Onion

Onion is, along with garlic, the food most commonly identified as an IBS trigger in low-FODMAP studies. Both contain fructans — chains of fructose molecules that humans can't break down and that gut bacteria ferment heavily in the colon. For people with FODMAP-sensitive guts, the result is bloating, gas, abdominal pain, and altered bowel habits within hours of eating.

Like garlic, onion is everywhere in cooking — soups, stocks, sauces, dressings, packaged seasonings, "natural flavors" in many processed foods. This makes onion one of the hardest FODMAPs to fully eliminate, and accidental exposure during an elimination trial is common.

FODMAP impact

All standard onion varieties — yellow, white, red, sweet, shallots — are high in fructans throughout the bulb. Cooking does not break down fructans, so caramelized onion is just as triggering as raw. Even small amounts (a quarter of a medium onion) can trigger symptoms in sensitive individuals.

Cooking tips

Onion-infused oil works the same way garlic-infused oil does — fructans are water-soluble and don't pass into oil, so you get the flavor without the FODMAPs. The green tops of scallions, chives, and leek greens are low in fructans and can replace the savory base in many recipes. Asafoetida (hing) provides a similar pungent flavor in tiny amounts. For broader recipes, just leaving onion out and leaning more on herbs, fennel bulb, and celery often works better than expected.