Medically reviewed on September 17, 2018
Scientific Name(s): Allium cepa L. Family: Liliaceae (lilies).
Common Name(s): Onion , Bulbus Allii Cepae , common onion , garden onion . Topical commercial preparations include Contractubex and Mederma .
Onion has potential in treating cardiovascular disease, hyperglycemia, and stomach cancer, although few quality clinical trials are available to support these uses. Topical preparations have been evaluated for the prevention of surgical scarring with varying results.
Clinical trials are lacking to provide dosage recommendations. Average daily doses of 50 g of fresh onion, 50 g of fresh onion juice, or 20 g of dried onion have been suggested. Topical onion extract gels have been used in studies evaluating the effect on scarring and are generally applied 3 times daily.
Contraindications have not been identified.
Generally recognized as safe when used as food. Avoid dosages above those found in foods because safety is unproven.
None well documented.
Information is limited.
The onion plant is a perennial herb growing to about 1.2 m in height, with 4 to 6 hollow, cylindrical leaves. On top of the long stalks, greenish-white flowers are present in the form of solitary umbels growing to 2.5 cm wide. The seeds of the plant are black and angular. The underground bulb, which is used medicinally, is comprised of fleshy leaf sheaths forming a thin-skinned capsule, and varies greatly in size (2 to 20 cm). Shape (flattened, spherical, or pear-shaped) and color depend on the variety. 1 , 2 , 3
The onion is one of the world's leading vegetable crops, believed to have been domesticated in central and western Asia. Onions were used as early as 5,000 years ago in Egypt, as depicted on ancient monuments; ancient Greek and Roman records also make references to the onion. Onions were consumed throughout Europe during the Middle Ages and were later thought to guard against evil spirits and the plague, probably because of their strong odor. Folk healers traditionally used onions to prevent infections, and an onion and garlic concoction cooked in milk was used as a European folk remedy for congestion. Onion skin dye has also been used for egg and cloth coloring in the Middle East and Europe. Christopher Columbus is said to have introduced the plant to North America on his 1492 expedition. Onions are routinely used in homeopathic medicine. 2 , 3 , 4
Onions contain 89% water, 1.5% protein, and vitamins B 1 , B 2 , and C, along with potassium and selenium. Polysaccharides such as fructosans, saccharose, and others are also present, as are peptides, flavonoids (mostly quercetin), and essential oil. Methods for the qualitative assessment of the flavonoids have been detailed, and quercetin glycosides have been shown to be heat-stable and transferable to cooking water.
Onion contains numerous sulfur compounds, including thiosulfinates and thiosulfonates; cepaenes; S-oxides; S,S-dioxides; mono-, di-, and tri-sulfides; and sulfoxides. Mincing or crushing the bulb releases cysteine sulfoxide from cellular compartments, making contact with the enzyme alliinase from the adjacent vacuoles. Hydrolysis results with the release of reactive intermediate sulfenic acid compounds and then to the various sulfur compounds. 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9
Uses and PharmacologyCancer
Large epidemiological studies infer a protective effect of onion (as well as garlic and leek) consumption against stomach and colorectal cancers. 5 , 10 , 11 Case-control (retrospective) data also suggest an inverse relationship between onion consumption and benign prostatic hypertrophy and endometrial cancer. 12 , 13 An overestimation of effect for these 2 cancers is possible due to publication and recall bias, and insufficient evidence exists for breast, lung, and other cancers. 5 , 10 , 11 , 14 In the case of GI-related cancers, proposed mechanisms of action for the Allium species include an inhibition of Helicobacter pylori and other bacterial activity, as well as a general decrease in the endogenous production of carcinogenic N-nitroso compounds. 10 , 11 In vitro and animal model studies describe the inhibition of mutagenesis, modulation of enzyme and cell signaling pathways, free-radical scavenging, apoptosis, and other effects on cell proliferation and tumor growth. 5 , 10 , 15 , 16 , 17 , 18 , 19 Prospective clinical trials are lacking.Cardiovascular disease
