Onion

High FODMAP

By Michael EisenwasserUpdated October 6, 2026

Serving sizes

Onion changes rating with portion size. These are the amounts tested in the literature behind our ratings.

About 1 small ring (10g) Low
Larger portions Moderate
Much larger portions High

Fructans GOS

Onion is High FODMAP at a usual serving, though about 1 small ring (10g) is low, driven by its Fructans and GOS.

Only about 1 small ring (10g) is low.
Fructans and GOS build fast, so larger portions are moderate and much larger portions are high.
Onion hides in soups, sauces, stocks and spice blends, so scan labels for onion and onion powder.

For savory depth without the onion itself, cook with garlic-infused or onion-infused oil and strain out the solids, or use the green tops of spring onions.

Why onion is rated high

Onion is rated high because of fructans and GOS, and it is one of the most concentrated fructan sources in an ordinary kitchen. The low serve is tiny: about 1 small ring (10g) is low, a larger portion is moderate, and a much larger portion, such as the diced onion that starts most recipes, is high. Because onion is usually the base of a dish rather than a side of it, the fructans end up spread through the whole pan, so a small ring on a burger is a very different thing from a softened onion in a sauce.

What changes your risk

Serving size, with a very low ceiling About 1 small ring (10g) is low, larger portions are moderate and much larger portions are high. Treat onion as a flavour problem to solve, not a portion to shrink, because the amount a recipe calls for is far past the low serve.
Cooking changes nothing Frying, caramelising, roasting or slow simmering does not break fructans down. Softened, sweet, browned onion carries the same fructans it started with.
Infused oil is the real workaround Fructans and GOS are water-soluble and do not dissolve into oil, so garlic-infused or onion-infused oil carries the savoury base flavour without them, as long as no solid pieces are left sitting in the bottle or the pan.
Simmering moves fructans into the liquid Water-soluble fructans leach out of onion into stock, soup and braising liquid. That does not make the pot any lighter, it spreads the problem into the broth, so fishing the pieces out at the end does not undo it.
Hidden onion on labels Onion powder, dried onion, onion salt, stock cubes and powders, gravy granules, most curry pastes, taco and fajita seasoning, soup bases and many crisps and crackers all carry the same fructans. Read the ingredient list, not the front of the pack.
Stacking with other fructan sources A low serving of wheat bread, leek leaves or a fructan-carrying vegetable in the same meal adds to whatever else is on the plate, so a day built on savoury sauces adds up even when no whole onion is involved.

Swaps

For the savoury base, garlic-infused oil is the closest thing to a straight swap, and it works in exactly the place a recipe says "soften the onion". Beyond it, build depth from the green tops of spring onions, the green leaves of leek, chives, and a pinch of asafoetida stirred into hot oil for that cooked-allium note. For the bulk and sweetness onion brings to a sofrito or a bolognese, finely diced carrot and zucchini sweated in the infused oil do a lot of the same work. What does not help is swapping to another allium bulb: shallot, red onion, white onion, leek bulb and the white part of spring onion all carry the same fructans.

How to lower the load

Buy a bottle of garlic-infused oil that lists oil and flavour only, and check nothing solid is floating in it, since fructans sitting in the oil defeat the point. When a recipe starts with diced onion, start it with the infused oil instead and add the chopped green tops of spring onions near the end so they keep their bite. Buy asafoetida with care, as it is often bulked out with wheat flour, which brings fructans back in through the side door. Eating out is where onion is hardest to see, so ask whether the sauce, marinade or stock is built on onion rather than asking for it to be left off the top, and pick grilled meat, fish, rice and plain vegetables where the kitchen cannot answer.

Common questions

how much onion is low fodmap

About 1 small ring (10g) is low. Larger portions are moderate and much larger portions, such as a diced onion in a sauce, are high, driven by fructans and GOS.

is onion powder low fodmap

No. Onion powder is dried onion, so it carries the same fructans in a more concentrated form, and it turns up in stock cubes, gravy granules, spice blends and savoury snacks.

does cooking onion remove fodmaps

No. Heat does not break fructans down, so frying, roasting and caramelising leave them intact. Fructans are water-soluble, so some move out of the pieces into soup or braising liquid, but that just puts them in the broth.

what can i use instead of onion for flavour

Garlic-infused oil is the main one, because fructans do not dissolve into oil. The green tops of spring onions, leek leaves, chives and a pinch of asafoetida fill out the rest of that savoury base.

Learn more

Where this rating comes from

Onion is rated from published lab measurements of its FODMAP content, using the low, moderate and high cutoffs of Varney et al. (2017).

Feedback sent through the FODMAP Foods app has led to changes in this list.

See our full rating method →

Sources

Fructan and free fructose content of common Australian vegetables and fruit. Muir et al. (2007), J. Agricultural and Food Chemistry 55(16):6619-6627.
Measurement of short-chain carbohydrates in common Australian vegetables and fruits by high-performance liquid chromatography (HPLC). Muir et al. (2009), J. Agricultural and Food Chemistry 57(2):554-565.
Quantification of fructans, galacto-oligosaccharides and other short-chain carbohydrates in processed grains and cereals. Biesiekierski et al. (2011), J. Human Nutrition and Dietetics 24(2):154-176.
Dietary sorbitol and mannitol: food content and distinct absorption patterns between healthy individuals and patients with irritable bowel syndrome. Yao et al. (2014), J. Human Nutrition and Dietetics 27(Suppl 2):263-275.
FODMAPs: food composition, defining cutoff values and international application. Varney et al. (2017), J. Gastroenterology and Hepatology 32(Suppl 1):53-61. Supplies the low, moderate and high cutoffs used on this page.