Whole and halved yellow and red onions on a cutting board

Are Onions Good for Digestion? What the Evidence Says

ByCraig Brockie Last updated

The verdict: good for most guts, hard on some

For most people, onions are good for digestion. They add fiber and are one of the richest everyday sources of fructans, a carbohydrate that feeds helpful gut bacteria (Muir et al., 2007). The same fructans can cause gas, bloating and pain in people with irritable bowel syndrome (IBS), and onions may worsen heartburn in people who already get it. So the fair answer is yes for most people, and “test your own tolerance” if your gut is sensitive.

What the research shows

Onions bring fiber and a lot of fructans

USDA FoodData Central lists raw onion at 40 calories, 1.7 g of fiber and 4.2 g of sugars per 100 g. A medium onion (110 g) works out to about 44 calories and 1.9 g of fiber, and a cup of chopped onion (160 g) to about 2.7 g of fiber (USDA FoodData Central). That is modest next to the 25 to 38 g of fiber a day that Mayo Clinic cites for adults (Mayo Clinic).

The fiber number undersells onions, though. Onions carry inulin-type fructans that standard fiber tests do not measure, which is why they are often missing from food tables. A 1995 review estimated that people eating a Western diet get roughly 1 to 10 g of these fructans a day (van Loo et al., 1995). When Monash University researchers measured 60 vegetables, onions were among the highest in fructans, alongside garlic, artichoke, shallots and leeks (Muir et al., 2007).

Fructans feed helpful bacteria

Fructans are not digested in the small intestine. They travel to the colon, where bacteria ferment them. A 2018 meta-analysis of 64 randomized trials with 2,099 healthy adults found that fiber supplements raised levels of Bifidobacterium and Lactobacillus, and that fructans and galacto-oligosaccharides had the clearest effect (So et al., 2018). You can read more about the difference in prebiotics vs probiotics.

One caution: those trials mostly used purified inulin or oligofructose powders, not onions. We could not find a human trial that fed people onions and measured their gut bacteria, so the benefit of onions themselves is inferred from their fructan content.

The same fructans can cause gas and bloating

Fermentation makes gas. In a small, controlled trial of 26 healthy adults, 5 to 10 g of chicory inulin or oligofructose mildly increased gas and bloating, and 10 g of oligofructose clearly increased symptoms (Bonnema et al., 2010). People with IBS react more strongly.

In a placebo-controlled rechallenge trial, 25 people with IBS were given fructans in drinks: 77% said their symptoms were not adequately controlled, compared with 14% on glucose (Shepherd et al., 2008). In another crossover trial of 30 people with IBS, 21 days on a low-FODMAP diet cut overall symptom scores roughly in half compared with a typical Australian diet (Halmos et al., 2014). NIDDK lists onions and garlic among high-FODMAP vegetables (NIDDK), and Monash names onion as a fructan-rich food (Monash University).

Onions and heartburn

The best-known study is small and old. In 1990, researchers gave 16 people with frequent heartburn and 16 without it a plain hamburger on one day and the same burger with a slice of raw onion on another, and tracked acid with a probe for two hours. Onion did not change reflux in the people without heartburn, but it increased reflux episodes, acid exposure and heartburn in those who had it (Allen et al., 1990). Cleveland Clinic notes that garlic and onions, in larger amounts, can relax the valve at the top of the stomach (Cleveland Clinic). See our GERD diet guide.

Caveats to keep in mind

  • Most of the evidence is indirect. Prebiotic benefits come from fructan supplement trials, not onion trials.
  • The IBS studies are small. They are well controlled but involve dozens of people, and they test FODMAPs as a group or as purified fructans.
  • The heartburn study is from 1990. It used one meal, 32 people and raw onion only.
  • Gas is not harm. Some extra gas after a high-fiber meal is normal fermentation, and tolerance varies with portion size.

How to use onions if your gut is sensitive

Small bowl of thinly sliced onions beside a vegetable stir-fry
  1. Start small. A few slices in a dish is not a bowl of onion soup.
  2. Build up slowly. Mayo Clinic advises adding fiber over a few weeks so gut bacteria can adjust (Mayo Clinic).
  3. Keep a short food and symptom diary for a couple of weeks.
  4. Watch sauces and marinades. Monash notes that onion- and garlic-based marinades and sauces are high-FODMAP options too (Monash University).
  5. Try a structured approach if you have IBS. NIDDK says a doctor may suggest a low-FODMAP diet for a few weeks, then adding foods back gradually. A dietitian can help you find your onion threshold rather than cutting it out for good.
  6. If onions trigger heartburn, try them earlier in the day, in smaller amounts, and not right before lying down.

If onions bother you, other foods can supply fiber. See easy-to-digest vegetables and what to eat when gas hurts.

When to see a doctor

This article is general information, not medical advice. NIDDK suggests talking with a doctor if gas bothers you, your symptoms change suddenly, or gas comes with abdominal pain, constipation, diarrhea or weight loss (NIDDK). Mayo Clinic lists weight loss, diarrhea at night, rectal bleeding, iron deficiency anemia, unexplained vomiting, and pain not relieved by passing gas or stool as more serious signs (Mayo Clinic). For heartburn, Mayo advises seeing a clinician if symptoms are severe or frequent, or you use heartburn medicine more than twice a week, and getting help right away for chest pain with shortness of breath or jaw or arm pain (Mayo Clinic).

Frequently asked questions

Are onions a prebiotic?

Onions are rich in fructans, the same type of carbohydrate used in most prebiotic trials, though onions themselves have not been tested that way in humans (Muir et al., 2007).

Why do onions give me gas?

Fructans reach the colon undigested, where bacteria ferment them and produce gas. People with IBS tend to feel this more.

Are cooked onions easier to digest than raw?

We found little research comparing the two directly, so adjusting portion size is the more reliable step.

Should I stop eating onions if I have IBS?

Not necessarily. NIDDK describes a low-FODMAP trial of a few weeks followed by adding foods back (NIDDK), ideally with a doctor or dietitian.

Can onions cause heartburn?

In a small 1990 study, a slice of raw onion increased reflux in people who already had heartburn but not in those who did not (Allen et al., 1990).

Related topics

Sources

This article is for general information and is not medical advice. Talk to your doctor or a registered dietitian before changing your diet, supplements or medication, especially if you have a health condition.

Craig Brockie is the founder of Ultimate Health Solutions, the company behind Ultimate Gut, and the author of Ultimate Health, The Ultimate Gut Health Guide and The Unlimited Probiotics Guide. After injuries and chronic health problems in his twenties that conventional care didn’t resolve, he began researching health on his own and has studied natural health for 30 years. He is not a medical professional, and his articles are general information, not medical advice.

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