The three short-chain fatty acids
Fermentation yields three short-chain fatty acids in quantity. They are usually described in one breath, as though they were interchangeable. They are not — they differ in how much is made, where each one goes, and what is known about it.
Acetate: the most abundant
Acetate is typically produced in the largest amounts of the three. It is the smallest and simplest, and many different organisms make it, which is part of why there is so much of it.
Much of it is absorbed and enters the general circulation, so acetate is the one most likely to reach tissues beyond the gut. Various tissues can take it up and use it.
Abundance is not superiority, though, and this is worth saying plainly because the word "most" invites the wrong conclusion. Acetate's other role is as raw material: other organisms take it up and build it into butyrate, which is the relay you met in Chapter 5. A large acetate pool is partly interesting because of what gets made from it.
Propionate: the one the liver sees
Propionate is absorbed from the colon into the portal circulation — the blood supply that runs from the intestine directly to the liver before going anywhere else.
That routing is the defining fact about it. The liver gets first sight of propionate, and much of it is taken up there rather than continuing into general circulation.
What follows from that is where the honesty has to come in. Propionate is studied for effects on liver metabolism and on appetite signalling, and there is genuine mechanistic work behind that interest. It does not follow that eating a propionate-yielding fibre produces a particular metabolic outcome in a particular person. The mechanism is real; the outcome is not something we can promise, and anyone telling you otherwise is ahead of the evidence.
Butyrate: the one that stays
Butyrate is different in a way that matters more than its quantity: it is largely consumed where it is made.
The cells lining your colon take it up and burn it as their primary fuel. Comparatively little escapes into general circulation, because the local cells get to it first.
So of the three, butyrate is the least systemic and the most local — and it is the one whose job in the colon is best characterised. It gets its own chapter next.
Why the ratio is not fixed
The mixture depends on what was fermented and by whom. Different substrates favour different outputs, and different communities produce different ratios from the same substrate.
This is the reason a fibre cannot be summarised by a single number. Two fibres can both be "fermentable" and produce meaningfully different mixtures — which is the thread Part IV picks up.
Go deeper: what "short-chain" means
It refers to the length of the carbon backbone. Acetate has two carbons, propionate three, butyrate four. That is the entire distinction in the name — and yet those one-carbon differences change how each is transported, which tissues take it up, and what it can be used for.
It is a useful reminder that in biochemistry a small structural difference is not a small functional difference. You met the same principle in Chapter 3, where one bond geometry decided whether a chain could be digested at all.
Acetate is the most abundant and the most systemic. Propionate goes to the liver first. Butyrate mostly stays and feeds the colon. Three different molecules doing three different things.
What to hold on to
Where a metabolite goes determines what it can plausibly affect. That is the discipline to bring to any claim about short-chain fatty acids: ask where the molecule ends up before believing what it is said to do.