Dietary trimethylamine oxide alters digestibility, intestinal histopathology,and gene expression in soy fed rainbow trout (Oncorhynchus mykiss)

Authors: BOCKUS, ABIGAIL - Us Fish And Wildlife Service; POWELL, MADISON - University Of Idaho; Sealey, Wendy; ZITO, PHOEBE - University Of New Orleans; KUREVO, YURI - University Of New Orleans; SBISA, TROY - University Of New Orleans; LONGORIA, ALONSO - University Of Idaho; GAYLORD, T - Us Fish And Wildlife Service

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2024
Publication Date: 2/15/2025
Citation: Bockus, A.B., Powell, M.S., Sealey, W.M., Zito, P., Kurevo, Y., Sbisa, T., Longoria, A.C., Gaylord, T.G. 2025. Dietary trimethylamine oxide alters digestibility, intestinal histopathology,and gene expression in soy fed rainbow trout (Oncorhynchus mykiss). Aquaculture. 596 (2025) 741810. https://doi.org/10.1016/j.aquaculture.2024.741810.
DOI: https://doi.org/10.1016/j.aquaculture.2024.741810

Interpretive Summary: Soybean meal is a cost-effective protein source for aquaculture feeds. However, feeding high levels of soybean meal to rainbow trout can damage the intestine and result in decreased feed efficiency and growth. Various mechanisms have been employed to combat this issue, including the use of dietary additives. In this study, USDA ARS scientists collaborated with scientists from the US Fish and Wildlife Service and the University of New Orleans to assess the ability of novel feed additive, trimethylamine oxide to reduce soybean meal induced damage in rainbow trout. The effect of the additive at various dietary inclusion levels (3, 10, 30 g kg-1) on growth and fish health was examined. Results indicate that dietary trimethylamine oxide supplementation improved rainbow trout intestinal health and increased lipid digestibility of fish fed high levels of soybean meal. Additives that can improve the use of plant feedstuffs by rainbow trout can reduce feed cost and increase sustainability by reducing the industry's dependence on wild-caught fish meal.

Technical 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 novel feed additive, trimethylamine oxide (TMAO). TMAO is a small intracellular molecule and its accumulation has been shown to enhance protein stability and membrane integrity across aquatic taxa. Here, we conducted a 12-week feeding trial to examine the effects of chemical TMAO, and fish solubles as a source of TMAO, on growth performance, soy enteritis, and gene expression in soy-based diets fed to rainbow trout. Rainbow trout were stocked into 320 L tanks and triplicate tanks fed one of six 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; TMAO30, 40% soybean meal + 30 g kg-1 TMAO; FS, 40% soybean meal + 10% fish solubles. TMAO was stable through extrusion and accumulated in fish muscle and distal intestine in a dose-dependent manner. 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. TMAO did not affect many of the inflammatory or oxidative genes examined. However, TMAO10 and TMAO30 upregulated the expression of TNF-a in red blood cells and TGF-ß in the distal intestine, respectively. 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. Although the fish solubles product tested here (Neptide' fish solubles, Midwest Ag) contained negligible levels of TMAO other fish solubles products measured contained higher levels. Further examination is needed 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.