Is onion high FODMAP?

The bulb is a fructan battery, and the green tops aren't

Onion is high FODMAP because it is one of the densest food sources of fructans, a chain of fructose molecules your small intestine has no enzyme to split. The chain passes through undigested, reaches the colon whole, and your gut bacteria ferment it into gas. That is the established mechanism, and it is why Monash never found a low-FODMAP serving for brown or white onion. They tested it and it ran high at every amount they measured.

Think of the onion bulb as a battery the plant charges over winter, storing energy as fructan instead of as the starch a potato uses. The charge is packed tightest in the white storage bulb. The green shoots a spring onion or leek sends up are mostly leaf, not storage, so they hold far less fructan. That single fact is the whole escape hatch: the part of the plant that banks winter fuel is the part that wrecks a sensitive gut.

Fructans dissolve in water but not in oil. Steep onion or garlic in oil, then strain the solids out, and the flavor crosses over while the fructans stay behind in the discarded pieces. The infused oil tastes of onion and carries almost none of the FODMAP. This is the rare case where a cooking technique, not a smaller portion, is what makes the food safe.

Where your line actually sits is not on the chart

The chart says red onion turns high somewhere past 10 g, but that cutoff is a population average, set so most people stay under threshold. Your own line is set by your gut sensitivity, your transit speed, and how much other fermentable load already arrived that day. Fructans also stack: the trace of onion in a stock, the garlic in a dressing, and the wheat in your bread all draw on the same fructan budget, so onion rarely flares you alone. The only way to find your number is to watch this food against how you actually feel, not against the label.

This is the kind of pattern memory loses. A teaspoon of onion powder hidden in a sauce two hours before a flare is exactly the detail you forget by the time symptoms land. Logging what you ate against how your gut responded turns that into something you can see, and Bellyweather adds up the fructan load across your day from a photo so the hidden onion shows up as a number, a lead to test, not a verdict.

• Swap the bulb for the greens: cook with the green tops of spring onions or leeks, which Monash clears up to about 75 g.
• Use onion- or garlic-infused oil for the flavor; the fructans are water-soluble and stay behind in the strained solids.
• Read labels for hidden onion: 'onion powder,' 'dehydrated onion,' 'vegetable stock,' and 'natural flavor' are common stealth sources.
• If you want a tiny amount, red onion at roughly 10 g (a thin slice) is the only common raw onion with a tested low serving.

Frequently asked questions

Sources

1. Monash University — High and low FODMAP foods (the FODMAP food list and app testing program)
2. Monash University — All about onion, garlic and infused oils on the low FODMAP diet (fructans; why the bulb runs high; infused oil)
3. Varney et al. — FODMAPs: food composition, defining cutoff values and international application, J Gastroenterol Hepatol (2017)