More evidence of the crucial role of gut bacteria in the digestion of dietary nutrients is seen in the lower urinary level of several microbial co-metabolites (hippurate, 4-HPP and 3-HCA) in GF mice (Figure 1C). Indeed, it has been reported that gut microbiota are able to metabolize polyphenols, such as chlorogenic acids, into more absorbable compounds such as 4-HPP, 3-HCA and benzoic acid (Goodwin et al, 1994; Manach et al, 2004). Benzoic acid is then detoxified through conjugation with glycine in the liver and the kidney to form hippurate (benzoylglycine), a more hydrophilic metabolite that is then secreted by the renal tubular cells and excreted in the urine (Goodwin et al, 1994; Williams et al, 2002; Nicholls et al, 2003). Another microbial co-metabolite, PAG, was also found in lower concentration in the urinary profile of GF animals (Figure 1C), illustrating that microorganisms are crucial actors in the production of these urinary metabolites through the modulation of food processing.