Figure 2.

Segmental transit time and pH throughout the gastrointestinal tract and its association with gut environment and gut microbial metabolism. The transit time varies throughout the gastrointestinal tract with substantial interindividual differences in gastric emptying time (GET), small intestinal transit time (SITT) and colonic transit time (CTT), which account for most of the whole gut transit time. The segmental transit time ranges show the minimum and maximum transit times reported for each segment. Long gut transit time has been associated with higher faecal pH, reduced faecal water content, higher microbial cell density and diversity, and a shift in microbial metabolism from saccharolysis towards proteolysis as reflected by reduced levels of short-chain fatty acids (SCFA) and increased levels of branched-chain fatty acids (BCFA). It is likely that once easy accessible carbohydrate sources become scarce in the colon, the gut microbes switch to ferment dietary and mucin-derived proteins. While saccharolysis by the gut microbiota gives rise to SCFA that are beneficial for the host and a source of energy for the colonocytes, proteolysis can lead to the accumulation of compounds such as BCFA, phenols, indoles, ammonium (NH₃) and hydrogen sulphide (H₂S) that are generally considered detrimental for health. Moreover, hydrogen (H₂) with carbon dioxide (CO₂) or formate can be converted into methane (CH₄) by methanogenic archaea, which are also linked to slower transit time. In addition, the production and circulation of secondary bile acids and hydrolysis of host-derived glucuronides excreted via bile can also be affected by alterations in gut transit time. Whether microbiota-derived trimethylamine (TMA), produced from mainly choline and carnitine, is linked to transit time remains unknown. Created with Biorender.com.