The sustainable development of modern aquaculture must rely on a significant reduction of the fish meal (FM) used in aquafeed formulations. However, FM substitution with alternative ingredients in diets for carnivorous fish species often showed reduced nutrient absorption, significantly perturbed metabolisms and histological changes at both hepatic and intestinal level. In the present study, adult rainbow trout (Oncorhynchus mykiss) were fed three different experimental aquafeed formulations. A control diet with higher FM content (27.3%) than two test formulations in which fish meal was substituted with two more sustainable and promising alternatives: insect meal (Hermetia illucens larvae=10.1%, FM=11.6%) and poultry by-products meal (PBM=14.8%; FM=11.7%). Combined metabolomics and proteomics analyses of fish liver, together with histological examination of liver and intestine demonstrated that a well balanced formulation of nutrients in the three diets allowed high metabolic compatibility of either substitutions, paving the way for innovative and sustainable use of novel raw materials for the fish feed industry. Results show that the main metabolic pathways of nutrient absorption and catabolism were essentially unaltered by alternative feed ingredients, and also histological alterations were negligible. It is demonstrated that substitution of fish meal with sustainable alternatives does not impact on fish metabolism, given proper efforts are put in fulfilling nutritional requirements of rainbow trout.