Fig. 1.

Simplified scheme of the metabolites produced by the interplay between the microbial and human metabolome. After passing through the small intestine, unabsorbed dietary substrates (mainly unabsorbed carbohydrates and proteins) along with other endogenous substrates (such as bile enzymes, mucus and exfoliated cells) reach the large intestine, where they are fermented by the intestinal microbiota. These microbial metabolites are excreted in feces or absorbed and transported to the liver. Subsequently, these absorbed metabolites are subject to further human metabolism and can be excreted in urine and breath (50).