Certain onion genotypes containing higher contents of sulfur in the bulb correlate with greater antiplatelet activity. Thiosulfinates dimethyl- and diphenyl-thiosulfinate, for example, are known to slow thrombocyte biosynthesis. The least polar fraction of onion extract was associated with the most inhibitory activity toward platelet aggregation; thus, a greater inhibition of thromboxane synthesis was reported. 11 , 20 , 21Animal data
Hypolipidemic effects of onion sulfur-compounds, including S-methyl cysteine sulfoxide and allylpropyl disulfide, have been demonstrated in rats and rabbits, and include effect on diet-induced atherosclerosis, hypolipidemic action, and inhibitory effects on platelet formation. Raw, but not cooked, onion demonstrated antithrombotic effects in rats. 22 , 23 , 24 , 25 , 26Clinical data
Quercetin, onion extract, and onion soup have been evaluated in vitro and in vivo for effect on platelet aggregation in limited studies with mixed results. 2 , 11 , 27 , 28 Quercetin, but not raw onion, decreased blood pressure in hypertensive subjects in one study, 29 and mixed results have been found for effects on hyperlipidemia. 5 , 30 , 31 Trials have generally been conducted with small populations, and evidence is lacking to support claims of cardiovascular benefits.Dermatology
Limited clinical trials have been conducted on the effects of onion extract gels on scarring, with equivocal results. Inhibition of human skin fibroblasts and keloidal fibroblasts has been demonstrated in vitro. 2 In certain populations, a reduction in the effects of surgical procedures, such as redness, itching, burning, and pain, has been demonstrated, 32 , 33 , 34 , 35 while in others there was no difference over standard petroleum-based emollient therapy. 33 , 36Diabetes
Hypoglycemic effects of onion extracts have been demonstrated in animal models. 2 , 37 , 38 , 39 Quality clinical trials are lacking; however, limited trials have shown a decrease in blood glucose levels in healthy volunteers and non-insulin–dependent diabetic subjects with aqueous onion extract and raw dietary onion. 2 , 11Other uses
In vitro studies have shown onion to possess antibacterial (including H. pylori ), antiparasitic, and antifungal activity. 1 , 5 , 40 , 41 Clinical applications for this activity have not been determined, and use as a food preservative is limited by the strong odor and instability of sulfur compounds. 11
Clinical trials are lacking to provide dosage recommendations. An average daily dose of 50 g of fresh onion, 50 g of fresh onion juice, or 20 g of dried onion have been suggested. 2 , 3 Topical onion extract gels have been used in studies evaluating the effect on scarring, and are generally applied 3 times daily. 33 , 36
Generally recognized as safe when used as food. Avoid dosages above those found in foods because safety and efficacy are unproven.
None well documented. No potentiation of antiplatelet effects has been reported.
Ingestion of onion and onion extract appears to be relatively safe. 2 , 3 Onion seeds have been reported as occupational allergens, but frequent contact with onion rarely causes allergic reaction. 42 Certain sulfur compounds, such as propanethial-S-oxide, escape from onion in vapor form and hydrolyze to sulfuric acid when cut, causing eye irritation and lacrimation. 43 Using a sharp knife minimizes the crushing of onion tissue and liberation of volatiles, and cutting an onion under running water reduces lacrimation.
Mutagenicity of Bulbus Allii Cepae was not demonstrated in vitro. 2 Low doses of onion (50 mg/kg) given to rats had little effect on lung and liver tissues, but high doses (500 mg/kg) resulted in histological changes in these organs. Intraperitoneal administration was more damaging than oral administration, resulting in 25% mortality in rats. 44 Poisoning in cattle fed large quantities of onion has been reported. 45 Food colorant extracted from the tan onion bulb covering had no acute or subacute toxic effects in mice. 46
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