Not at typical servings. Onion is high-FODMAP and is usually best avoided or strictly limited during the elimination phase. Safe serving: 0 (raw or cooked); green tops of scallions are unlimited.

What's a safe serving of Onion?

0 (raw or cooked); green tops of scallions are unlimited.

Which FODMAPs are in Onion?

Mainly Fructans. Based on Monash University testing data. These are the carbohydrate groups that can trigger IBS symptoms when poorly absorbed.

Is onion low FODMAP?

After garlic, onion is the second food most likely to wreck a low-FODMAP day. Like garlic, the dose-response is unforgiving and it hides in places you wouldn’t think to look.

Which onions count?

All of these are high-FODMAP and behave similarly:

Yellow onion, white onion, red onion
Shallots
Leek bulb (the white part)
Spring onion / scallion bulb (the white part only)
Pearl onions
Onion powder, dehydrated onion, onion salt

Cooking does not reduce fructans. Caramelizing them, sautéing them, or putting them in soup all keep the FODMAP load fully present.

How much is too much?

Per Monash data, 2 grams of yellow onion (about ⅛ of a small onion) is already over the threshold. There is no realistic kitchen portion that’s safe.

The one good loophole: scallion green tops

The green leaves of spring onions / scallions are low-FODMAP at any reasonable portion. Use them like you’d use chives — sliced thin into stir-fries, scrambled eggs, congee, garnishes. Just stop where the white turns into green.

This is the trick that makes Asian cooking (Chinese, Japanese, Korean) far more workable than Italian or Mexican on low-FODMAP — Asian cuisines use a lot of scallion, and the green parts are free.

Common hidden sources

Onion is in nearly every savory packaged product:

Stocks, broths (chicken, beef, vegetable — almost all)
Soups, jarred and boxed
Marinara and tomato sauces
Salsa
Most BBQ sauces and ketchups (onion powder)
Falafel, hummus
“Vegetable seasoning” mixes
Frozen prepared meals — anything with “natural flavors”
Bouillon cubes

If a savory product comes from a box and tastes like food, assume onion until proven otherwise.

Low-FODMAP swaps

Green parts of scallions (best 1:1 swap for raw onion flavor)
Chives (similar but milder)
Asafoetida (tiny pinch — gives onion-like aromatic depth)
Fennel bulb (small portions are low-FODMAP and have a sweet onion-like quality when sautéed)

Restaurant strategy

Like garlic, “easy on the onion” doesn’t work — you need full omission:

“Could the kitchen leave out the onion entirely? Even a small amount triggers symptoms for me.”

This is harder in cuisines built on a sofrito (Spanish, Italian, Latin American), Punjabi/Mughal Indian (which uses an onion bhuna base), or French (onion is in almost every stock). It’s easier in Japanese, Korean BBQ, Cantonese, and Thai — those cuisines often add aromatics late or rely on different bases.

Phase 2 testing tips

Onion is in the same FODMAP family as garlic (fructans), so they’re often tested together during reintroduction. If you tolerate one, you usually tolerate the other to a similar degree. Many people discover they can have green parts unlimited, and tolerate very small amounts of cooked onion (say, what’s in a single restaurant portion) without major symptoms — but this varies.

Source: Monash University FODMAP team, peer-reviewed measurements 2014–2024. Educational content; please consult a registered dietitian for personalized guidance.