Contents
What it is
The onion is the bulb of a plant in the onion genus, a relative of garlic, leek and chive, native to Central Asia. Cutting one makes your eyes water because the ruptured cells let sulphur compounds meet an enzyme and convert into a volatile that irritates the eye; chilling the onion or using a sharp knife reduces it. Raw it is sharp and harsh, but heat breaks those sulphur compounds down and caramelises the sugars until it is remarkably sweet — the same ingredient tastes like its own opposite depending on how it is cooked.
Scientific name Allium cepa
Onions are how most people get quercetin. Quercetin is a flavonoid studied for anti-inflammatory and antioxidant activity, and for its effect on blood pressure.
What matters is where it sits. Quercetin concentrates in the outer layers and falls off towards the centre. Red onions contain more than white ones.
Key benefits
- Moderate evidence
Quercetin modestly lowers blood pressure
A meta-analysis of 7 randomised trials (587 patients) found quercetin supplementation lowered systolic pressure by 3.04 mmHg and diastolic by 2.63 mmHg. The effect only appeared at 500 mg a day or more; below that it was not significant. A single onion supplies quercetin in the tens of milligrams — these figures describe a supplement dose, not what you get from eating onions.[1]
- Not established
Blood sugar and lipids are not confirmed
Several small studies report lower fasting glucose and total cholesterol with onion intake. But a meta-analysis of randomised quercetin trials found that while blood pressure fell, lipid profiles and glucose did not change significantly. On current evidence, only the blood pressure claim holds.[2]
- Moderate evidence
Prebiotic fibre feeds beneficial gut bacteria
Onions contain inulin and fructo-oligosaccharides, which pass undigested to the colon where bifidobacteria and others feed on them.[3]
- Early research
Sulphur compounds are antimicrobial and anti-inflammatory
The compounds that sting your eyes belong to the same family as garlic's. Laboratory studies confirm antibacterial activity.[4]
How to eat it well
- Do not discard the two layers just under the papery skin. A large share of the quercetin is there.
- Choose red onions to get quercetin and anthocyanins together.
- Chop and wait ten minutes before cooking, as with garlic, to let more sulphur compounds form.
- Long caramelising sweetens them while retaining much of the quercetin; boiling in water leaches it out.
- To reduce eye sting, chill the onion for 30 minutes before cutting; fewer volatiles are released.
How to read the grades
Each benefit carries a label showing how far research on people supports it. Grades differ between items even within the same ingredient.
- Nutrition data
- A fact verifiable from food composition databases or health authorities. This is a different kind of statement from a clinical effect.
- Strong evidence
- Consistently confirmed across randomised controlled trials or meta-analyses.
- Moderate evidence
- Human studies exist, but effect sizes are small or results are mixed.
- Early research
- Cell or animal work, or small preliminary studies.
- Not established
- Commonly claimed, but not confirmed in studies on people.
What to watch out for
- Onions are a classic high-FODMAP food and readily cause bloating and gas in irritable bowel syndrome.
- With acid reflux, raw onion can worsen symptoms.
- If you take anticoagulants, discuss large intakes with your clinician.
- They are toxic to dogs and cats — never feed them to pets.
Nutrition
Approximate values per 100 g edible portion.
- Calories
- 40 kcal2%
- Carbohydrates
- 9.3 g3%
- Fiber
- 1.7 g6%
- Sugars
- 4.2 g
- Protein
- 1.1 g2%
- Vitamin C
- 7.4 mg8%
- Potassium
- 146 mg3%
- Quercetin
- 20–50 mg
Percentages are of the US FDA Daily Value for adults and children 4 and over. National reference intakes differ by sex and age.
Calculate this alongside other ingredients →
Key nutrients
By health concern
Dishes with this ingredient
- Tofu with stir-fried kimchi20 min
- Lentil and tomato soup35 min
- Chicken and chilli stir-fry20 min
- Tuna and cabbage salad10 min
- Herring braised in tomato30 min
- Barley and mushroom risotto45 min
- Chickpea and kale stew30 min
- Bell pepper and tofu stir-fry20 min
- Squid and bell pepper stir-fry20 min
- Potato and kale soup35 min
- Coconut chickpea curry30 min
- Turmeric and pepper fried rice40 min
- Boiled soybean and tomato salad25 min
- Lamb and vegetable skewers30 min
- Saffron chicken rice40 min
- Oyster mushrooms stir-fried with sesame oil15 min
Compare with
Sources
The databases and health authorities this article's nutrition figures and health statements draw on.
- 1Effects of Quercetin on Blood Pressure: A Systematic Review and Meta-Analysis of Randomized Controlled TrialsMeta-analysisSerban MC, et al. J Am Heart Assoc 2016PMID 27405810
- 2Effect of quercetin supplementation on plasma lipid profiles, blood pressure, and glucose levels: a systematic review and meta-analysisMeta-analysisHuang H, et al. Nutr Rev 2020PMID 31940027
- 3Effect of chicory-derived inulin-type fructans on abundance of Bifidobacterium and on bowel function: a systematic review with meta-analysesMeta-analysisLe Bastard Q, et al. Crit Rev Food Sci Nutr 2023PMID 35833477
- 4Beneficial Effects of Organosulfur Compounds from Allium cepa on Gut Health: A Systematic ReviewSystematic reviewsGuillamón E, et al. Foods 2021PMID 34441457
- 5USDA FoodData CentralFood composition database
- 6NIH Office of Dietary SupplementsHealth authority
- 7PubMedResearch literature
