Calories in Onion: Nutritional facts (Carbs, Fiber, Protein, Fat) for Onion Types

According to My Food Data, a 100-gram onion has 40 calories.

One kilogram of onion has 400 calories.

A medium red onion contains approximately 41 calories, while a medium green onion has around 4.8 calories.

The health benefits of onion are listed below:

• Supports Heart Health: Onions contain antioxidants and compounds that may help reduce the risk of heart disease by fighting inflammation and lowering triglyceride and cholesterol levels. Quercetin, a flavonoid antioxidant in onions, may aid in reducing high blood pressure. A 2015 study in 70 people with overweight and hypertension found that 162 mg of quercetin-rich onion extract daily lowered systolic blood pressure by 3.6 mmHg. 
• Contains Plenty of Antioxidants: Onions contain at least 17 types of flavonoids, which help protect cells from oxidative damage linked to conditions like cancer, diabetes, and heart disease. Red onions are rich in anthocyanins, which are pigments that may lower the risk of diabetes and certain cancers. A 2016 study involving 43,880 males found that consuming up to 613 mg of anthocyanins was associated with a 14% reduction in nonfatal heart attack risk. 
• Has Anticancer Properties: Onions and other allium vegetables may help lower the risk of stomach and colorectal cancers. A 2015 review titled “Allium vegetable intake and gastric cancer: a case-control study and meta-analysis” of 26 studies found that those with the highest allium vegetable intake had a 22% lower risk of stomach cancer. A 2014 review involving 13,333 people suggested that high onion consumption was linked to a 15% reduced risk of colorectal cancer. 
• Supports Blood Sugar Control: Onions may help manage blood sugar levels, which is crucial for individuals with diabetes or prediabetes. In a  2010 study of 84 people with type 1 or type 2 diabetes found that consuming 100g of raw red onion significantly lowered fasting blood sugar levels after four hours. 
• Enhances Bone Density: A 2016 study found that consuming 100ml of onion juice daily for eight weeks improved bone mineral density. Another 2009 study showed that daily onion consumption increased bone density by 5% in 507 perimenopausal and postmenopausal females. 
• Possesses Antibacterial Properties: Onions may help fight harmful bacteria, including E. coli, S. aureus, and Bacillus cereus. A 2010 study found that onion extract inhibited the growth of Vibrio cholerae. Quercetin in onions may also reduce bacteria growth, with research suggesting onions can inhibit strains like H. pylori, which is linked to stomach ulcers and digestive cancers. 

Onions are very healthy but come with some drawbacks, as shown in the list below:

• May Worsen IBS Symptoms: IBS affects 10–15% of the U.S. population, causing bloating, abdominal pain, cramping, and irregular bowel movements. According to a 2015 study, a low FODMAP diet is often recommended as FODMAPs are poorly absorbed short-chain carbohydrates that can trigger symptoms. Onions are high in FODMAPs, and research indicates that avoiding foods like onions, garlic, and wheat-based products may help manage IBS symptoms. 
• Allergies: Some individuals may be allergic to onions based on a 2020 study titled “New Perspectives in Food Allergy”. People with onion allergies experience symptoms like asthma, nasal congestion, itchy eyes, or contact dermatitis. Onions share allergenic properties with other Liliaceae plants, potentially causing cross-reactions.
• May irritate the Eyes: Chopping onions releases a lachrymatory factor (LF) according to a 2008 study. This sulfur compound evaporates quickly and irritates the eyes. Chilling onions, cutting them under water, or wearing goggles may help reduce irritation.

Yes, onions are beneficial for health. One of the benefits of eating onions is that it helps  regulate blood sugar. Onions also support bone health by protecting against bone loss and increasing bone density, based on findings from a large observational study in women over 50 years of age. Studies also suggest that regular consumption has been linked to a reduced risk of certain cancers including stomach, breast, colon, and prostate cancer.

Onions are good for cooking, health, and household uses. Onions add flavor to any dish, preserve food, and tenderize meat. This vegetable is rich in antioxidants, vitamins, and sulfur compounds, which support heart health, regulate blood sugar, strengthen bones, reduce cancer risk, and boost immunity. Onions also repel insects, soothe bites and burns, remove stains, and work as a natural dye.

Medical professionals recommend consuming 1 medium sized onion per day as part of a meal. One study in people who have been diagnosed with T2 diabetes found that eating 100 grams or 3.5 ounces of onion per day led to a huge decrease in blood sugar levels. A 2014 study involving 54 females with polycystic ovary syndrome (PCOS) found that eating 80–120g of red onions (raw) per day for 8 weeks reduced their total cholesterol, particularly LDL cholesterol. One medium sized onion is equivalent to 2 servings of vegetables.

There is no set limit to the amount of onions one can consume in a day but excessive consumption may cause bloating, stomach cramps, allergic reactions, or bad breath, especially in those with IBS, allergies, or sensitivities.

Calorie content varies by onion type due to differences in water content. Red onions have the highest calories at 44 kcal per 100g. This is followed by yellow onions at 38 kcal, and white onions at 36 kcal. Onions with more water tend to have fewer calories. White onions have the most water at 91.2g, while red onions have the least at 88.6g. Fat and protein content differ slightly but have little impact on calorie variation.

Onions likely originated in Central Asia, with early cultivation in Iran and West Pakistan. Onions have been cultivated for over 5,000 years and have remained a staple in many food preparations because of their long shelf life, adaptability, and nutritional benefits. Onions have long been used in food, medicine, art, and mummification during ancient times.

China consumed the most onions in 2013, with an annual total of 19,163 kilotons. India followed, consuming 14,000 kilotons per year. The United States ranked third, with 3,000 kilotons consumed annually. These countries have high onion consumption due to their large populations and the prominence of onions in their cuisines.