Is onion low FODMAP?
Why onion is a special case
Nearly every food on the low FODMAP diet has a threshold — a serving size below which it stops being a problem. Avocado is fine at an eighth of a fruit, cauliflower at a few florets. Onion doesn't work that way. Its fructan content is high enough that even the amount used to flavour a dish sits above the low FODMAP threshold, which is why it is usually the first food removed at the start of an elimination phase.
This applies to brown, white and red onion, and to shallots — shallots are often assumed to be gentler, and are not.
What still works
- Green tops of spring onions — low FODMAP, and the closest substitute for the flavour. Discard the white bulb.
- Chives — low FODMAP and useful for finishing dishes.
- Onion-infused oil — fructans are water-soluble, not oil-soluble, so they do not transfer into oil. Onion cooked in oil and then removed leaves the flavour behind without the FODMAPs. The onion itself must be taken out, not left in.
- Asafoetida — a pungent spice used in Indian cooking that approximates onion and garlic flavour. Check the label: it is often bulked out with wheat flour.
Where onion hides
Onion is one of the most common hidden FODMAPs in packaged food, because it appears as seasoning rather than as an ingredient anyone would notice:
- Stock cubes, liquid stock and gravy mixes
- Packet seasoning, marinades and rubs
- Sausages, processed meats and crumbed products
- Sauces, chutneys, relishes and most commercial soups
- Ingredient lists showing "natural flavour", "vegetable powder" or "spices" without a breakdown
If a meal that should have been safe causes symptoms, a stock or seasoning containing onion is one of the most likely explanations.
Onion and garlic together
Onion and garlic are the two foods most often eliminated first, and they behave similarly — both are fructan-heavy and both survive cooking. The oil-infusion trick works for both, for the same reason. See our is garlic low FODMAP guide for the detail on that, and the low FODMAP food list guide for the wider picture.
Frequently asked questions
Is there any amount of onion that is low FODMAP?
No. Onion is one of the few foods with no low FODMAP serving size at all — even a small amount used for flavour is high in fructans. Most high FODMAP foods become low FODMAP in a small enough portion; onion is an exception.
Is onion powder low FODMAP?
No. Onion powder is dehydrated onion, so the fructans are concentrated rather than removed. It is high FODMAP in the quantities used in seasoning blends, stocks and packet mixes.
Can I use spring onions?
The green tops only. The green part of spring onions (scallions) is low FODMAP and works well as a substitute; the white bulb is high in fructans, the same as any other onion. Chives are also low FODMAP.