Onion: Nutrition, and the Real Reason It Makes You Cry
Key takeaways
- Onion has about 40 calories per 100g and is a genuinely good source of quercetin, a flavonoid antioxidant.
- Research has found quercetin from onions is absorbed about twice as well as the same compound from tea, and more than three times as well as from apples.
- Onions make you cry through a real, two-step enzyme reaction discovered by Japanese researchers in 2002 — not simply from a single irritant chemical already present in the onion.
- Chilling an onion in the fridge for 10-15 minutes before cutting genuinely slows this reaction down, since the enzymes involved work more slowly when cold.
- Onion's fibre is largely fructans, a soluble fibre type that can cause noticeable gas for some people, particularly if they're not used to eating onion regularly.
Source: USDA FoodData Central
Onion is the base of nearly everything cooked at home — the first thing into the pan, chopped through watering eyes most mornings without a second thought about why exactly it happens. Looking into the actual chemistry behind both the crying and the nutrition turned out to be more interesting than either fact alone.
What’s actually in an onion
A 100g onion has about 40 calories, a reasonable 7.4mg of vitamin C, and roughly 1.7g of fibre — mostly fructans, a soluble fibre type. The standout nutrient, though, is quercetin, a flavonoid antioxidant onion carries in genuinely generous amounts.
The absorption fact worth knowing
This is a specific, verified detail worth knowing: research has found that quercetin from onions is absorbed roughly twice as well as the same compound from tea, and more than three times as well as from apples. This means onion isn’t just “one of several” quercetin sources — it’s genuinely one of the more efficiently absorbed ones, which matters more than the raw quercetin content alone would suggest.
Why onions actually make you cry
This turns out to be a genuine two-step chemical reaction, only properly understood since a 2002 study by Japanese researchers published in Nature. Cutting an onion ruptures its cells, releasing an enzyme called alliinase, which reacts with a sulfur compound to produce an intermediate substance. A second enzyme, discovered specifically in that 2002 research (lachrymatory-factor synthase), then converts that intermediate into the actual irritant gas — syn-propanethial-S-oxide — which drifts upward and irritates the nerves in your eyes, triggering the reflex tears. It’s a genuinely more complex process than “onions just contain an irritant chemical,” involving two separate enzymatic steps rather than one.
The fix that actually works, and why
Chilling an onion in the fridge for 10 to 15 minutes before cutting genuinely slows down this whole reaction, because the enzymes involved — like most biological reactions — work more slowly at colder temperatures. This isn’t a folk remedy with no real basis; it’s a direct application of basic enzyme kinetics to a specific, well-understood chemical reaction.
Other things worth knowing
Onion and digestion. Onion’s fructan content, the same soluble fibre responsible for some of its health benefits, can cause noticeable gas or bloating for people not used to eating it regularly, or for anyone with a sensitivity to FODMAPs specifically — this generally settles with more consistent intake for most people.
Onion during pregnancy. Onion is a genuinely reasonable, nutrient-dense food during pregnancy in normal cooking amounts. Our guide to foods and drinks to limit or avoid during pregnancy covers broader dietary guidance if you have other specific concerns.
The biggest misconception about onions
Treating the crying reaction as a simple, single-chemical irritation misses a genuinely interesting piece of real chemistry — it took dedicated scientific research to work out that a second enzyme was involved at all, discovered only relatively recently in food-science terms. It’s a good reminder that even a completely ordinary, everyday kitchen experience can have real, actively-researched science behind it.
How I actually cook with onions at home
Onion, cooked down properly in oil before adding garlic and tomato, is the starting point for nearly every dal and curry I make — the same base used across most Indian households, cooked slowly enough to properly soften and sweeten before the rest of a dish builds on top of it.
Priyanka’s tip: If chilling the onion beforehand isn’t practical, cutting it under a kitchen extractor fan or near an open window genuinely helps disperse the irritant gas faster, reducing how much reaches your eyes even without changing the onion’s temperature.
Frequently asked questions
Is onion’s quercetin actually well absorbed by the body?
Yes, genuinely well — research has found it’s absorbed about twice as well as quercetin from tea, and more than three times as well as from apples, making onion one of the more efficient sources of it.
Why exactly do onions make you cry?
Cutting an onion triggers a real two-step enzyme reaction: one enzyme creates an intermediate compound, and a second enzyme (only properly identified in 2002) converts that into the actual irritant gas that reaches your eyes and triggers tears.
Does chilling an onion before cutting it actually reduce crying?
Yes — the enzymes responsible for producing the irritant gas work more slowly at colder temperatures, so chilling an onion for 10-15 minutes before cutting genuinely slows the whole reaction down.
Why do onions sometimes cause gas or bloating?
Onion’s fibre is largely fructans, a soluble fibre type that can cause noticeable gas for people not used to eating it regularly, or for those with a specific sensitivity to that fibre category.
Is onion safe to eat regularly during pregnancy?
Yes — normal cooking amounts of onion are a genuinely reasonable, nutrient-dense part of a pregnancy diet.
References
- USDA FoodData Central — Onions, raw.
- Imai, S., et al. Plant biochemistry: an onion enzyme that makes the eyes water. Nature, 2002.
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