Amaryllidaceae
Allium cepa
Onion
- Part used
- bulb
- Native range
- A cultigen of Central Asian origin; not known in the wild
- Morphology
- A biennial grown as an annual, with hollow tubular leaves and a swollen bulb of fleshy concentric leaf bases in a papery skin.
What is in it
Evidence
Small studies on lipids and blood glucose with inconsistent results. Topical onion extract gel for scars has modest and mixed trial evidence.
Safety
A food. High fructan content makes it a significant trigger in irritable bowel syndrome. Anticoagulant activity at high intake. Notably toxic to dogs and cats, which is a genuine veterinary emergency.
Interactions: Additive with anticoagulants at high intake; May lower blood glucose
Regulatory: Permitted as a food.
How the traditions classify it
Three systems, rating the same plant. Where they disagree, none of them is correcting the others — they are separate frameworks that arrived at their categories independently.
Historical use
Cultivated for at least five thousand years, fed to the builders of the pyramids, and used medicinally across every tradition it reached — for the chest, for wounds, and as a poultice.
Recorded as history and attributed where possible. Long use establishes that a plant was widely used, not that it worked, and not that it was safe.
Worth knowing
The onion is one of very few plants that manufactures its irritant on demand, in a two-step reaction, at the moment it is damaged.
Cutting ruptures cells and releases an enzyme, alliinase, which converts a stored precursor into an unstable intermediate; a second enzyme rearranges that into syn-propanethial-S-oxide, a volatile lachrymator that reaches your eyes and produces sulphuric acid on contact with the tear film. The whole sequence takes seconds and does not exist in the intact bulb.
That is the same defensive architecture as garlic, and it is the reason Galen graded both hot in the fourth degree. A humoral framework with no chemistry detected, correctly, that these plants produce something burning only once you injure them.
Nomenclature verified against GBIF backbone taxonomy.