Onion and garlic are flavour heroes, which makes them a genuine loss when they don't suit you. The reason they so often cause trouble is fructans— a chain-type carbohydrate that our gut can't break down, so gut bacteria ferment it instead, producing gas. Monash University rates both as high-FODMAP, and fructans are water-soluble, which is the key to working around them.
Because fructans dissolve in water but not in oil, a garlic- or onion-infused oil carries the aroma into a dish while leaving the fructans behind in the discarded solids. (Make infused oils safely — use them fresh and keep them chilled.) The green tops of spring onions and chives are also much gentler than the white bulb.
| Form of onion / garlic | Roughly how it behaves |
|---|---|
| Garlic / onion-infused oil | Flavour without the fructans — the gentlest way in. |
| Green tops of spring onion / chives | Low in fructans — usually well tolerated. |
| Cooked-and-drained onion (small amount) | Some fructans leach into the water — gentler than raw. |
| Raw onion / garlic | Full fructan load — a common trigger. |
| Onion / garlic powder | Concentrated — and often hidden in sauces and seasonings. |
The hidden powders are the real catch — they turn up in stocks, gravies, crisps, marinades and spice mixes even when you can't see any onion. Our hidden high-risk ingredients page lists the usual suspects, and you can scan a product's barcode to check its ingredient list.
Cooking for the family? Make a gentle base with infused oil that everyone eats, then let the others add raw or pickled onion to their own plates at the end — same meal, yours is just the gentle version.