Is onion low FODMAP? Safe swaps for flavour

Onion is the quiet base of almost every savoury dish. It sits in soups, sauces, curries, stock cubes, salad dressings and spice mixes. A large US food survey found that onions supplied about a quarter of all the fructans people ate. Wheat supplied most of the rest.

So is onion low FODMAP? No, and that is why it trips up so many people with IBS. The good news is that you can keep the flavour. Here is what the research says, why onion upsets a sensitive gut and the swaps I trust.

Is onion low FODMAP?

No, onion is high FODMAP. It is rich in fructans, a type of fermentable carbohydrate that your small intestine cannot break down. Monash University, the team that created the low FODMAP diet, lists onion and garlic as high in fructans.

A Monash lab study measured fructans in 60 vegetables. Onion was among the richest sources, along with garlic, artichoke, shallots and leek bulb. These vegetables held between 1.2 and 17.4 g of fructans per 100 g, about 3.5 ounces.

A 2024 Slovenian study tested 40 of the most common foods in the national diet. Onion had the most fructans of all of them, at 1.97 g per 100 g. That was more than double the amount in wheat flour.

Why does onion cause bloating and pain?

Fructans pass through your small intestine without being absorbed. In the colon, gut bacteria ferment them and make gas. This happens in everyone. In IBS, the gut feels that stretch much more strongly.

A 2017 UK trial used MRI scans to watch this happen. Twenty-nine people with IBS and 29 healthy people drank inulin, a type of fructan, and plain glucose on different days.

Inulin filled the colon with gas in both groups. But 13 of the 29 people with IBS had clear symptoms after inulin, compared with 6 after glucose. The real problem was a sensitive gut, not extra gas.

How common is an onion or fructan trigger?

Fructans are the most common FODMAP trigger in recent research. In a 2024 Belgian trial, 117 people with IBS tested hidden FODMAP powders one at a time. Fructans brought symptoms back in 56% of them.

That also means plenty of people with IBS cope with fructans. Your own reaction is what counts.

Onion and wheat often travel together, in pizza, burgers, pasta and sandwiches. A 2018 Norwegian trial of 59 people who thought they were sensitive to gluten found that fructans caused more bloating than gluten did. If bread and onion both bother you, fructans may be the link. Please ask your doctor about a coeliac test before you cut out gluten.

Does cooking onion make it low FODMAP?

No. Cooking does not make onion safe, and water makes things worse. Fructans dissolve in water, so they leak into soups, stews and sauces. Monash explains that lifting the onion out of a soup later does not help.

Oil is different. Fructans do not dissolve in fat, but the flavour does. Monash suggests adding a large piece of onion to the oil in a stir-fry, then removing it before you add anything else.

Pickled onions are a strange case. In 2017 Monash tested two German products. Large pickled onions were low FODMAP, perhaps because fructans leaked into the pickling liquid. Small pickled onions were high, in fructose this time. Monash admitted it was not sure why, so treat pickles with care.

What can I use instead of onion?

You have more choices than you might think. These are the swaps I lean on most.

• The green tops of spring onions, sliced, without the white bulb
• Chives, fresh or dried, added near the end of cooking
• Onion-infused oil. Monash lists infused oils as low FODMAP because fructans are not fat soluble. Look for a certified brand.
• Asafoetida, a strong spice from traditional kitchens. A tiny pinch fried in oil tastes like onion and garlic. Some brands mix it with wheat flour, so read the label.
• Fresh ginger, cumin, herbs and lemon or lime juice to brighten a dish

Try this. Warm 2 tablespoons of olive oil. Add a few thick slices of onion and cook gently for 2 minutes. Lift out every piece, then cook the rest of your meal in that oil. Finish with sliced spring onion greens.

Garlic follows the same rules, and my guide is garlic low FODMAP covers safe homemade infused oil. For full meals built without onion, see my low FODMAP Indian food guide. Global traditional Ayurveda has cooked this way for centuries, as sattvic kitchens skip onion and garlic.

Can you spot onion hiding in everyday foods? Try the Trigger or Safe game, then build a calm plate in Feed the Tummy.

Can onion cause heartburn too?

For some people, yes. In a small 1990 US study, 16 people with heartburn and 16 without ate a plain burger, with and without a slice of raw onion. The onion did not change anything in the healthy group. In the heartburn group, it raised acid reflux and heartburn.

This was a small and old study. Still, if raw onion gives you heartburn as well as bloating, you are not imagining it.

Is onion good for gut health?

For many people, yes. I strongly believe gut health plays a major role in nearly every health condition, and onion fructans are food for helpful gut bacteria.

In a 2025 Korean lab study, scientists added onion extract to stool samples from 19 healthy people. Bacteria that make short-chain fatty acids, such as Bifidobacterium and Faecalibacterium, grew in response. These fatty acids help feed and protect the gut lining. This was a lab test, not a human trial, so it only hints at what happens inside us.

Cutting fructans has a cost. A 2022 review of 9 trials with 403 people found that a low FODMAP diet consistently lowered Bifidobacteria. That is one reason I never keep clients off onion for longer than they need.

Can I eat onion again?

Many people can, at least in small amounts. The low FODMAP diet is a short test, not a forever diet. A 2022 review of 13 trials with 944 people ranked it first for easing IBS symptoms. The aim is to find your own limit and then widen your food.

Try this. Once your symptoms have settled, test onion on its own. On day 1, cook 1 tablespoon of chopped onion into a meal. On day 2, try 2 tablespoons. On day 3, try a quarter of an onion. Keep everything else low FODMAP and note how you feel. If symptoms return, stop and wait until you feel normal again.

My full IBS diet guide walks you through every phase. If bloating is your main problem, my guide to bloating causes covers the other common culprits. Wondering about fruit too? Read is banana low FODMAP next.

Before you change your diet, please see a doctor about any blood in your stool. The same goes for weight loss without trying, even 3 kg (about 7 lb).

Frequently asked questions

Is red onion low FODMAP?

No. Red, white, brown and yellow onions are all bulb onions, and bulb onions are rich in fructans. Treat every colour as high FODMAP during the strict phase, then test your own tolerance later.

Is onion powder low FODMAP?

No. Onion powder is dried onion, so the fructans are still there and even more concentrated. It hides in stock cubes, spice blends, crisps, sauces and ready meals, so check ingredient lists.

Are spring onions low FODMAP?

Only the green tops. Monash suggests the green parts of spring onions as an easy swap for onion. The white bulb holds fructans like a regular onion, so leave it out during the strict phase.

Are shallots low FODMAP?

No. A Monash lab study placed shallots among the richest fructan sources, next to garlic and onion. Use spring onion greens or chives for a similar mild flavour.

Is onion-infused oil low FODMAP?

Yes. Fructans dissolve in water but not in oil, so the oil carries the flavour without the fructans. Choose a certified low FODMAP brand, or cook onion briefly in oil and remove every piece before eating.

What I want you to remember

Onion is high FODMAP because it is packed with fructans, the most common IBS trigger in recent trials. Cooking does not remove them, and soups soak them up. During the strict phase, use spring onion greens, chives, infused oil and spices instead.

Then test onion again, because many people can enjoy some later, and their gut bacteria will thank them.

For a calmer gut every day, join my free 30-Day Calm Gut Challenge.