<Table> <Tr> <Td> Tryptophan metabolism by human gastrointestinal microbiota (v t e) This diagram shows the biosynthesis of bioactive compounds (indole and certain derivatives) from tryptophan by bacteria in the gut . Indole is produced from tryptophan by bacteria that express tryptophanase . Clostridium sporogenes metabolizes indole into 3 - indolepropionic acid (IPA), a highly potent neuroprotective antioxidant that scavenges hydroxyl radicals . In the intestine, IPA binds to pregnane X receptors (PXR) in intestinal cells, thereby facilitating mucosal homeostasis and barrier function . Following absorption from the intestine and distribution to the brain, IPA confers a neuroprotective effect against cerebral ischemia and Alzheimer's disease . Lactobacillus species metabolize tryptophan into indole - 3 - aldehyde (I3A) which acts on the aryl hydrocarbon receptor (AhR) in intestinal immune cells, in turn increasing interleukin - 22 (IL - 22) production . Indole itself acts as a glucagon - like peptide - 1 (GLP - 1) secretagogue in intestinal L cells and as a ligand for AhR . Indole can also be metabolized by the liver into indoxyl sulfate, a compound that is toxic in high concentrations and associated with vascular disease and renal dysfunction . AST - 120 (activated charcoal), an intestinal sorbent that is taken by mouth, adsorbs indole, in turn decreasing the concentration of indoxyl sulfate in blood plasma . </Td> </Tr> </Table> <Tr> <Td> Tryptophan metabolism by human gastrointestinal microbiota (v t e) This diagram shows the biosynthesis of bioactive compounds (indole and certain derivatives) from tryptophan by bacteria in the gut . Indole is produced from tryptophan by bacteria that express tryptophanase . Clostridium sporogenes metabolizes indole into 3 - indolepropionic acid (IPA), a highly potent neuroprotective antioxidant that scavenges hydroxyl radicals . In the intestine, IPA binds to pregnane X receptors (PXR) in intestinal cells, thereby facilitating mucosal homeostasis and barrier function . Following absorption from the intestine and distribution to the brain, IPA confers a neuroprotective effect against cerebral ischemia and Alzheimer's disease . Lactobacillus species metabolize tryptophan into indole - 3 - aldehyde (I3A) which acts on the aryl hydrocarbon receptor (AhR) in intestinal immune cells, in turn increasing interleukin - 22 (IL - 22) production . Indole itself acts as a glucagon - like peptide - 1 (GLP - 1) secretagogue in intestinal L cells and as a ligand for AhR . Indole can also be metabolized by the liver into indoxyl sulfate, a compound that is toxic in high concentrations and associated with vascular disease and renal dysfunction . AST - 120 (activated charcoal), an intestinal sorbent that is taken by mouth, adsorbs indole, in turn decreasing the concentration of indoxyl sulfate in blood plasma . </Td> </Tr> <P> Without gut flora, the human body would be unable to utilize some of the undigested carbohydrates it consumes, because some types of gut flora have enzymes that human cells lack for breaking down certain polysaccharides . Rodents raised in a sterile environment and lacking in gut flora need to eat 30% more calories just to remain the same weight as their normal counterparts . Carbohydrates that humans cannot digest without bacterial help include certain starches, fiber, oligosaccharides, and sugars that the body failed to digest and absorb like lactose in the case of lactose intolerance and sugar alcohols, mucus produced by the gut, and proteins . </P> <P> Bacteria turn carbohydrates they ferment into short - chain fatty acids (SCFAs) by a form of fermentation called saccharolytic fermentation . Products include acetic acid, propionic acid and butyric acid . These materials can be used by host cells, providing a major source of useful energy and nutrients for humans, as well as helping the body to absorb essential dietary minerals such as calcium, magnesium and iron . Gases and organic acids, such as lactic acid, are also produced by saccharolytic fermentation . Acetic acid is used by muscle, propionic acid helps the liver produce ATP, and butyric acid provides energy to gut cells and may prevent cancer . Evidence also indicates that bacteria enhance the absorption and storage of lipids and produce and then facilitate the body to absorb needed vitamins like vitamin K . </P>

The vitamin produced by the normal flora of the colon