Short-chain fatty acidsSCFAs · Butyrate

What gut bacteria make from fibre, and a main fuel for the colon lining.

Short chain fatty acids are the main products of fibre fermentation in the large intestine: chiefly acetate, propionate and butyrate, produced by gut bacteria from the carbohydrate your own enzymes could not digest.

They are the mechanism by which fibre stops being an inert bulking agent and becomes a signalling story. Butyrate in particular is the preferred fuel of the cells lining the colon, supplying a large share of their energy, which is a rare case of your microbes feeding your tissue directly.

Beyond the gut wall, they enter the circulation and act on receptors involved in appetite regulation, insulin sensitivity and immune signalling. That is the plausible route from a high fibre diet to the metabolic and mortality associations seen in the epidemiology.

They are also part of why fibre type matters. Different fermentable fibres feed different organisms and yield different proportions of these acids, which is the reasoning behind advice to eat a wide variety of plants rather than a large amount of one.

The honest boundary is between mechanism and product. The biology is well described in humans and animals, and that has not translated into evidence for supplements: butyrate capsules and similar products have far weaker human outcome data than simply eating more fibre, which produces the same compounds in the place they are meant to act.

Fermentation also produces gas, which is why a sudden increase in fermentable fibre causes bloating. That settles as the community adjusts, provided the increase is gradual.

In practice, the intervention is fibre from varied plant sources, at the 30 g a day most people fall short of, rather than anything sold in a bottle.

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