When gut bacteria ferment fibre, they break down the fibre your small intestine could not digest. This happens in your large intestine, or colon. The process makes short-chain fatty acids and gases. What you get depends on the fibre and on the bacteria already in your gut, so fermentation does not promise a change you can feel.

Maybe you have started eating more fibre and noticed more wind. It is natural to wonder whether that means it is “working”. Gas alone cannot tell you that. This guide explains what really goes on in your gut, in plain words, and how to add fibre without the discomfort.

What actually happens when gut bacteria ferment fibre

Most food is broken down into small pieces that your body can absorb. Fibre is different. It gets through your small intestine without being digested. When fibre that bacteria can use reaches your colon, they feed on it. That feeding is fermentation.

Think of it as bacterial food processing. The bacteria turn fibre into several things, including short-chain fatty acids. Your body absorbs some of what they make. The rest leaves in your stool or as gas.

Not every fibre ferments to the same extent. Fermentation is also only one of the ways fibre works. Fibre can hold water and add bulk to stools too. So “contains fibre” on a label does not tell you how a food or supplement will behave. Our guides to soluble or insoluble fibre and what decides how a fibre behaves in your gut explain the basics.

You will often see the word “prebiotic” on fibre packs. It has a specific meaning. The ISAPP consensus defines a prebiotic as “a substrate that is selectively utilized by host microorganisms conferring a health benefit”. In other words, feeding bacteria is not enough. There must also be a health benefit.

Fibre, prebiotics and probiotics are not the same thing:

• Dietary fibre is the broad group. Not all fibre counts as a prebiotic.
• Prebiotics are substances used by the microorganisms already living in you.
• Probiotics contain live microorganisms.

Our article on your resident gut microbiome versus probiotic supplements covers the difference in more detail. Neither has to be ranked above the other.

Short on fibre most days? ADVAGEN Prebio is a soluble prebiotic fibre in 5 g sachets that you stir into the drink or food of your choice. See today’s price and stock.

Meet the three short-chain fatty acids

Short-chain fatty acids, or SCFAs, are small compounds that gut bacteria make when they ferment fibre. Three come up most often:

• Acetate: one of the main products of fermentation. It plays a part in how your body uses energy.
• Propionate: another fermentation product. It takes part in metabolism and in signalling between cells.
• Butyrate: a source of energy for the cells lining your colon. Scientists also study it for its signalling roles.

The Koh review describes how SCFAs can switch on certain cell receptors, affect enzymes linked to gene control, and act as fuel. These processes may play a part in health and disease.

But knowing how something works is not the same as proving it helps you. A compound can take part in a process without more of it making that process better for everyone. A change measured in a lab is not automatically a health benefit.

Measuring SCFAs is also harder than it sounds. A stool test shows what is left over. It does not show how much the bacteria made, or how much your body took in. The den Besten review points to this gap, and to some findings that disagree with each other.

Does every fibre raise short-chain fatty acids?

No. Different fibres can give different results. The bacteria already in your gut can shape your response too.

The Baxter trial shows this well. It gave 174 healthy young adults one of four things for two weeks: potato resistant starch, maize resistant starch, inulin, or a corn-starch control.

Potato resistant starch gave the biggest rise in total SCFAs, including butyrate. Maize resistant starch and inulin did not significantly raise butyrate in stool. People’s own gut bacteria also affected how they responded.

That does not crown one fibre the winner. It shows that fibres we lump together as “fermentable” do not all act the same way.

What researchers measure matters as well. In one 2010 trial of soluble fibre dextrin and soluble corn fibre, total SCFAs did not change, but some individual ones did. Research on resistant dextrin has looked at SCFAs less often, and the results have been mixed.

So when you read about the benefits of a prebiotic fibre, ask three simple questions:

• Which fibre was tested?
• What did the researchers actually measure?
• Who took part? A result in healthy young adults may not apply to someone older or with a health condition.

Why fermentation also means gas

Gas is a normal by-product when gut bacteria break down certain carbohydrates. You may notice more of it when you change what you eat.

Fermentation is not the only source. According to NIDDK, gas also comes from air you swallow. The rest comes from bacteria breaking down undigested carbohydrates in your large intestine.

That explains why fibre can make you gassy. It does not make gas a useful scorecard, though. More wind does not mean more helpful fermentation. Less wind does not mean fermentation has stopped.

SingHealth notes that a sudden jump in fibre may leave you bloated from too much gas. It suggests increasing your intake gradually over two to three weeks.

Discomfort is not a sign to push harder. Notice how you feel, and avoid making several big changes at once. Say you start a fibre supplement, eat far more beans and switch every grain to wholegrain in the same week. If you then feel uncomfortable, you will not know which change caused it.

For more on this, read our guide to the common causes of bloating. Symptoms that are severe or do not go away need a doctor, not a guess about fermentation.

A practical way to add fibre: slowly, food first

You do not need to chase a number. Start with food, build up slowly, and pay attention to how your body copes.

