What Are Prebiotics? A Simple Guide

If you have read anything about gut health lately, you have probably seen the word "prebiotic." It is often used next to "probiotic," sometimes interchangeably, which is where much of the confusion starts. This guide explains what prebiotics actually are, where you find them, and how they relate to fiber and probiotics.

What Prebiotics Are

A prebiotic is a compound - most commonly a type of fiber - that resists digestion in the small intestine and reaches the colon largely intact. There, it acts as a substrate (in plain terms, a food source) for the microorganisms that live in your gut. The most widely cited working definition, from an international panel of scientists, describes a prebiotic as "a substrate that is selectively utilized by host microorganisms conferring a health benefit."

Two parts of that definition matter for everyday understanding:

1. Prebiotics are used by your gut microorganisms, not digested by you the way ordinary carbohydrates are.
2. The definition includes a health benefit, which is why researchers are careful about which compounds they call prebiotics and which they do not.

In practice, most ingredients described as prebiotics are carbohydrates: fibers and sugars such as inulin, fructooligosaccharides (FOS), galactooligosaccharides (GOS) and resistant starch.

Prebiotic vs. Fiber

The relationship is easiest to picture as overlapping circles.

• All prebiotics are substrates for gut microorganisms, but not every fiber meets the stricter scientific criteria for being called a prebiotic.
• Many fibers function in a prebiotic-like way because they are fermented in the colon, but researchers debate exactly where the prebiotic line should be drawn.
• Fiber has other roles too, such as adding bulk and supporting regular digestion, which are separate from the "food for microbes" role.

Food Sources of Prebiotics

You do not need a supplement to encounter prebiotic-like compounds. Common food sources include:

• Chicory root - one of the richest natural sources of inulin
• Jerusalem artichoke (also called sunchoke) - high in inulin-type fibers
• Onions and garlic - contain fructans such as inulin and FOS
• Leeks and asparagus - also members of the allium family, rich in fructans
• Bananas - especially slightly green ones, which contain resistant starch
• Oats and barley - contain beta-glucan, a soluble fiber
• Apples - a source of pectin, a fermentable fiber
• Legumes - beans, lentils and chickpeas contain varied fermentable fibers

Cooking, ripeness and processing all change how much fermentable fiber a food contains. A ripe banana has less resistant starch than an underripe one, for example.

Prebiotics vs. Probiotics

This is the pairing most people mix up:

• Probiotics are live microorganisms - typically bacteria or yeasts - that, when consumed in adequate amounts, may confer a benefit. They are found in fermented foods like yogurt, kefir and some sauerkrauts, and in probiotic supplements.
• Prebiotics are not living things. They are compounds - usually fibers - that feed the microorganisms already living in your gut.

A common way to remember it: probiotics add microbes, prebiotics feed microbes. For a detailed side-by-side comparison, see Prebiotics vs. Probiotics: What's the Difference?.

Common Supplement Ingredients Described as Prebiotics

When you read a supplement label, you will most often see these names:

• Inulin - usually derived from chicory root; see What Is Inulin?
• Fructooligosaccharides (FOS) - short-chain fructans, often from chicory or cane sugar
• Galactooligosaccharides (GOS) - often derived from lactose

Resistant starch - starch that escapes digestion, from sources like green bananas or potatoes

Some products combine several of these, sometimes alongside probiotics. Products that combine both are sometimes called synbiotics; see What Are Synbiotics? for what that term does and does not mean.

How to Read a Prebiotic Claim on a Label

• Look for named ingredients and amounts rather than vague phrases like "prebiotic blend."
• Check the serving size - the listed amount of inulin or FOS is per serving, not per container.
• Remember that a blend that lists no individual amounts may make it impossible to know how much of each fiber you are getting; see What Does 'Proprietary Blend' Mean on a Supplement Label?
• Note other ingredients such as sweeteners, flavorings or digestive enzymes, which are listed separately.

What Prebiotics Are Not

• Prebiotics are not live bacteria - that is what probiotics are.
• Prebiotics are not a single ingredient; the term covers a category of compounds.
• Prebiotics are not automatically right for everyone. Large amounts of fermentable fiber can cause gas or bloating in some people, and anyone managing a digestive condition should discuss changes with a healthcare professional.
• Prebiotics are not a replacement for a varied diet. Foods provide many types of fiber and nutrients at once.

Putting It Together

Prebiotics are best understood as the "food" side of the gut-microbe relationship: compounds, mostly fibers, that reach your gut microbes largely undigested. They are abundant in ordinary foods - onions, garlic, bananas, oats, legumes - and they also show up on supplement labels under specific ingredient names. Knowing the ingredient names and checking serving sizes puts you in a much better position to evaluate any product that uses the word "prebiotic" on its label.

If you want to see how these terms look on a real label, the Pro-Active Greens category on MarketPlace Global lists the current formula and ingredient details.