Colonic Bacterial Fermentation and Short-Chain Fatty Acids

Colonic bacterial fermentation is the anaerobic breakdown, by the dense resident microbiota of the large intestine, of dietary carbohydrate and resistant starch that escaped digestion and absorption in the small bowel. The principal products are the short-chain fatty acids acetate, propionate, and butyrate, which acidify the lumen, provide energy to the colonocytes and the host, and act as signalling molecules.

Definition

Colonic bacterial fermentation is the microbial anaerobic metabolism of undigested carbohydrate in the large intestine, yielding short-chain fatty acids (chiefly acetate, propionate, and butyrate) together with gases.

Scope

This topic covers the substrates and microbial communities involved in colonic fermentation, the production and approximate proportions of the major short-chain fatty acids, their absorption and metabolic fates, and butyrate's role as the primary colonocyte fuel. It is reference material on normal physiology.

Core questions

• Which dietary substrates reach the colon and are fermented there?
• What short-chain fatty acids are produced, and in what approximate proportions?
• How are short-chain fatty acids absorbed and metabolised by the host?
• Why is butyrate especially important for the colonic epithelium?

Key concepts

• Resistant starch and non-starch polysaccharides as substrates
• Anaerobic saccharolytic fermentation
• Acetate, propionate, and butyrate
• Approximate 60:20:20 molar ratio of the major SCFAs
• Butyrate as the preferred colonocyte energy source
• Luminal acidification by SCFAs
• Free fatty acid receptors (FFAR2/FFAR3) as SCFA sensors
• Hepatic and peripheral metabolism of absorbed SCFAs

Mechanisms

Dietary carbohydrate that is not digested in the small intestine - resistant starch, non-starch polysaccharides, and some oligosaccharides - reaches the colon, where the anaerobic microbiota ferments it. The dominant end products are the short-chain fatty acids acetate, propionate, and butyrate, typically present in roughly a 60:20:20 molar ratio, together with the gases hydrogen, carbon dioxide, and in some individuals methane. Concentrations are highest in the proximal colon, where substrate is most abundant, and fall distally as substrate is consumed. The acids are largely absorbed across the colonic epithelium; butyrate is preferentially taken up and oxidised by the colonocytes as their principal energy source, while acetate and propionate pass into the portal blood, where propionate is largely cleared by the liver and acetate reaches the peripheral circulation (Cummings et al., 1987; Bergman, 1990). Beyond their caloric contribution, short-chain fatty acids lower luminal pH and act as signalling molecules through free fatty acid receptors and other pathways (den Besten et al., 2013; Koh et al., 2016).

Clinical relevance

Fermentation and short-chain fatty acid production connect dietary fibre intake, the gut microbiota, and colonic epithelial energy supply, and they form the physiological background for interest in fibre and the microbiome. This entry describes normal physiology and is not a basis for individual dietary prescription or treatment.

Evidence & guidelines

The quantities and metabolic fates summarised here derive from human measurements of short-chain fatty acids in colonic contents and portal blood (Cummings et al., 1987) and from physiological and biochemical reviews (Bergman, 1990; den Besten et al., 2013; Koh et al., 2016); no clinical practice guideline is invoked for this normal-physiology topic.

History

Recognition that the human colon is a fermentative organ producing short-chain fatty acids was consolidated by Cummings and colleagues' 1987 measurements across human colonic contents and portal, hepatic, and venous blood, building on comparative work on volatile fatty acid energetics (Bergman, 1990). Subsequent reviews integrated these findings with the gut microbiota and host signalling (den Besten et al., 2013; Koh et al., 2016).

Key figures

• John H. Cummings
• George T. Macfarlane
• E. N. Bergman
• Fredrik Bäckhed

Seminal works

• cummings-1987
• bergman-1990
• koh-2016

Frequently asked questions

What are short-chain fatty acids and where do they come from?

They are small organic acids - mainly acetate, propionate, and butyrate - produced when colonic bacteria ferment dietary carbohydrate and resistant starch that were not absorbed in the small intestine.

Why is butyrate considered important for the colon?

Butyrate is the preferred energy source of the colonocytes, the cells lining the colon, which take it up and oxidise it in preference to glucose; it is one of the main products of bacterial fermentation.