Onion

Why onion is rated high

Onion is high FODMAP because of fructans, and it is one of the most concentrated fructan sources in an ordinary kitchen. The only serving in the published data, ½ medium (75g), already comes in high, and there is no smaller tested rung that lands lower, so "just use a little" is not a portion strategy the data supports. Anything above that tested serving only carries more fructan, and because onion is usually the base of a dish rather than a side of it, the fructans end up spread through the whole pan.

What changes your risk

• Serving size, with no lower rung ½ medium (75g) is the only tested serving and it is already high, so there is no published amount of onion that comes in lower. Treat onion as a flavour problem to solve, not a portion to shrink.
• 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 are water-soluble and do not dissolve into oil, so garlic-infused or onion-infused oil carries the savoury base flavour without the fructans, 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 safer, 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, red bell pepper 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

There is no low serving of onion in the published data. The tested serving, ½ medium (75g), is high in fructans, and no smaller amount has been tested at a lower level.

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.