Prebiotic

What it is

A prebiotic is a substance — usually a type of dietary fiber — that your own body can't digest, but that feeds the beneficial microorganisms in your gut. Where a probiotic adds live microbes, a prebiotic is the food that helps the microbes you already have thrive. Common examples include inulin, fructooligosaccharides (FOS), and galactooligosaccharides (GOS).

What it does

• Nourishes helpful gut bacteria. Prebiotics reach the colon undigested, where resident microbes ferment them.
• Supports production of short-chain fatty acids (SCFAs). That fermentation produces SCFAs, which nourish the cells lining the colon and support the gut barrier.
• Complements probiotics. Prebiotics and probiotics are sometimes combined (a "synbiotic") so the added microbes have fuel.

What it does NOT do

• A prebiotic is not a probiotic. It contains no live organisms; it feeds the ones already present.
• It's not a cure-all. Effects depend on the specific fiber, the dose, and your existing gut community.
• More isn't always comfortable. Ramping up prebiotic fiber too quickly commonly causes gas and bloating — a tolerance issue, not a sign it's "working harder."

Forms and doses

• Found naturally in foods like onions, garlic, leeks, asparagus, bananas, oats, and legumes — and added to supplements as inulin, FOS, or GOS.
• Typical supplemental doses range from a few grams per day; start low and increase gradually to limit gas and bloating.
• Pair with adequate fluids as you increase fiber.

Sources

1. National Institutes of Health, Office of Dietary Supplements. Probiotics fact sheet for health professionals.
2. International Scientific Association for Probiotics and Prebiotics. Prebiotics.