Why do raw onions bother my stomach?

January 9, 2026 •

4 min read

According to research on food intolerances, millions of people worldwide struggle with digestive issues triggered by common foods. For many, the seemingly innocuous raw onion is a primary culprit, leaving them to wonder, "Why do raw onions bother my stomach?".

Quick Summary

Raw onions can cause stomach discomfort due to fructans, a fermentable carbohydrate poorly absorbed by some people, and sulfur compounds that can irritate the gut. The fermentation of fructans by gut bacteria produces gas, leading to bloating, pain, and other symptoms. Cooking onions can often alleviate these issues.

Key Points

Fructan Intolerance: Raw onions are high in fructans, a type of fermentable carbohydrate that many people, especially those with IBS, cannot properly digest.
Gas and Bloating Cause: The fermentation of undigested fructans by gut bacteria creates gas, leading to bloating, pain, and flatulence.
Sulfur Sensitivity: Onions contain sulfur compounds that can irritate the gut and worsen digestive symptoms in sensitive individuals.
Cooking Alleviates Symptoms: Heating onions breaks down the fructans and reduces the potency of sulfur compounds, often making them easier to digest.
Raw Onions and Heartburn: For people prone to acid reflux, raw onions can relax the lower esophageal sphincter, allowing stomach acid to enter the esophagus.
Portion and Preparation Matter: Experimenting with smaller portions or using methods like oil infusion can help you enjoy onion flavor without digestive distress.
Allergy vs. Intolerance: An onion intolerance is a digestive issue, while an onion allergy is a more serious immune response that requires complete avoidance.

The Core Culprit: Fructans and FODMAPs

At the heart of many raw onion-related digestive issues lies a type of carbohydrate called fructans. Fructans belong to a group of fermentable carbohydrates known as FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols). For individuals with a sensitive digestive system, such as those with Irritable Bowel Syndrome (IBS), fructans are not easily absorbed in the small intestine.

Instead, they travel to the large intestine where resident gut bacteria begin to ferment them. This process, while normal, can produce excess gas in large quantities for those who are sensitive, leading to the familiar symptoms of bloating, abdominal pain, and flatulence. This is not an allergy but an intolerance based on the digestive system's inability to break down these particular carbs.

The Impact of Sulfur Compounds

Beyond fructans, onions are part of the Allium family and are rich in organosulfur compounds, which are responsible for their distinct pungent aroma and flavor. For some individuals, these compounds can further irritate the digestive tract. This sensitivity to sulfur can also manifest as symptoms such as nausea, cramping, and indigestion. In addition, when raw onion is chopped, a chemical reaction occurs that releases a volatile sulfur compound, which is what makes your eyes tear up. This same compound can contribute to digestive discomfort in susceptible individuals.

Raw vs. Cooked Onions: Why Cooking Helps

Many people notice that they can eat cooked onions without any issue, but raw onions cause significant discomfort. This difference is due to the effects of heat on the problematic compounds within the vegetable.


Feature	Raw Onions	Cooked Onions
Fructan Content	High concentration, especially in the white bulb.	Cooking, especially boiling, can reduce the fructan content as it leaches into the cooking liquid.
Digestibility	Can be difficult for sensitive guts to process, leading to fermentation.	The heat breaks down some of the complex fibers, making them easier to digest for many people.
Heartburn Trigger	A known trigger for acid reflux as it can relax the lower esophageal sphincter.	Less likely to trigger heartburn, particularly if sautéed or caramelized.
Sulfur Compounds	More concentrated and potent, contributing to irritation.	The heating process reduces the potency of some of the volatile sulfur compounds.

Strategies for Managing Onion Intolerance

If you find yourself sensitive to raw onions, you don't have to eliminate them entirely from your diet. Here are some strategies you can use to minimize the impact on your stomach:

Embrace cooked onions: Swap raw onion in salads or sandwiches for sautéed, roasted, or caramelized versions. The cooking process breaks down the fructans, making them much easier to digest.
Consider onion-infused oil: Fructans are water-soluble, not oil-soluble. You can sauté onions in oil and then discard the onion pieces, leaving behind a delicious, flavorful oil that won't cause digestive upset.
Reduce portion sizes: Instead of a large portion of raw onion, try a small amount to see if your digestive system can tolerate it. Your sensitivity might be dose-dependent.
Try milder varieties: Some individuals report better tolerance to milder varieties, like green onion tips or shallots, which may contain lower concentrations of fructans.
Look for hidden onion: Be mindful of packaged and processed foods, which often contain onion powder or dried onion flakes. Reading labels can help you avoid unexpected triggers.
Consult a dietitian: If symptoms persist, a dietitian can help you follow a low-FODMAP diet to systematically identify your specific triggers and reintroduce foods properly.

Conclusion

In summary, the next time you ask, "Why do raw onions bother my stomach?", remember that fructans and sulfur compounds are the most likely reasons for your discomfort. By understanding the science behind your digestive system's reaction, you can make informed choices about how you prepare and consume onions. Simple changes, like cooking or reducing your portion size, can allow you to enjoy the flavor of onions without the unpleasant aftermath. It's a matter of listening to your body and finding what works for your unique digestive system. For more detailed information on managing food intolerances, consult resources like the Monash University Low FODMAP Diet website, which provides extensive information on FODMAPs and digestive health.(https://www.monashfodmap.com/blog/all-about-onion-garlic-and-infused-oils-on-the-low-fodmap-diet/)

Common Digestive Symptoms Associated with Raw Onions

Many people experience a range of gastrointestinal issues after consuming raw onions. These can vary in severity and may include:

Bloating and excess gas.
Abdominal pain and cramping.
Nausea or general discomfort.
Heartburn and acid reflux.
Altered bowel habits, such as diarrhea or constipation.

If you experience any of these symptoms, especially consistently after eating raw onions, it's a strong indicator of an intolerance.

Frequently Asked Questions

Raw onions are harder to digest primarily because the fructans and other fibers are in their raw, more complex state. Cooking breaks down these compounds, making them less likely to cause gas and bloating during digestion.

An onion intolerance is a digestive system issue caused by difficulty processing specific compounds like fructans, leading to symptoms like gas and bloating. An onion allergy is an immune system reaction to onion proteins, which is much rarer and can cause more severe symptoms, including hives or breathing difficulty.

Yes, different onion varieties have varying levels of fructans. Some, like shallots, have a higher concentration of fructans, while others, like Spanish onions, may have lower levels. Many individuals find green onion tips are a well-tolerated alternative.

Yes, it is possible for an onion or other food intolerance to develop later in life. This can be due to changes in gut health, diet, or other digestive conditions that affect your body's ability to process certain foods.

Yes. A great way to get the flavor without the FODMAPs is to use onion-infused oil. Fructans are not oil-soluble, so you can infuse oil with onion and then remove the solids before adding it to your cooking.

Keeping a food diary is an effective way to pinpoint whether raw onions are the cause of your discomfort. If you notice a pattern of symptoms like bloating, gas, or pain appearing within a few hours of eating raw onions, you may have an intolerance.

For flavor, you can use the green tips of spring onions or leeks, which contain fewer fructans. A pinch of asafoetida can also provide an onion-like taste without the digestive issues.