What Are Prebiotics?

A prebiotic is a substrate that is selectively utilised by host microorganisms and confers a health benefit — that's the official definition from the International Scientific Association for Probiotics and Prebiotics (ISAPP), updated in 2017. In simpler terms: prebiotics are the food your gut bacteria eat.

The Formal Definition (And Why It Matters)

The ISAPP definition has three criteria for something to qualify as a prebiotic:

1. Resists digestion in the upper GI tract — prebiotics reach the colon intact, which is where most of the microbiome lives.
2. Is fermented by gut microorganisms — beneficial bacteria in the colon break it down.
3. Selectively stimulates beneficial bacteria — the growth benefit is specific, not general. Prebiotics preferentially feed bacteria like Bifidobacterium and Lactobacillus species.

This third criterion is what separates prebiotics from general dietary fibre. All prebiotics are fibre (broadly speaking), but not all fibre qualifies as prebiotic — it has to specifically benefit the microbiome.

How Prebiotics Work

When prebiotic fibres reach the colon, bacteria ferment them through anaerobic metabolism, producing:

• Short-chain fatty acids (SCFAs) — primarily butyrate, acetate, and propionate. Butyrate is the primary energy source for colonocytes (colon cells) and has potent anti-inflammatory and anti-cancer properties. Propionate travels to the liver and influences lipid metabolism. Acetate enters systemic circulation.
• Gases — carbon dioxide, hydrogen, and sometimes methane. This explains the bloating and flatulence that accompanies a sudden increase in prebiotic intake.
• Reduced colonic pH — the acidic environment from fermentation inhibits pathogenic bacteria.

Collectively, this fermentation supports:

• Stronger intestinal lining integrity (reduced permeability)
• More diverse microbial community
• Reduced systemic inflammation
• Improved bowel regularity
• Potentially improved insulin sensitivity and lipid profiles

Types of Prebiotics

Several specific compounds have sufficient evidence to be classified as prebiotics:

Fructooligosaccharides (FOS) and inulin — Found in chicory root (the most concentrated source), Jerusalem artichoke, garlic, onion, leeks, and asparagus. The most studied prebiotic class. Inulin is often added to processed foods labelled as prebiotic.

Galactooligosaccharides (GOS) — Found in legumes and naturally in human breast milk (which is why the infant gut develops Bifidobacterium-dominant microbiota). Commonly added to infant formula.

Resistant starch — Starch that resists small intestinal digestion. Found naturally in green (unripe) bananas, cold cooked potato and rice, and legumes. Cooking and then cooling starches increases resistant starch content (retrograde starch).

Beta-glucan — Found in oats and barley. Well-evidenced for both microbiome benefit and independent cardiovascular effects (cholesterol reduction).

Pectin — Found in apples, citrus peel, and berries. Fermented in the colon; shows prebiotic properties in several studies.

Arabinoxylan — Found in wheat bran and cereal grains. Fermented by Bifidobacterium species.

Best Prebiotic Foods

You don't need supplements to get adequate prebiotics — whole foods provide them naturally:

Prebiotics vs Probiotics: The Key Difference

This is the most common point of confusion. See our dedicated guide for the full comparison: prebiotics vs probiotics.

The short version: probiotics add live beneficial bacteria to your gut (via fermented foods or supplements). Prebiotics feed the bacteria already there. Both support gut health, but through different mechanisms.

An analogy: if your gut microbiome is a garden, probiotics are seeds and prebiotics are fertiliser. Fertiliser without seeds grows weeds; seeds without fertiliser struggle to take root. The combination — sometimes called a synbiotic — is what you get from fermented foods like kimchi and kefir.

How Much Do You Need?

Current intake guidelines suggest 5–8 g of prebiotic fibre per day for measurable health effects. Most people in Western countries consume around 3–4 g daily. Getting to 5–8 g is achievable through diet:

• 1/2 cup oats: ~2 g beta-glucan
• 1 medium banana (slightly green): ~3–5 g resistant starch
• 100g cooked and cooled lentils: ~3–5 g GOS + resistant starch
• 2 cloves garlic: ~1–2 g FOS

If you're starting from a low-fibre diet, increase prebiotic intake gradually over 2–3 weeks to minimise gas and bloating. The digestive symptoms typically reduce as the microbiome adapts.

Prebiotic Supplements

Inulin and FOS powders, GOS supplements, and resistant starch supplements (usually sold as green banana flour or potato starch) are widely available. These can help reach therapeutic doses if dietary intake is consistently low.

However, whole food sources are generally preferable: they come packaged with vitamins, polyphenols, and other compounds that have independent benefits, and they're far cheaper.

The Broader Gut Health Picture

Prebiotics work best as part of a diet rich in diverse plant foods. The gut microbiome thrives on variety — different bacterial species prefer different substrates, so eating a wide range of plant foods feeds a wider range of beneficial bacteria.

For the full approach to supporting your microbiome, see our guides on how to improve gut microbiome, fermented foods for gut health, and signs of poor gut health.

Frequently Asked Questions

Are prebiotics safe for everyone? For most people, yes. Those with FODMAP sensitivities (IBS) may experience worsening symptoms, since many prebiotic foods (garlic, onion, FOS) are high-FODMAP. In this case, a low-FODMAP approach to identifying tolerances is recommended before increasing prebiotics.

Can I take prebiotics with antibiotics? You can, but it's less impactful while antibiotics are reducing microbial diversity. The better approach is to continue eating prebiotic foods throughout antibiotic treatment, then increase intake afterward to help microbiome recovery.

Do prebiotics cause weight loss? Not directly. However, prebiotics increase SCFA production, which influences satiety hormones (PYY and GLP-1) and improves insulin sensitivity — both of which support healthy weight over time. The effect on weight is real but indirect and modest.

Are prebiotic supplements the same as fibre supplements? Not exactly. All prebiotics are technically fibre, but most fibre supplements (psyllium husk, methylcellulose) aren't classified as prebiotics because they don't meet the selective stimulation criterion. Inulin, FOS, and GOS supplements are specifically prebiotic.

What happens if you eat too many prebiotics? Primarily: gas, bloating, and loose stools. This is the fermentation byproduct. It's unpleasant but not harmful. The threshold varies — people with a healthy, diverse microbiome typically tolerate more before experiencing symptoms.

Sources & References

• Gibson GR, et al. "Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics." Nature Reviews Gastroenterology & Hepatology, 2017.
• Sonnenburg ED, Sonnenburg JL. "Starving our microbial self: the deleterious consequences of a diet deficient in microbiota-accessible carbohydrates." Cell Metabolism, 2014.
• Bindels LB, et al. "Towards a more comprehensive concept for prebiotics." Nature Reviews Gastroenterology & Hepatology, 2015.
• Dahl WJ, Auger J, Alyousif Z. "Resistant starch: sources, types and health effects." Applied Physiology, Nutrition, and Metabolism, 2020.
• Swanson KS, et al. "The International Scientific Association for Probiotics and Prebiotics consensus statement on the definition and scope of synbiotics." Nature Reviews Gastroenterology & Hepatology, 2020.

Reviewed by Nicole Adams, MPH, RD, CPT — 27 April 2026