Why humans cannot digest fibre
Fibre is made of sugars. You digest sugars. So why can you not digest fibre? The answer is not about the sugars — it is about how they are joined.
The bond, not the building block
Starch and cellulose are both long chains of glucose. Chemically, the units are identical. What differs is the linkage between them: the geometry of the bond that joins one glucose to the next.
Your amylase recognises one geometry and cuts it. It does not recognise the other. Cellulose passes through you not because it is exotic, but because the enzyme you have is the wrong shape for the join.
This is the whole reason dietary fibre exists as a category. Fibre is not a substance. It is a description of plant carbohydrate that human enzymes cannot break down.
Three fates for what you swallow
It is worth separating three things that get muddled:
- What your enzymes can digest. Absorbed in the small intestine. This feeds you.
- What reaches the colon. Anything your enzymes could not handle, plus anything they did not get to in time.
- What microbes can use. A subset of the above. Reaching the colon is not the same as being usable once there.
That last distinction is the one most fibre marketing skips. Two fibres can both arrive in your colon intact and behave completely differently once they do — one broken down enthusiastically, the other passing through almost unchanged.
Which is where the word prebiotic comes in
Now the term can be defined without hand-waving. A prebiotic is a substrate that resident microorganisms use, in a way that benefits the host.
Note what that does and does not require. It requires that something living can metabolise it. It does not require that the fibre forms a gel, adds bulk, or does anything mechanical. And it does not follow that every fibre is a prebiotic — a fibre that neither you nor your microbes can break down is still fibre, and still useful, but it is not feeding anything.
Go deeper: why humans never evolved the enzyme
Some animals do digest cellulose, but almost none do it with their own enzymes. Ruminants and termites host microbial communities that do the work, in specialised chambers, over long residence times. The strategy is outsourcing, not invention.
Humans use a smaller version of the same strategy. Our fermentation chamber is the colon rather than a multi-compartment stomach, and it sits after absorption rather than before it — which is why microbial products made there reach us as metabolites rather than as our primary meal.
You cannot digest fibre because your enzymes cannot read the bond. Something in your colon can. That handover is the foundation of everything in this course.
What to hold on to
Fibre is defined by what your enzymes cannot do, not by what it is made of. And "fibre" covers materials with almost nothing in common beyond that. Keep that in mind next time you see a single number for fibre on a label — it is a category, not an ingredient.
If we cannot digest it, and something in the colon can — what exactly is down there?