What Really Happens to Fibre in Your Gut?

GOOD GUT WEDNESDAY · 03
The Nourishment Edit

We hear a lot about eating more fibre.

Usually, the conversation revolves around digestion, regularity or simply reaching a certain number of grams each day.

But fibre has another story — one that begins when some of it reaches a place our own digestive enzymes cannot fully access.

The large intestine.

And waiting there is an extraordinary microbial community.

Some members of that community can do something we cannot do on our own:

ferment certain fibres.

What happens next is one of the most fascinating conversations taking place between food, our microbes and our own biology.

Not all of your food is digested by you

When we eat digestible carbohydrates, proteins and fats, much of their digestion and absorption occurs before they reach the large intestine.

Certain dietary fibres and other carbohydrates, however, resist digestion in the upper gastrointestinal tract.

They continue into the colon.

And there, they encounter microorganisms equipped with enzymes capable of accessing carbohydrates that our own digestive machinery cannot fully break down.

In a sense, part of the nutritional story of these foods is only beginning.

Your microbes are metabolically active

As we explored in Good Gut Wednesday · 01 — Your Gut Isn’t Just Home to Bacteria, the microbiome is more than a simple collection of bacteria.

We often talk about the gut microbiome as though it were simply a collection of microorganisms living inside us.

But those microorganisms are doing things.

They consume substrates.

They interact.

They compete.

And they produce metabolites.

When certain fibres and other fermentable carbohydrates are metabolised by gut microbes, one important group of compounds produced is known as short-chain fatty acids, or SCFAs.

The three most commonly discussed are:

Acetate.
Propionate.
Butyrate.

These molecules are produced in different proportions depending on the substrates available and the microorganisms involved.

And they do not simply remain irrelevant by-products sitting inside the colon.

They can interact with human physiology in remarkably interesting ways.

Meet butyrate

Of the short-chain fatty acids, butyrate has attracted enormous scientific interest.

One particularly important reason is that butyrate can serve as a major energy source for colonocytes — the cells lining the colon.

Research has also explored the role of short-chain fatty acids in intestinal barrier function, immune signalling and metabolic regulation.

But this is where nutrition conversations can easily become oversimplified.

It would be inaccurate to say:

“Eat fibre and you will produce more butyrate.”

The reality is considerably more complex.

Different fibres can be fermented differently.

Different microorganisms have different metabolic capabilities.

Microbial communities interact with one another.

And individuals can respond differently to the same dietary intervention.

The interesting lesson is therefore not that one particular fibre creates one particular outcome.

It is that our microbes actively transform components of our food into other biologically relevant compounds.

Fibre isn’t one thing

The word fibre sounds singular.

Scientifically, it isn’t.

Dietary fibres vary enormously in their structure, solubility, viscosity, fermentability and physiological effects.

A fibre found in oats is not necessarily behaving in exactly the same way as one found in beans, onions, seeds or other plant foods.

Some fibres are readily fermented.

Others are fermented more slowly.

Some may be only minimally fermented.

And different microbial species possess different collections of enzymes that allow them to access different carbohydrates.

This is another reason why reducing nutrition to a simple statement such as “eat more fibre” misses some of the richness of the story.

Both the amount and type of fibre can matter.

And that brings us back to something we explored in Good Gut Wednesday · 02:

diversity.

Different plant foods bring different fibres and other compounds to the table.

An ecosystem fed by what reaches it

Imagine your gut microbiome not as a passive collection of bacteria, but as a metabolically active ecosystem.

What reaches that ecosystem influences the resources available within it.

When our diet changes, that microbial environment can change too.

Research has repeatedly demonstrated that dietary patterns can influence the composition and metabolic activity of the gut microbiome — sometimes remarkably quickly — although individual responses vary.

This doesn’t mean we can micromanage individual microbes simply by eating one particular food.

Microbiome science isn’t that simple.

But it does mean that what we eat becomes part of the ecological environment in which our microorganisms live.

And that is a very different way of thinking about food.

More than “roughage”

For a long time, fibre was popularly thought of largely as roughage — the part of plants that helped food move through the digestive system.

That role remains important.

But modern microbiome science has revealed a much richer picture.

Certain fibres can become substrates for microbial fermentation.

Microbial fermentation can produce metabolites.

And those metabolites can interact with our own cells.

Food.

Microbe.

Metabolite.

Human.

An extraordinary chain of interactions taking place inside us every day.

A different way to look at your plate

Perhaps this is why fibre deserves to be thought about differently.

Not simply as:

“How many grams did I eat today?”

But also:

Where did my fibre come from?

Beans?

Lentils?

Whole grains?

Vegetables?

Fruits?

Nuts?

Seeds?

And how much variety exists across those foods?

There is still a great deal we do not understand about the microbiome, and researchers continue to investigate how different fibres, microbial communities and metabolites influence human health.

But one thing is increasingly clear:

our gut microorganisms are not simply passengers.

They are metabolically active participants in what happens to some of the food we eat.

So the next time you eat a fibre-rich plant food, remember:

the story doesn’t necessarily end when you swallow it.

For some of its components, an entirely new story may be beginning.

Scientific Evidence

Mann ER, Lam YK, Uhlig HH. (2024).
Short-chain fatty acids: linking diet, the microbiome and immunity.
Nature Reviews Immunology, 24, 577–595.
DOI: 10.1038/s41577-024-01014-8

Makki K, Deehan EC, Walter J, Bäckhed F. (2018).
The Impact of Dietary Fiber on Gut Microbiota in Host Health and Disease.
Cell Host & Microbe, 23(6), 705–715.
DOI: 10.1016/j.chom.2018.05.012

Koh A, De Vadder F, Kovatcheva-Datchary P, Bäckhed F. (2016).
From Dietary Fiber to Host Physiology: Short-Chain Fatty Acids as Key Bacterial Metabolites.
Cell, 165(6), 1332–1345.
DOI: 10.1016/j.cell.2016.05.041

Deehan EC, Mocanu V, Madsen KL. (2024).
Effects of dietary fibre on metabolic health and obesity.
Nature Reviews Gastroenterology & Hepatology, 21, 301–318.
DOI: 10.1038/s41575-023-00891-z

The Nourishment Edit is Lynn’s exploration of food, nourishment and the science behind how we eat. Our articles are for educational purposes and are not intended as medical advice.

Researched and edited by Lynn Glenda Pinto
Founder, Lynn’s