Onions: Nutrients, What Research Shows and Safety

Onions are the bulbs of Allium cepa, a plant in the onion family (Amaryllidaceae, Allium group). People eat the bulb raw, cooked or pickled, mostly for flavor. Some also eat it for health because it contains sulfur compounds and a plant pigment called quercetin. This page keeps the food apart from supplements and from hype, and says plainly where the evidence is thin. For the whole onion family, see our overview of onions, garlic and allium vegetables.

Read this first

• Gut comfort: onions are the highest-fructan food in a 2024 measurement of 40 common foods. Fructans are a type of FODMAP (short carbohydrates that ferment quickly) and can worsen symptoms in some people with irritable bowel syndrome (IBS). [1][2]
• Allergy is uncommon but real: one report describes anaphylaxis after cooked onion. For trouble breathing or swelling of the face or throat, call 911. [3]
• Pets: the ASPCA lists onion as toxic to dogs, cats and horses. Do not feed it to them. [4]
• Evidence in one line: onions are a low-calorie flavoring vegetable; quercetin supplements lowered blood pressure a little in trials, but we found no trial showing that eating onions treats or prevents a disease.

What is in it (USDA values)

Source: USDA FoodData Central, onions, raw (SR Legacy, item 170000). [5] USDA household portions include 1 cup chopped (160 g), 1 medium onion (110 g) and 1 large onion (150 g). Per-serving figures are our own arithmetic from the per-100 g values. % Daily Value uses the FDA 2016 Daily Values; values vary with variety, ripeness and storage.

Reviews describe onions as containing organosulfur compounds and flavonoids, and lab chemistry confirms quercetin and related flavonols in minced onion. [6][7] Those are mostly laboratory and mechanism findings; the 2025 review itself lists low bioavailability (how little reaches the body) and limited clinical validation as problems. [6] USDA lists no meaningful vitamin A and only trace carotenoids in raw onion. [5]

What research says

Eaten as food vs taken as a supplement or extract. The blood pressure and weight results come from quercetin capsules or onion peel extracts. A result for a capsule does not carry over to the onion on your plate. The food studies (cancer, mood) are observational, which means they find patterns but cannot prove cause: people who eat more onions may also eat and live differently in other ways. [12][14] We found no trial of eating whole onions that gave a clear health result.

How much is a normal serving

• The WHO healthy-diet fact sheet (26 January 2026) says people older than 10 should eat at least 400 g of fruits and vegetables a day. [15]
• The NHS "5 A DAY" guide (last reviewed 9 June 2026) counts 80 g of fresh, canned or frozen vegetables as one portion. [16]
• USDA's household portions for onions are listed above (for example 1 medium onion, 110 g). [5]
• These are official figures for information, not a prescription for you.
• Cooking changes allium compounds in ways that depend on the method, time and temperature, and the authors say more research is needed for each vegetable. [17] We found no source on how cooking changes the vitamins in onions, so we make no claim.

Safety, allergy and medicines

• FODMAPs and IBS. A 2024 study measured the fructan content of 40 common foods and found it highest in onion (1.97 g per 100 g fresh weight). [1] Fructans ferment quickly and can worsen symptoms in people with IBS. [1] A 2024 umbrella review of trials found a low-FODMAP diet improved IBS symptoms overall, though it noted possible effects on gut bacteria. [2] The NHS says a dietitian may recommend this diet if general changes have not helped. [18] Whether a given person reacts, and to how much onion, is individual: ask a dietitian instead of cutting foods out alone.
• Food allergy. Handling onion can cause skin rash, runny nose or asthma, but reactions to eating it are reported only rarely. A 2019 report describes a 35-year-old man with anaphylaxis after cooked onion and suggests some allergen parts survive heat. [3] See food allergy basics and emergency signs. Call 911 for trouble breathing or swelling of the face or throat. Because this is one case report, we cannot say how common it is.
• Pets. The ASPCA lists onion as toxic to dogs, cats and horses; signs include vomiting, red blood cell breakdown, blood in urine, weakness and a fast heart rate. [4] A laboratory study fed cooked onions to dogs and found hemolytic anemia (red blood cells breaking down). [19] The ASPCA page does not say whether raw, cooked or powdered onion is worse. If a pet ate onion, call a veterinarian or ASPCA Poison Control at (888) 426-4435 (a fee may apply). [4]
• Blood sugar and diabetes. USDA lists 9.34 g of carbohydrate and 4.24 g of sugars per 100 g of raw onion. [5] We found no onion-specific human trial on blood sugar that we could read, so we make no claim that onions lower it. People taking diabetes medicines should keep to their care team's advice; see foods for blood sugar control.
• Medicines and supplements. Quercetin or onion-peel capsules are different products from the food. We read no source on interactions with onions as food, so we make no claim either way; a pharmacist can check any supplement.
• Kidney disease and potassium. USDA lists 146 mg of potassium per 100 g (3% of the Daily Value). [5] People on a restricted-potassium plan should follow their own team's list; see foods, kidney disease, potassium and oxalates.
• Pregnancy. The sources we read did not single out onions eaten in meals; ask your prenatal care provider about your whole diet.
• Pesticide residues. We did not read an onion-specific source, so we say nothing about residue levels.

Choosing, storing and food safety

• Washing. The FDA says to rinse produce under running water, not with soap, detergent or produce washes, and to dry it with a clean towel. Washing reduces germs but does not remove them. [20]
• Cut onion. Refrigerate cut or packaged produce, and keep perishable produce at 40°F or below. [20]
• Cutting boards. After handling raw meat, wash boards and knives before cutting vegetables. [20]

Related pages

• Onions, garlic and allium vegetables as food
• Garlic
• Leeks
• Prebiotic foods: inulin, onions, garlic and more
• Food allergy basics and emergency signs
• Foods for blood sugar control