Overview
About Onion
Onion is graded E for fructans and E for total FODMAPs, the top of our scale, which is why it is the single most common trigger people uncover once they start logging food against symptoms.
The reaction is a carbohydrate problem rather than an allergy: onion is A for histamine, A for oxalates, A for lectins and A for nightshades, so if onion bloats you the fructans are almost certainly the mechanism. It also grades E for low fermentation SIBO plans. The frustrating part is that fructans are water soluble and heat stable, so caramelizing, slow simmering or fishing the pieces out of a sauce does not make onion any safer, and onion hidden in stock, gravy and spice blends is why so many people react to restaurant food they cannot account for.
Sensitivities
How Onion scores
Watch for
What trips people up
- Fructans and total FODMAPs both graded E, the highest tier we use, and the grade does not soften between raw and cooked onion
- Cooking is not a fix: fructans dissolve out of the onion into the pan and stay in the liquid, so straining the pieces out leaves the trigger in the sauce
- Hidden onion drives most unexplained flare-ups: stock cubes, gravies, curry pastes, spice rubs, chips and anything listed as seasoning or natural flavor
- Graded E for low fermentation SIBO, so onion is usually one of the first things pulled when working through a SIBO protocol
- Clean on the rest of the chemistry (histamine A, oxalate A, lectin A, nightshade A, with only a borderline B for histamine liberation), which is why an onion reaction is nearly always intolerance rather than allergy
Diet fit
Where it fits
Swap ideas
More alternatives
FAQ
Common questions about Onion
Is onion low FODMAP?
No. Onion grades E for fructans and E for FODMAPs overall, and unlike most high FODMAP foods there is no small serving that comes out low. Raw, cooked, dried and powdered forms all count. During an elimination phase the usual replacements are the green tops of spring onions and chives, which carry allium flavor at a far lower fructan load.
Why does onion upset my stomach when other vegetables are fine?
Onion is dense in fructans, chains of fructose that the human small intestine has no enzyme to split. They arrive in the colon intact and gut bacteria ferment them into gas. That is ordinary digestion, but with IBS or a sensitized gut the extra gas and the water drawn in are felt as pain and distension. It is carbohydrate intolerance, not allergy, and the grades support that: onion is A for histamine and A for oxalates.
Does cooking or caramelizing onion make it easier to digest?
No. Fructans are water soluble and survive heat, so they leach out of the onion into the stock, soup or sauce. Removing the pieces takes away the texture, not the fructans. Caramelizing concentrates what is there rather than breaking it down. The one exception is fat: fructans do not dissolve into oil, so frying onion in oil and discarding the solids leaves most of the load behind.
Are spring onions and scallions the same as onion for FODMAPs?
Not quite, and the difference is which end you use. Whole scallions grade C for FODMAPs and D for fructans because the white bulb is the loaded part. The green tops are the low FODMAP portion and the standard way to keep onion flavor during elimination. Chives sit at A for fructans and B for FODMAPs because they are leaves rather than bulbs.
References
Read the background
Ratings reflect the Triggerbites ingredient database and may vary with portion size and preparation.