Figure 1. An expanded model for the contribution of the gut microbiota to energy harvest from dietary lipids, carbohydrates, and proteins.

In this issue, Semova et al. (2012) show that the gut microbiota stimulates lipid absorption in the zebrafish proximal intestine. Compared with germ-free animals fed the same diet, enterocytes of animals conventionalized with a gut microbiota accumulated larger and more numerous lipid droplets. Increased lipid accumulation was also observed in the liver of conventionalized animals, suggesting greater uptake of lipids into systemic circulation. Microbial processes in the distal gut (colon) are also known to contribute to host energy gain. Carbohydrates and protein that resist digestion in the small intestine are fermented by the colonic microbiota, producing short-chain fatty acids (SCFA) that can be assimilated and used as energy by the host (Martens et al., 2011). The H₂ and CO₂ released during fermentation are consumed as substrates by some microbes in acetogenesis (Rey et al., 2010), which produces a secondary source of SCFA, as well as methanogenesis (Samuel and Gordon, 2006). Consumption of H₂ and CO₂ reaction products helps to drive the continued production of SCFA.