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Onion benefits: the quercetin sits in the outer two layers

Onion's antioxidant quercetin concentrates in the outer layers. Peeling two extra layers out of habit throws much of it away.

Published Jun 2, 20261 min read

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

  1. Quercetin modestly lowers blood pressure

    Randomised trials giving quercetin-rich onion extract to adults with hypertension found significant reductions in systolic pressure. In people with normal pressure, little changed.

  2. It may help blood sugar and lipid markers

    Several studies report lower fasting glucose and total cholesterol with onion intake, though effect sizes are small and the findings are not fully consistent. Treat it as supportive, not decisive.

  3. 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.

  4. 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.

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.

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 kcal
Carbohydrates
9.3 g
Fiber
1.7 g
Sugars
4.2 g
Protein
1.1 g
Vitamin C
7.4 mg
Potassium
146 mg
Quercetin
20–50 mg

Key nutrients

  • Quercetin
  • Vitamin C
  • Fiber
  • Potassium
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