Short-chain fatty acids (SCFAs) are carboxylic acids with aliphatic tails fewer than six carbons. These SCFAs are produced in the gut by enteric bacteria that ferment non-digestible carbohydrates. Some of these SCFA molecules stay local, exerting their effects in the gut; others are messengers, taking part in complex interactions that produce various effects on human physiology.
The SCFAs acetate, propionate, and butyrate, account for roughly 90 to 95% of the SCFAs in the gut.2 3 Although the colon comprises the highest concentration of SCFAs, they are also present in the cecum and small intestines.4 Butyrate is the major energy source for colonocytes and, as such, is mainly consumed locally. The vast majority of propionate produced in the colon is absorbed; after passing the colonocytes and viscera, it drains into the portal vein.5 It’s thought that about 90% of propionate is metabolized by the liver with the remainder being transported into the peripheral bloodstream.6 Propionate serves largely as a substrate for hepatic gluconeogenesis, but it also serves as a gluconeogenic substrate in the intestines before reaching the liver.1 3 Acetate, the most abundant SCFA in the colon, comprises more than half of the total SCFA detected in feces. It is also the most abundant SCFA in peripheral circulation.1 4 6 Despite lower peripheral concentrations, butyrate and propionate indirectly affect organs through the hormonal and nervous systems.1 SCFA production is influenced by the pattern of food intake and diet-mediated changes in the gut microbiota. Purposeful manipulation of the microbiota, such as through probiotic or fiber supplements or fecal transplant, can also alter SCFA production.
Fiber, resistant starches, and complex carbohydrates are fermented in the cecum and the large intestine by the anaerobic cecal and colonic microbiota. Thus, diets high in these plant polysaccharides, oligosaccharides, and resistant starches will stimulate SCFA production.