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Health 28/12/2025 22:23

3 Reasons Onions Might Upset Your Stomach

You may love onions, but that doesn't mean they love you back. For some, these often pungent and aromatic bulb-shaped veggies cause more than just watery eyes and bad breath.

Onions can trigger gas, bloating and an upset stomach, according to Robin Foroutan, RDN, a registered dietitian nutritionist at the Morrison Center in New York City and spokesperson for the Academy of Nutrition and Dietetics.

So does that mean onions are hard to digest or take longer to digest than other foods? "It is not that onions take longer to digest, it's that they have compounds in them that can be difficult to digest," she says. On average, the entire digestive process takes about one to three days from the time you swallow to the time something leaves your body.

Onions have healthy perks, too — they're rich in vitamin C and many B vitamins, potassium and a potent heart-healthy antioxidant called quercetin, Foroutan says. Onions can also help lower your blood sugar, strengthen your bones and ward off disease.

 If you can safely eat onions, it's well worth it.

Here’s why onions may upset your stomach – and what to do about it.

1. Onions Contain FODMAPs, Which Can Trigger Gas and Bloating

If you find yourself wondering "Why can't I eat onions anymore?," it could be the FODMAPs in onions. FODMAP stands for fermentable oligosaccharides, disaccharides, monosaccharides and polyols.

Onions contain fructans, an oligosaccharide, or carbohydrate, that's not broken down well by the gut for some people, so it sits and ferments there. This can disrupt the balance of good and bad bacteria in your gut and cause gas and bloating. If you also get these symptoms from other foods high in fructans, like garlic, wheat products, artichokes, and beans, FODMAPs could be the cause of your discomfort.

Fix it: Temporarily limit or avoid FODMAPs in your diet and then strategically add them back with the help of a dietitian or doctor, Foroutan says. "The net-net is that there might be one or two categories of FODMAPs that you don't tolerate very well."

Supplemental digestive enzymes may help you break down these carbohydrates in general more efficiently, Foroutan says. But products like these are not regulated closely by the Food and Drug Administration, so it's best to work with your doctor for specific advice on any supplements that may help you.

2. Onions Are Rich in Sulfur, Which Can Cause Stomach Upset

Onions, along with other members of the allium family like shallots, chives and garlic, are high in sulfur, which is what gives onions their strong smell and taste.

 Some people may have a food intolerance or allergy to it, Foroutan says. "If you have an allium or sulfur intolerance or allergy, it can manifest as nausea, cramping, indigestion, diarrhea or even breathing difficulty, itching, hives and/or headache," Foroutan says.
Fix it: See an allergist to find out if you have an intolerance or allergy to sulfur or another component of onions. If you’re allergic, you’ll need to avoid all onions and any prepared, processed, and packaged foods that contain onions, because food allergies can be life-threatening.

Food intolerance, on the other hand, is not life-threatening, but can cause stomach discomfort.

 If you have a food intolerance, experiment to see the amounts you can eat before symptoms kick in. If you love the taste and flavor of onions, but still can't tolerate them, try sautéing onion in a healthy oil, then discard the onions themselves, and use the infused oil to flavor your favorite dishes, Foroutan says.

3. Raw Onions Can Trigger Heartburn

Other compounds in onions can trigger heartburn shortly after eating, especially if they're raw, Foroutan says. "Raw onions are a common heartburn trigger because they can relax the sphincter muscle between the stomach and esophagus," she says.

The sphincter acts as a valve to keep the acid in the stomach and out of the esophagus, but when it relaxes inappropriately, stomach acid can flow backward into the esophagus, causing heartburn.

Fix it: Remove onions from your diet for a few weeks and then add back a small amount of cooked onions, which are easier to digest and less likely to cause heartburn than raw ones, Fourotan says.

"If that goes well, try a small amount of raw onion and see how it feels," she says. "It may be that you can tolerate small amounts of onion, but not large amounts." It may require trial and error to find your personal upper limit.

The Takeaway

  • If you're experiencing digestive discomfort after eating onions, you may want to consider a low-FODMAP diet to help manage symptoms, as onions contain fructans, a type of FODMAP known to provoke gas and bloating.
  • For those sensitive to sulfur, found widely in onions and similar vegetables, it may be beneficial to consult with an allergist to determine if you have an intolerance or allergy to sulfur or onions, which can cause stomach upset.
  • Onions, especially raw onions, are a common heartburn trigger. You may want to consider temporarily removing them from your diet and reintroducing cooked onions in moderation to see if symptoms improve.

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