One of the short chain fatty acids produced in lower bowel microbial fermentation. Butyric acid production in the human lower gut is promoted by the ingestion of resistant starch [RS]: that is, starch that is resistant to digestion in the preceding elements of the digestive tract via either acidic or enzymatic hydrolysis. There are different types of RS:
RS1 is starch physically occluded [hidden] by other plant anatomical structures, e.g. when consuming unmilled whole grains;
RS2 is raw granular starch of poor digestibility often with B-type amylopectin crystal structures [potato & banana starches], RS2 starch becomes digestible on cooking after gelatinization;
RS3 is most commonly based on normal, or preferably high-amylose, starch sources. The need for amylose in RS3 is the enhanced capability of amylose to quickly and strongly re-crystallize (retrograde) on cooling of a cooked starch matrix. The strong tendency of amylose to re-crystallize and the ability to grow and make the amylose crystallites more perfect during repeated heating and cooling cycles is exploited in the food industry in order to create sources of RS3 for addition to foods
Back to butyric: the higher levels of butyric acid that arise from the RS fermentation (compared to the higher amount of propionic acid fermented from non-starch fiber sources) are believed to be the genesis of the protective effects of RS against colo-rectal cancers. Butyric acid is believed to act as a cell growth regulator for the cells in the bowel epithelium, but also contributes to other more general factors that improve bowel function such as lowered fecal pH. RS appears to be fermented in the distal (descending) colon, as opposed to non-starch fibers that are fermented in the ascending and transverse colons, it extends these beneficial attributes further along the digestive tract.