Onion is not low FODMAP. It is one of the highest FODMAP foods commonly eaten, and even small amounts can trigger symptoms in people with IBS or fructan sensitivity. The culprit is fructans, a type of carbohydrate chain that humans cannot digest. However, several practical workarounds let you get onion flavor into your cooking without the digestive consequences.
Why Onion Is High FODMAP
Onions are loaded with fructans, which are short chains of fructose molecules linked together by bonds that human digestive enzymes simply cannot break. Because your small intestine lacks the tools to process them, fructans pass through it intact and arrive in the large intestine whole. There, gut bacteria ferment them rapidly, producing gas. Fructans also draw water into the bowel through osmosis. The combination of excess gas and fluid is what causes bloating, cramping, and changes in bowel habits.
This applies to all common varieties: white, brown, red, and Spanish onions are all high in fructans at typical serving sizes. Cooking onions does not remove fructans either. Sautéing, roasting, or caramelizing changes the flavor but leaves the fructan content essentially unchanged, because the fructans stay in the food as long as they’re surrounded by the water inside the onion itself.
Onion-Infused Oil: The Main Workaround
Fructans are water-soluble but not fat-soluble. This single chemical property is the key to getting onion flavor on a low FODMAP diet. When you gently cook onion pieces in oil, the flavor and aroma compounds transfer into the fat, but the fructans do not. You then discard the onion pieces and use the infused oil for cooking.
To make it at home, heat extra virgin olive oil on low, add sliced onion, let it cook gently for 10 to 15 minutes until fragrant, then strain out every piece of onion. The oil carries the taste without the FODMAPs. You can store it in a sealed jar in the fridge for about a week. Many grocery stores also sell pre-made garlic and onion infused oils, which save time. Just check the label to confirm the onion was infused and removed, not blended into the oil.
Green Parts of Spring Onions Are Safe
Spring onions (scallions) are a split ingredient on the low FODMAP diet. The white bulb at the base is high in fructans, just like regular onion. But the green tops are low FODMAP and safe to use freely. The fructans concentrate in the bulb, so the further up the green stalk you go, the lower the fructan content.
This makes green scallion tops one of the easiest onion substitutes. They add a mild onion flavor to stir-fries, salads, soups, and garnishes. Chives work similarly and are also low FODMAP. Leek leaves (the dark green part, not the white base) are another option, though they have a milder taste.
The Surprising Case of Pickled Onions
Pickling creates an interesting FODMAP situation. Monash University tested pickled onions and found that large pickled onions were low in fructans, likely because the water-soluble fructans leached out into the surrounding pickling liquid over time. Small pickled onions, however, tested high in fructose and remained high FODMAP overall.
The size matters because larger onions have a better surface-area-to-volume ratio for leaching. But this finding comes with a practical caution: the pickling liquid itself would contain the leached fructans, so you would not want to drink it or use it as a dressing. And since results varied by size, this is not as reliable a workaround as infused oil or scallion greens.
Low FODMAP Alternatives for Onion Flavor
Beyond infused oil and scallion greens, a few other ingredients can help fill the gap onion leaves in your cooking:
- Asafoetida (hing): A pungent spice used in Indian cooking that mimics the savory depth of onion and garlic. A tiny pinch goes a long way. It is low FODMAP in the small amounts typically used for seasoning.
- Chives: Milder than onion but useful as a finishing herb on eggs, potatoes, soups, and dips.
- Celery salt or fennel seeds: These do not taste like onion, but they add aromatic complexity that helps compensate when onion is missing from a recipe.
Testing Your Personal Tolerance
The low FODMAP diet is not meant to be permanent. After an initial elimination phase (typically two to six weeks), you systematically reintroduce individual FODMAP groups to find your personal thresholds. Onion falls into the fructan group, and your tolerance may be different from someone else’s.
Monash University recommends a three-day challenge for each FODMAP group. On the first day, you try a moderate amount of the test food. On the second day, you increase to a larger serving. On the third day, you go higher still or eat your usual portion. You stay on your baseline low FODMAP diet throughout and only test one FODMAP group at a time. If symptoms appear, you stop and wait two to three days for things to settle before trying the next group.
Some people find they can handle a small amount of cooked onion in a mixed dish without issues, even if a whole sautéed onion would cause problems. Others find fructans are their primary trigger and need to avoid onion long-term. The reintroduction phase is where you find that boundary, ideally with guidance from a dietitian experienced in the FODMAP protocol.