When You Eat Too Much Onion, This Is What Happens To You

Do you enjoy adding onions to most meals? That's a good habit to have, but you should still watch your portions. Eating too much onion can wreak havoc on your gut, among other side effects. Just like with everything else, moderation is key.

This veggie is a nutritional powerhouse, offering large doses of antioxidants. Fresh yellow onions provide about 270 to 1,187 milligrams of flavonols per kilogram, reports a 2007 review featured in the Journal of Agricultural and Food Chemistry. The same amount of red onions delivers 415 to 1,917 milligrams of flavonols, a type of flavonoid. These nutrients may reduce inflammatory markers, protect against cancer, and support immune function, according to a 2021 review published in Pharmaceutical Biology. Moreover, onion consumption may lower your risk of vascular and heart disease, improve eye health, and keep your brain sharp, note the study authors. 

Unfortunately, too much of anything can be harmful — and the humble onion is no exception. This vegetable may give you bad breath and body odor, affect digestion, and trigger allergies.  

Some people experience bloating and gas after eating onions — and for good reason. These veggies are rich in fructans, a group of carbohydrates that can ferment in the digestive system and are not well absorbed in the small intestine (via Eat This, Not That!). As a result, they may cause stomach pain, cramps, and other gastrointestinal issues, especially in those with irritable bowel syndrome. Both raw and cooked onions contain fructans, but you can simmer them in oil to minimize the side effects. 

This vegetable may also trigger heartburn, suggests a 2000 review published in the journal Nutrition and Cancer. The same goes for tomatoes, chocolate, garlic, and citrus fruits. These foods cause the lower esophageal sphincter to relax, allowing gastric acid to flow back up into the esophagus. Cooked onions are less problematic, but you should still consume them in moderation. Eat a small amount, see how your body reacts, and adjust your portions accordingly.

Apart from that, onions are generally safe for most people. It's possible to develop bad breath when you eat too much onion, but this side effect should subside within hours, according to Healthline. There is also a risk of developing allergic reactions, says Medical News Today. In this case, you may experience nausea, diarrhea, hives, difficulty breathing, itching, and other symptoms within two hours of eating onions — regardless of the amount consumed.