Any substance produced by the chemical reactions of metabolism, or a food-derived compound transformed by those reactions. Primary metabolites include glucose, amino acids, and fatty acids circulating after digestion; secondary metabolites encompass specialised compounds such as plant polyphenols and their breakdown products. In nutrition research, measuring metabolites in blood or urine reveals how foods are processed and what effects they have, forming the basis of metabolomics. Some metabolites are beneficial, like short-chain fatty acids from fibre fermentation, while others, such as certain advanced glycation end products, may be harmful. The metabolite profile reflects diet, genetics, and gut microbes.