loamWhat is prebiotic fiber?
Prebiotic fiber is a type of (mostly soluble) fiber that resists digestion in the small intestine and travels intact to the large intestine, where it's selectively fermented by beneficial bacteria — especially bifidobacteria and lactobacilli. That fermentation is the whole point: it's how these fibers actually feed your microbiome, rather than just adding bulk.
Inulin is the best-known example: a fructan naturally concentrated in chicory root and found in many everyday vegetables. When gut bacteria ferment inulin and other prebiotics, they produce short-chain fatty acids — compounds that go on to nourish the cells lining your gut.
Prebiotics vs. probiotics
Prebiotics are the fibers that feed your good bacteria. Probiotics are the live bacteria themselves — found in foods like yogurt, kefir and sauerkraut, or in supplements. Put simply: prebiotics are the food, probiotics are the diners. You benefit from both, and they work best together.
Best food sources of prebiotic fiber
- Chicory root & Jerusalem artichoke — the richest natural sources of inulin.
- Garlic — a concentrated everyday source, even in small amounts.
- Onion — raw or cooked, a reliable daily contributor.
- Leek — similar profile to onion and garlic.
- Asparagus — a good source alongside other nutrients.
- Slightly-green (under-ripe) bananas — prebiotic content drops as bananas ripen.
- Oats & barley — rich in beta-glucans, a different but well-studied prebiotic fiber.
- Legumes — beans and lentils bring prebiotic fiber along with plenty of protein.
- Wheat bran — an easy addition to baking or cereal.
One thing to know: many of the richest prebiotic foods — garlic, onion, wheat bran — are also higher in FODMAPs, so people with IBS or a sensitive gut may need smaller portions to stay comfortable. See our guide to low-FODMAP and fiber.
How much do you need — and a gentle warning
There's no official RDA for prebiotic fiber specifically. In practice, eating a variety of plants — vegetables, fruit, whole grains, legumes — naturally covers your prebiotic intake without needing to track a separate number.
One caveat: because gut bacteria ferment prebiotics enthusiastically, adding a lot at once — a big bowl of Jerusalem artichoke, say, or a inulin supplement — can cause gas and bloating. The fix is the same as with fiber generally: increase gradually. See how to increase fiber without bloating.
Prebiotic fiber, tracked on its own
loam treats prebiotic fiber as its own metric — not just lumped into "fiber." On the microbiome goal, it targets roughly 12 g of prebiotic fiber a day (against a 30 g total fiber goal) and flags when prebiotic is specifically your gap, suggesting foods like garlic, leeks and oats to close it. Free, no account, private by design.
Download loam on the App Store →Frequently asked
What foods are high in prebiotic fiber?
Chicory root and Jerusalem artichoke are the richest sources of inulin, followed by garlic, onion, leek, asparagus, under-ripe banana, and oats.
What's the difference between prebiotics and probiotics?
Prebiotics are the fibers that feed your existing gut bacteria. Probiotics are the live bacteria themselves, found in fermented foods or supplements. Prebiotics are the food; probiotics are the diners — you benefit from both.
Is inulin good for you?
Inulin is a well-studied prebiotic that feeds beneficial gut bacteria and supports short-chain fatty acid production. It's generally considered beneficial — just start with small amounts to avoid gas.
Sources: research on inulin and fructans as prebiotics, and on short-chain fatty acid production from prebiotic fermentation. loam supports general wellness and education — it is not medical advice.