Onions are one of the single best prebiotic foods you can eat — raw onion contains up to 7.5g of inulin per 100g, making it one of the most concentrated prebiotic sources in any kitchen. They also contain quercetin, a powerful polyphenol that has anti-inflammatory, antiviral, and antioxidant properties. Their pungent flavour comes from sulphur compounds similar to garlic.
Why it matters
Highest Inulin Density
Up to 7.5g inulin per 100g — one of the most concentrated prebiotic foods available.
Quercetin Content
Powerful anti-inflammatory polyphenol that also inhibits histamine release.
Feeds Bifidobacterium
Selectively fuels beneficial bacteria without feeding pathogenic strains.
Sulphur Compounds
Similar to garlic — provide antimicrobial and cardiovascular benefits.
How to eat it
Eat them raw for maximum prebiotic impact — finely sliced in salads, salsas, or sandwiches. Cooked onions still provide fibre and flavour but lose some prebiotic potency. Red onions have higher quercetin content than white. Spring onions and leeks (same family) are equally effective and milder in flavour.
Inulin from onions has been shown in multiple controlled trials to significantly increase Bifidobacterium longum populations — one of the key species linked to reduced anxiety, improved digestion, and stronger gut barrier function.
Source: Kelly, Journal of Nutrition, 2009
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