1. Know the target. HealthHub suggests 20 g of fibre a day for women and 26 g for men. SingHealth gives 30 g, and reports that the average Singaporean eats 13 g. The sources differ a little, and an average cannot tell you what you eat.
2. Put food first. Think oats, wholemeal bread, brown rice, beans or dhal, fruit and vegetables. Add something that fits the meals you already eat. There is no need to change everything overnight.
3. Build up over two to three weeks. Give each change time, so you can tell how it feels. A big jump to hit a number fast is more likely to leave you uncomfortable.
4. Drink fluids with your fibre. SingHealth explains that fibre takes up water in the gut, making stools softer and bulkier. So fluids matter as you eat more fibre. If your doctor has told you to limit fluids, follow that advice.
5. Notice how you feel. Watch for changes in gas, stool and your usual bowel pattern. These tell you how well you are coping. They cannot tell you your SCFA levels.
6. Change one thing at a time. Jot down what you changed and when. If symptoms carry on, that note can help your doctor.

When gas or bowel changes need a doctor

Do not assume it is just your gut getting used to fibre. See your GP or a polyclinic if gas or bowel changes come with any of these:

• Blood in your stool, or black stool.
• An unexplained drop in body weight.
• A change in bowel habit that lasts for weeks.
• Pain that is severe or will not go away.
• Symptoms in a baby.

Advice for adults should not be used for infants. A doctor can check your symptoms and decide what needs looking into. Eating more fibre does not replace that check.

Where ADVAGEN Prebio fits

Food should always come first. If your meals already give you enough fibre, you may not need a supplement at all. But on busy days, many of us fall short. That is where a simple sachet can help.

ADVAGEN Prebio helps you add soluble fibre to your daily diet. Its one named fibre is resistant dextrin, also called resistant maltodextrin. Prebio is not a probiotic. It is easy to fit into your day because it does not form a gel and does not change the taste or texture of your food and drinks.

To be clear about what it is not: the fermentation facts above are about fibres in general. It does not show that Prebio will change your short-chain fatty acids, or produce any particular result for you.

ADVAGEN Prebio at a glance

• What it is: a plant-based soluble prebiotic fibre supplement. Ingredient: resistant dextrin (soluble prebiotic fibre from corn).
• What it does: helps you add soluble fibre to your daily diet.
• How it mixes: soluble, does not form a gel, and does not affect the taste or texture of food and drinks.
• Label: sugar-free and gluten-free, with no artificial colourings, flavourings or preservatives. Made in a GMP-compliant facility, according to the product page.
• How to take it (product page directions): adults 1 to 4 sachets a day; children ½ to 1 sachet a day; for children under 6, ask a paediatrician first.
• Pack: 30 sachets of 5 g powder, about SGD 0.99 a sachet at today’s shop price.

As with any fibre, start with one sachet a day and build up slowly. If you are pregnant, take medication or manage a health condition, check with your doctor or pharmacist first.

FAQ

Is fibre a prebiotic?

Not always. Under the ISAPP definition, a prebiotic must be used selectively by the microorganisms in your body and bring a health benefit. Being fermentable is not enough on its own. Fibre is a broader group, so “fibre” and “prebiotic” do not mean the same thing.

Why does fibre make me gassy at first?

Gut bacteria can make gas when they break down fermentable fibre. This is more likely after a sudden increase. Adding fibre gradually may make the change easier. Gas does not prove a benefit, and you do not need to push through it. Severe or lasting symptoms need a doctor.

Do I need a supplement to make short-chain fatty acids?

No. Gut bacteria can ferment fibre from food to make short-chain fatty acids. Food comes first, and a supplement is optional.

Is resistant dextrin the same as resistant starch?

No. The names sound alike and both reach the colon undigested, but they are different ingredients. Resistant dextrin, also called resistant maltodextrin, is a soluble fibre. The one in ADVAGEN Prebio comes from corn. Resistant starch is a form of starch. So a result for potato resistant starch, like the Baxter trial above, cannot simply be applied to resistant dextrin or to a product that contains it.

Read next: what resistant dextrin is and how much is too much, or how to choose a fibre supplement.

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References

1. Gibson GR, Hutkins R, Sanders ME, et al. (2017). Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nat Rev Gastroenterol Hepatol, 14(8), 491-502. PubMed
2. Koh A, De Vadder F, Kovatcheva-Datchary P, et al. (2016). From Dietary Fiber to Host Physiology: Short-Chain Fatty Acids as Key Bacterial Metabolites. Cell, 165(6), 1332-1345. PubMed
3. den Besten G, van Eunen K, Groen AK, et al. (2013). The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism. J Lipid Res, 54(9), 2325-40. PubMed
4. Baxter NT, Schmidt AW, Venkataraman A, et al. (2019). Dynamics of Human Gut Microbiota and Short-Chain Fatty Acids in Response to Dietary Interventions with Three Fermentable Fibers. mBio, 10(1). PubMed
5. Stewart ML, Nikhanj SD, Timm DA, et al. (2010). Evaluation of the effect of four fibers on laxation, gastrointestinal tolerance and serum markers in healthy humans. Ann Nutr Metab, 56(2), 91-8. PubMed
6. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Gas in the digestive tract. Accessed 19 October 2026. Link
7. SingHealth. High fibre diet. Accessed 19 October 2026. Link
8. HealthHub Singapore. Boost your health: the essential benefits of dietary fiber. Accessed 19 October 2026. Link

This article is general information, not medical advice. ADVAGEN Prebio is a dietary fibre supplement and is not intended to diagnose, treat, cure or prevent any disease. Speak to a doctor or pharmacist about your own health.