The main objective of this thesis was to evaluate metabolomics changes in humans and fish as a response to food/feed consumption. To alleviate the environmental impact of animal production and maximize the use of resources, the valorization of meat by-products might be an attractive alternative. A meat product containing heart and aorta tissue from pork was designed and analyzed for fatty acid and metabolite composition. In comparison with a control of similar qualities, the designed meat product (or test product) showed higher monounsaturated fatty acid and tyramine levels and lower levels of sugars. The test meat product was used in a randomized controlled clinical trial to test for potential health effects in patients showing atherosclerosis symptoms. Patients receiving the test product showed a decrease in blood levels of low-density lipoproteins, total cholesterol, atherogenic index and triacylglycerols. To reduce the impact of animal production on ecosystems, the replacement of feed ingredients by a microbial alternative was realized. In this study, vegetable oils included in the feed of Arctic char (Salvelinus alpinus) were replaced by biomass of the oleaginous yeast (Rhodotorula toruloides). The analysis of the yeast biomass showed safe levels of pollutants and heavy metals. Fish growth and muscle fatty acid profile were similar to the control. A higher liver weight and hepatosomatic index were observed in fish fed including the yeast biomass, albeit no significant difference in liver fat content or in hepatic enzyme activity was observed. Quantification of plasma metabolites revealed higher levels of metabolites involved in energy pathways such as one-carbon metabolism and gluconeogenesis. In conclusion, this thesis showed that metabolomics can be applied to evaluate effects of food/feed at the molecular level in complex systems. It adds knowledge on the effects of meat by-product consumption in the particular case of atherosclerosis symptoms. The fish feed trial showed the possibility of feed modification with a specific yeast.