What do gut microbes make from the fibre you can't digest?
Your own enzymes cannot break down fibre, so it arrives in the colon almost untouched, and the microbes there turn it into fuel for your gut wall.
▶ Start the storyGut microbes turn the fibre you cannot digest into short-chain fatty acids, fuel that your own body then uses. Dietary fibre is the part of plant food that human digestive enzymes cannot completely break down, so it passes down to the colon. There, gut bacteria ferment it. Soluble, fermentable fibre is generally in the colon into gases and active by-products, such as short-chain fatty acids. Some insoluble fibres, like , can be fermented too, while others are inert and simply pass through.
The main products are three short-chain fatty acids, acetic, propionic and butyric acid. Butyrate is the main energy source of , the cells that line the colon, which makes it crucial to the health of the gut. Almost all of these fatty acids are absorbed from the colon. Acetate is the one that reaches the bloodstream in appreciable amounts, and the liver can use it for energy.
Step 1: Fibre arrives
Human enzymes cannot completely break it down.
Step 2: Fermentation
Colon bacteria ferment and some resistant starches.
Step 3: Acids and gas
Acetic, propionic and butyric acid, plus carbon dioxide, hydrogen and methane.
Step 4: Fuel
Butyrate fuels the colon lining; acetate reaches the blood and the liver can use it.
The same fermentation also makes gas, mainly carbon dioxide, hydrogen and methane. That is why fermentable fibre can bring bloating or flatulence when a lot of it ferments quickly. The gas is the microbes' exhaust, and the fatty acids are the dividend you receive.
You are doing in miniature what a cow does at scale: cattle depend on microbes in their to convert cellulose into proteins, short-chain fatty acids and gases. Your own enzymes lack that chemistry, and your microbes supply it.
Quiz me
0/3
Recap
Fibre that human enzymes cannot digest is fermented by colon bacteria into short-chain fatty acids, mainly acetate, propionate and butyrate.
💡 A trick to remember it · Fibre goes down, microbes cook it, and the colon dines on the butyrate, with gas as the cooking smell.
Surprising fact · Butyrate, made by microbes from fibre, is the main energy source of the cells lining your colon.
Connects to
- 🦠 Do the microbes on your body really outnumber your own cells ten to one?
- 💊 How can a medicine that kills germs help one germ take over?
- 🦷 Is the plaque on your teeth a tiny city of microbes?
- 🏃 Why do your muscles keep working when you can't breathe fast enough?
- 🦠 What do the trillions of microbes in your gut actually do?
Sources (3)
No source, no claim. Every fact in this lesson (14 claims) cites at least one of these.