Fibre isn’t just for digestion. We’re often told that it acts like a scrubbing brush, cleaning out your bowels. But it’s also food, just not food for you. Your gut bacteria rely on fibre to sustain themselves. Without it, their levels can become dysregulated (known as dysbiosis).

Dietary fibre is fermented by the gut bacteria, producing short-chain fatty acids (SCFAs) which help with everything from maintaining gut barrier integrity to regulating inflammation. SCFAs are a big part of your health. If your gut microbiome is affected, it’s often one of the first things to be affected.

Below, we explore what fibre is, how it supports the gut microbiome, and where short-chain fatty acids fit into everything.

What Is Fibre and Why Does It Matter for Gut Health?

Fibre is an indigestible plant carbohydrate. Unlike other carbohydrates (like sugars and most starches), fibre is not digested or absorbed in the small intestine. Instead, it reaches the colon intact.

That’s where the gut bacteria take over.

Types of Fibre

Fibre comes in three different forms:

• Soluble fibre (feeds bacteria, forms gels)

• Insoluble fibre (adds bulk, supports bowel movements)

• Resistant starch (important for SCFA production)

The two big categories are soluble and insoluble fibres. However, fibres can vary in their solubility depending on their source and level of processing.

What Are Short-Chain Fatty Acids (SCFAs)?

SCFAs are a byproduct of fibre fermentation. Two of the most common SCFAs are acetate and butyrate, known for their ability to reduce inflammation.

But they’re only produced when we eat the right foods. Oats, rice, and potatoes become rich in resistant starch when they are cooked, cooled, and reheated, while unripe (green) bananas are naturally a good source. These forms of resistant starch help promote the production of SCFAs. In contrast, when whole grains are ground into flours, they tend to be lower in this type of fibre and so produce fewer SCFAs.

When you eat more fibre, your gut produces more SCFAs which can protect the intestinal lining. It can also prevent harmful bacteria from overgrowing and help absorb certain minerals like calcium.

Butyrate: The Key Player

Butyrate is one of the most important short-chain fatty acids your gut produces. It acts as the primary fuel source for the cells lining the colon, helping them function properly and stay healthy.

This supports the integrity of the gut barrier, prevents ‘leaky gut’, and therefore reduces the risk of unwanted substances passing through. It also has a natural anti-inflammatory effect, making it a key part of maintaining overall gut balance.

How Fibre Feeds Your Gut Microbiome

The Fermentation Process

Most foods are digested as they pass through the stomach and small intestine. Fibre is different. It reaches the colon (large intestine), where bacteria ferment it. SCFAs are produced as a result, alongside certain gases like H2 (hydrogen) and CO2 (carbon dioxide).

You might think the fermentation process is bad. But it’s actually extremely beneficial. Without it, the “good” bacteria wouldn’t survive, and no SCFAs would be produced.

Why Diversity of Fibre Matters

Each bacterium relies on a specific fibre (or a handful). The more sources of dietary fibre you introduce, the more variety of bacteria you support.

Relying on just a single bacterium for SCFA production is risky. It’s better to rely on lots of different species, as this creates a more resilient microbiome. It also means there isn’t a single “superfood.” Rather, it’s about eating a varied, balanced diet to cater to the varied bacteria species.

Benefits of Short-Chain Fatty Acids for Gut Health

Short-chain fatty acids have numerous beneficial functions. These include:

• Strengthened gut lining (reduce “leaky gut” risk)

• Reduced inflammation

• Supported immune function

• Regulated metabolism and blood sugar levels

• Regulated appetite and satiety

Signs You May Not Be Eating Enough Fibre

Most people probably don’t get enough fibre. Even when they do, it’s often from processed foods, which could harm beneficial bacteria.

Common signs your fibre intake is poor include:

• Constipation or irregular bowel movements

• Bloating after low-fibre meals

• Low energy or poor satiety

• Limited variety in diet

• Digestive discomfort

How to Increase Fibre to Support SCFA Production

Focus on Whole Food Sources

Vegetables, fruits, legumes, and whole grains are excellent sources of soluble fibres, which all support different microbial populations. Oats, rice, and potatoes, and unripe bananas (as mentioned above) are especially useful for SCFA production.

Add Fibre Gradually

Sudden increases in fibre can cause bloating and flatulence. Gradually increase your intake so your microbiome has time to adapt.

Stay Hydrated

It’s also incredibly important to make sure you’re drinking enough water when eating more fibre. If you’re eating lots of fibre or increasing the amount you’re having, drinking enough water ensures the smooth running of your digestive system, and helps reduce the risk of dehydration and or constipation.

Get Your Microbiome Checked

If you’re dealing with ongoing digestive or hormonal symptoms, it may be worth going beyond guesswork.

The Functional Gut Clinic offers personalised microbiome testing to assess the balance of your gut bacteria and how well it’s functioning. From there, you will receive targeted guidance on diet and lifestyle changes to support SCFA production and your overall gut health.

It’s a more precise way to understand what’s actually going on and what to do about it.

Read the next article: The Microbiome and Your Immune System