📊 Full nutrition facts — per 100g
| Nutrient | Amount | % Daily value | Level |
|---|---|---|---|
| Calories | 40 kcal | 2% | |
| Carbohydrates | 9.3g | 3% | |
| Dietary fibre | 1.7g | 6% | |
| Fructooligosaccharides | ~2.8g | — | |
| GI | ~10 — Near zero | — | |
| Quercetin | 23–60mg | — | |
| Vitamin C | 7.4mg | 8% | |
| Folate | 19µg | 5% | |
| Potassium | 146mg | 3% | |
| Vitamin B6 | 0.12mg | 7% | |
| Chromium | ~0.02mg | — | |
| Allicin precursors | present | — |
Based on Australian NRV. Source: FSANZ Australian Food Composition Database.
📈 Glycaemic index (GI)
💊 Key vitamins & minerals
✅ Health benefits
Onions are the richest common dietary source of quercetin — a flavonol with documented anti-inflammatory, antioxidant, antihistamine and anti-cancer properties. Quercetin inhibits pro-inflammatory enzymes (COX-1, COX-2 — the same targets as ibuprofen), inhibits histamine release from mast cells (relevant for allergies), and has demonstrated anti-cancer activity in multiple cell and animal studies. Red onions have significantly more quercetin than white onions (located primarily in the outer layers). The quercetin is concentrated in the outermost layers — removing too many outer layers discards the most nutritious part.
Onions are one of the richest food sources of fructooligosaccharides (FOS) — indigestible carbohydrates that pass intact to the colon where they selectively feed Bifidobacterium and Lactobacillus species (beneficial gut bacteria). FOS from onions have been shown in clinical trials to significantly increase Bifidobacterium populations within 2 weeks of regular consumption. Robust populations of beneficial bacteria are associated with improved immune function, reduced colorectal cancer risk, better mood (via the gut-brain axis) and reduced inflammation. Onions are one of the most practical and affordable prebiotic foods available.
Multiple mechanisms support onion consumption for cardiovascular health: quercetin reduces LDL oxidation, inhibits platelet aggregation and relaxes arterial smooth muscle; allicin precursors (isoalliin, methiin) are converted to allicin-like compounds on cooking that reduce cholesterol synthesis; chromium improves insulin sensitivity and reduces cardiovascular risk markers. A meta-analysis of 12 randomised trials found allium vegetable consumption significantly reduced total cholesterol, LDL and triglycerides.
Onions contain chromium — a trace mineral that potentiates insulin's action by activating the insulin receptor. Quercetin additionally inhibits alpha-glucosidase (the digestive enzyme that breaks down starch to glucose) and has demonstrated blood glucose-lowering effects in type 2 diabetes patients in multiple trials. The combination makes onions particularly valuable as a dietary staple for people managing blood sugar — the GI of 10 reflects this minimal glycaemic impact, and the active compounds further moderate glucose metabolism beyond their simple GI contribution.
⚠️ Who should limit or avoid
Onions, along with all members of the Allium family (garlic, shallots, leeks, chives), are toxic to dogs and cats. Allium compounds (N-propyl disulfide) cause oxidative damage to red blood cells, leading to haemolytic anaemia. Both raw and cooked onions are toxic, and the effects are cumulative. Even onion powder in small amounts can cause serious harm. Never feed dogs or cats any food containing onions or onion derivatives. This is one of the most important pet food safety warnings.
Onions are one of the highest-FODMAP foods and a common trigger for IBS symptoms. The fructooligosaccharides (FOS) that make onions excellent prebiotics for most people are fermented by gut bacteria to produce gas in people with IBS, causing bloating, cramping and altered bowel habits. Cooking reduces but does not eliminate FODMAP content. People following a Low-FODMAP diet during IBS management should avoid onions entirely during the elimination phase and re-test carefully.
Onions are a recognised trigger for gastro-oesophageal reflux (GORD) and heartburn. They relax the lower oesophageal sphincter and their volatile compounds can irritate the oesophagus. Raw onions are generally more problematic than cooked. People with frequent reflux symptoms should observe their individual response to both raw and cooked onions and adjust intake accordingly.
🛒 How to select & buy onion
A good onion feels firm and solid throughout when squeezed — no soft spots. The papery outer skin should be dry, crackly and tight. Any wet patches, soft areas, dark mould spots or sprouting green shoots indicate age or improper storage. The neck (top) should be tight and dry — a thick or wet neck means the onion is past prime. Lighter outer skin generally means milder flavour; darker outer skin correlates with stronger flavour and higher quercetin content.
Red onions: highest quercetin, sweetest flavour, best raw (salads, salsas, pickling) or roasted. Brown onions: most common Australian variety — all-purpose, strong flavour when raw, sweet when caramelised — best for cooking. White onions: mild, crisp, best for Mexican and Asian cuisine, good raw in salsas. Shallots: most delicate flavour, best for dressings, sauces and quick sautés. For maximum health benefit, choose red onions and use the outer layers.
The compounds that cause eye irritation (syn-propanethial-S-oxide) are released when the cell walls are cut and the enzyme alliinase contacts the sulphur compounds. Cold slows alliinase activity: refrigerate onions for 30 minutes before cutting. Cut at the root end last (the enzyme concentration is highest there). Use a very sharp knife (less cell damage). Cut under running water or near a lit gas flame. None of these methods are perfect — the sharp knife approach is the most consistently effective.
🧊 Storage tips & shelf life
Whole unpeeled onions store best at room temperature in a cool, dry, well-ventilated spot — a basket, mesh bag or open bowl. Never store in a sealed plastic bag at room temperature — trapped moisture causes rapid mould. Keep away from potatoes (they emit ethylene that accelerates onion sprouting). The pantry is better than the fridge for whole onions.
Do not refrigerate whole unpeeled onions unless your pantry is warm — cold and humidity cause sprouting and softening. Cut onions: wrap tightly or seal in a container (cut onions will permeate other foods with their odour). Keep away from eggs, dairy and delicate foods in the fridge. Use within 7–10 days.
Onions freeze very well for cooking — texture changes on thawing (softer) but flavour is excellent. Dice, spread on a tray, freeze until solid, then bag in portions. No blanching needed. Frozen diced onion goes directly into soups, stews, curries and sauces from frozen — a huge time-saver. Quercetin and FOS are well preserved in freezing.
📖 About onion — complete guide
The onion (Allium cepa) is among the oldest cultivated plants in human history — archaeological evidence places onion cultivation in ancient Egypt at 3,500 BCE, and they appear in ancient Sumerian texts, Egyptian tomb paintings (workers building the pyramids were reportedly paid partly in onions, radishes and garlic), ancient Indian Vedic literature and the Hebrew Bible. The Allium genus is one of the most ancient food plant lineages, with evidence suggesting human consumption predating organised agriculture. In ancient Egypt, the onion's concentric rings were a symbol of eternity, and onions were placed in the body cavities of mummies and carved into tombs. The spread of onion cultivation is so ancient and so universal that it is genuinely difficult to identify a culinary tradition that does not use onions as a foundational ingredient.
The quercetin story in onions has practical implications that most people overlook: the vast majority of quercetin is in the outer 1–2 layers of the onion. Red onion outer layers contain 75–80mg of quercetin per 100g, while the inner rings contain only 0–5mg. This means that removing too many outer layers when peeling wastes the most nutritious part of the vegetable. For cooking, the outer layers are edible and fully safe — only the dry papery skin itself is discarded. Similarly, research shows that boiling onions leaches quercetin into the cooking water — if boiling onions in soup, the cooking liquid retains much of the quercetin and should be consumed as part of the dish rather than discarded.