat digestibility and does suppress soy-enteritis at moderate inclusion levels. Significant levels of TMAO were measured in two of the three fish solubles products tested. Further examination is warranted to determine TMAO's role in promoting digestibility and gut health in fish and how fish solubles, and other ingredients, can be used to incorporate TMAO into practical formulations.