Carbohydrates make up about 9–10% of both raw and cooked onions.
They consist mostly of simple sugars, such as glucose, fructose, and sucrose, as well as fiber.
A 3.5-ounce (100-gram) portion contains 9.3 grams of carbs and 1.7 grams of fiber, so the total digestible carb content is 7.6 grams.
Onions are a decent source of fiber, which accounts for 0.9–2.6% of the fresh weight, depending on the type of onion.
They are very rich in healthy soluble fibers called fructans. In fact, onions are among the main dietary sources of fructans.
Fructans are so-called prebiotic fibers, which feed the beneficial bacteria in your gut.
This leads to the formation of short-chain fatty acids (SCFAs), such as butyrate, which may improve colon health, reduce inflammation, and cut your risk of colon cancer.
Vitamins and minerals
Onions contain decent amounts of several vitamins and minerals, including:
- Vitamin C. An antioxidant, this vitamin is needed for immune function and maintenance of skin and hair.
- Folate (B9). A water-soluble B vitamin, folate is essential for cell growth and metabolism and especially important for pregnant women.
- Vitamin B6. Found in most foods, this vitamin is involved in the formation of red blood cells.
- Potassium. This essential mineral can have blood pressure-lowering effects and is important for heart health.
Other plant compounds
The most abundant plant compounds in onions are:
- Anthocyanins. Only found in red or purple onions, anthocyanins are powerful antioxidants and pigments that give these onions their reddish color.
- Quercetin. An antioxidant flavonoid, quercetin may lower blood pressure and improve heart health.
- Sulfur compounds. These are mainly sulfides and polysulfides, which may protect against cancer.
- Thiosulfinates. These sulfur-containing compounds may inhibit the growth of harmful microorganisms and prevent the formation of blood clots.
Red and yellow onions are richer in antioxidants than other types. In fact, yellow onions may contain almost 11 times more antioxidants than white onions.
Cooking can significantly reduce levels of some antioxidants.
Health benefits of onions
Onions have been shown to have strong antioxidant and anti-inflammatory properties.
Blood sugar regulation
Type 2 diabetes is a common disease, characterized primarily by high blood sugar levels.
One study in people with type 2 diabetes found that eating 3.5 ounces (100 grams) of raw onions per day led to a significant reduction in blood sugar levels.
Osteoporosis is a common health problem, especially in postmenopausal women. A healthy diet is one of the main preventive measures.
A large observational study in women over 50 years of age found that regular onion consumption is linked to increased bone density.
Reduction of cancer risk
Observational studies have linked increased consumption of onions to a reduced risk of several types of cancers, such as those of the stomach, breast, colon, and prostate.
Eating onions can lead to bad breath and an unpleasant body odor.
Onion intolerance and allergy
Onion allergy is relatively rare, but intolerance to raw varieties is fairly common.
Symptoms of onion intolerance include digestive disruption, such as upset stomach, heartburn, and gas.
Some people may experience allergic reactions from touching onions, whether or not they’re allergic to eating them.
Onions contain FODMAPs, which are a category of carbs and fibers that many people cannot tolerate.
They may cause unpleasant digestive symptoms, such as bloating, gas, cramping, and diarrhea.
Eye and mouth irritation
The most common issue with preparing and cutting onions is eye irritation and tear production. When cut, an onion’s cells to release a gas called lachrymatory factor (LF).
The gas activates neurons in your eyes that cause a stinging sensation, followed by tears that are produced to flush out the irritant.
Dangerous for pets
While onions are a healthy component of human diets, they can be deadly for some animals, including dogs, cats, horses, and monkeys.