A fascinating peek at digestion, fermentation, and microbial balance.

We’re often told that fibre is good for us, but few people know what actually happens after we eat it. Fibre doesn’t just bulk up our stool and keep things moving. It also plays a central role in feeding the trillions of bacteria that live in our gut and helping them maintain a healthy, balanced environment.

Let’s take a closer look at what fibre does behind the scenes once it enters your digestive system.

Fibre Travels Undigested

Unlike carbohydrates, proteins, and fats, fibre passes through the stomach and small intestine mostly undigested. It reaches the large intestine relatively intact. This is where the real action begins. The large intestine is home to a dense population of gut microbes that specialise in breaking down complex carbohydrates like fibre.

Some types of fibre are fermentable, meaning our gut bacteria can metabolise them. This process is called fermentation, and it produces beneficial by-products, most notably short-chain fatty acids.

Fermentation and Short-Chain Fatty Acids

When certain fibres reach the large intestine, they are fermented by bacteria to produce short-chain fatty acids such as acetate, propionate, and butyrate. These compounds are anti-inflammatory, help regulate the immune system, and even play a role in protecting against colorectal cancer. Butyrate, in particular, is the preferred fuel source for the cells lining your colon. It helps keep the gut barrier strong and supports a healthy inflammatory response.

Fermentation also helps modulate the pH of the colon, making it less hospitable to potentially harmful bacteria. This encourages a more balanced microbiome and may help reduce the risk of certain gut-related diseases over time.

Feeding Your Gut Microbiota

Think of fibre as a buffet for your gut microbiota. When you eat a variety of fibres from fruits, vegetables, legumes, nuts, seeds, and whole grains, you’re feeding a broader spectrum of bacterial species. This diversity is a key marker of a healthy gut.

A low-fibre diet, on the other hand, starves beneficial microbes and can lead to reduced diversity, which has been linked with issues like inflammation, constipation, and even mental health symptoms. The more varied your fibre intake, the more robust and resilient your microbiome is likely to be.

Not All Fibre Ferments

It’s worth noting that not all types of fibre are fermented. Some fibres, like sterculia, work by absorbing water and supporting bowel movements without feeding bacteria. These non-fermentable fibres are often better tolerated by people with sensitive guts and are used to improve stool consistency without the side effects of gas or bloating.

That said, both fermentable and non-fermentable fibres have an important role to play in gut health, one feeds your microbiota, the other supports regularity.

The Bigger Picture

Adding more fibre to your diet doesn’t just improve digestion. It helps shape your entire internal ecosystem, influencing immunity, inflammation, metabolic health, and even brain function through the gut-brain axis.

So next time you load your plate with fibre-rich foods, know that you’re not just doing something good for your bowels. You’re actively supporting a healthier, more balanced gut environment, one that benefits your whole body.

Founder Mind + Gut Clinic | Co-Founder Nerva | Co-Founder TUMME