The intestinal microbiota has been implicated in a wide range of diseases, including inflammatory bowel disease, obesity and cancer. Food ingredients are considered a major determinant of gut microbial composition, as exemplified by high-fat diet-induced dysbiosis that can affect host physiology. Accumulating studies show that microbial metabolites are key regulators of the intestinal epithelial barrier and gut immunity. In particular, short-chain fatty acids produced by bacterial fermentation of indigestible polysaccharides have profound impacts on host physiology beyond the gut. In this review, we describe the influences of the diet-microbiota-metabolite axis on host physiology, and especially on the immune and metabolic systems.