Know2eat

Onions

Allium cepa

Also known as Brown onion, Yellow onion, Red onion, White onion

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The bulb of a cultivated Allium, used as a base flavour in most of the world cuisines. Cutting the bulb releases enzymes that generate both its pungency and the compound that irritates the eyes.

Heart HealthGut & Digestive HealthImmune Health

Potential health benefits

Best supported: Limited

Ordered by how well the human research supports them — strongest first. A rating describes the state of the evidence, not how impressive the food is. How we rate evidence.

Blood lipid profile

Heart Health
Limited evidence

Evidence is limited for an effect on blood lipids. Some small trials of onion or onion extract report modest changes in cholesterol, with results varying widely between studies.

Limited evidence. Early human work, or mostly lab and animal research.

3 sources

Deep research mode adds each paper’s own conclusion and stated limitations.

Gut microbiome

Gut & Digestive Health
Limited evidence

Research suggests onion fructans act as a substrate for bifidobacteria and other colonic species, in line with the wider prebiotic literature on inulin-type fibres. Trials using onion rather than isolated inulin are scarce.

Limited evidence. Early human work, or mostly lab and animal research.

3 sources

Deep research mode adds each paper’s own conclusion and stated limitations.

Healthy blood pressure

Heart Health
Limited evidence

Trials of quercetin supplements report small reductions in blood pressure, mostly at doses far above what onions supply. Trials feeding onions themselves are few and inconsistent.

Limited evidence. Early human work, or mostly lab and animal research.

3 sources

Deep research mode adds each paper’s own conclusion and stated limitations.

Immune function

Immune Health
Insufficient evidence

Onions feature in a great deal of folk advice about colds and infections, including the belief that a cut onion in the room absorbs illness. None of that has dependable human research behind it.

Insufficient evidence. Not enough human research to say anything useful yet.

No source is attached to this association yet, so it is rated insufficient by default. It is listed because people commonly ask about it — not because there is evidence for it.

Ask about Onions

Ask about Onions

Answers are written only from the entries on this page — the same nutrients, compounds, claims and citations you can read above. Nothing is added from outside the database, and every answer lists what it was built from.

AI explanations are not switched on for this site, so there is nothing to ask here. Everything above is read straight from the database and does not need them.

Key nutrients

Nutrient content per 100 grams
NutrientAmount% DV
Macronutrients
Calories40 kcal2%
Protein1.1 g2%
Total fat0.1 g0%
Carbohydrate9.34 g3%
Fibre1.7 g6%
Total sugars4.24 g—
Water89.1 g—
Fats
Saturated fat0.042 g0%
Monounsaturated fat0.013 g—
Polyunsaturated fat0.017 g—
Cholesterol0 mg0%
EPA (omega-3)0 g—
DHA (omega-3)0 g—
DPA (omega-3)0 g—
Linoleic acid (omega-6)0.013 g—
Vitamins
Vitamin A0 µg0%
Vitamin C7.4 mg8%
Vitamin D0 µg0%
Vitamin E0.02 mg0%
Vitamin K0.4 µg0%
Thiamin (B1)0.046 mg4%
Riboflavin (B2)0.027 mg2%
Niacin (B3)0.116 mg1%
Pantothenic acid (B5)0.123 mg2%
Vitamin B60.12 mg7%
Folate19 µg5%
Vitamin B120 µg0%
Choline6.1 mg1%
Minerals
Calcium23 mg2%
Iron0.21 mg1%
Magnesium10 mg2%
Phosphorus29 mg2%
Potassium146 mg3%
Sodium4 mg0%
Zinc0.17 mg2%
Copper0.039 mg4%
Manganese0.129 mg6%
Selenium0.5 µg1%
Other measured compounds
Beta-carotene1 µg—
Alpha-carotene0 µg—
Lycopene0 µg—
Lutein + zeaxanthin4 µg—
Caffeine0 mg—
Ash0.35 g—

Nutrient values are per 100 g of the USDA reference entry named on this page and vary with variety, growing conditions, storage and preparation.

Source record: Onions, raw (USDA FoodData Central, FDC ID 170000).

% DV uses the FDA Daily Values for adults and children aged 4 and over. A dash means no Daily Value has been established for that nutrient.

Bioactive compounds

Compounds present in meaningful amounts. Presence is banded rather than quantified, because whole-food levels vary enormously with variety, growing conditions, storage and preparation.

How preparation changes things

Only effects with real support are listed. If a preparation method is missing, it is because nothing dependable is documented — not because it makes no difference.

  • crushedIncreases sulphur compound formation

    Chopping or crushing ruptures cells and lets alliinase act on sulphur precursors, generating the pungent thiosulfinates. Whole cooked onion produces far less of them.

  • boiledReduces fructans

    Fructans are water-soluble and leach into cooking liquid, which is why onion-infused oil carries the flavour without the FODMAP load while boiled-and-drained onion carries less than raw.

  • cookedReduces thiosulfinates

    Heat degrades the sharp sulphur compounds, which is why long-cooked onion tastes sweet rather than pungent. Some quercetin is also lost to cooking water.

Things to know

Allergies, interactions and intake concerns. Every food gets this section — a reference where everything is uniformly good is not a reference.

  • Too much

    High FODMAP content

    Onion is one of the single most frequent triggers of symptoms in irritable bowel syndrome because of its fructan content, and even small amounts in stock, sauces or seasoning blends can provoke bloating and pain. It is one of the first foods removed on a low-FODMAP plan.

  • Worth knowing

    Reflux symptoms

    Raw onion in particular is a commonly reported heartburn trigger in gastro-oesophageal reflux disease.

  • Interaction

    Possible additive antiplatelet effect

    Allium sulphur compounds have measurable antiplatelet activity in laboratory work. The effect from culinary amounts is small, but it is worth mentioning alongside warfarin or antiplatelet medication if intake is unusually high.

  • Allergy

    Contact dermatitis and allium allergy

    Hand dermatitis from handling raw onion and garlic is well documented in cooks and food handlers, and true allium food allergy, though rare, exists.