Scientific Name(s): Allium cepa L.
Common Name(s): Bulbus Allii Cepae, Common onion, Contractubex, Garden onion, Mederma, Onion
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 in the form of solitary umbels grow to 2.5 cm wide. The seeds of the plant are black and angular. The underground bulb, which is used medicinally and consumed as food, is comprised of fleshy leaf sheaths forming a thin-skinned capsule and varies greatly in size (2 to 20 cm). The shape (flattened, spherical, or pear shaped) and color of the bulb depends on the variety.Blumenthal 1998, Suleria 2015, USDA 2018, WHO 1999
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, and are 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.Blumenthal 1998, Lanzotti 2006, WHO 1999
Onions contain 89% water, 1.5% protein, and vitamins B1, B2, 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 trisulfides; and sulfoxides. When the onion bulb is minced or crushed, cysteine sulfoxides are released from cellular compartments and make contact with the enzyme alliinase from the adjacent vacuoles. Hydrolysis results in the release of reactive intermediate sulfenic acid compounds and then the various sulfur compounds.Arnault 2006, Blumenthal 1998, Graefe 2001, Lanzotti 2006, Nemeth 2007, Rose 2005, WHO 1999, Wiczkowski 2008
Uses and Pharmacology
Animal and in vitro data
In vitro data
In vitro studies have shown onion possesses antibacterial (including against Helicobacter pylori), antiparasitic, and antifungal activity.Elnima 1983, Rose 2005, USDA 2018, Zohri 1995 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.Griffiths 2002
Animal and in vitro data
In vitro and animal model studies describe inhibition of mutagenesis, modulation of enzyme and cell signaling pathways, free-radical scavenging, apoptosis, and other effects on cell proliferation and tumor growth.Bianchini 2001, Boyle 2000, Fukushima 1997, Herman-Antosiewicz 2004, Izawa 2008, Munday 2004, Nohara 2017, Rose 2005
Large epidemiological studies infer a protective effect of onion (as well as garlic and leek) consumption against stomach and colorectal cancers.Bianchini 2001, Griffiths 2002, Rose 2005 Case-control (retrospective) data also suggest an inverse relationship between onion consumption and benign prostatic hypertrophy and endometrial cancer.Galeone 2009, Galeone 2007, Suleria 2015 An overestimation of effect for stomach and colorectal cancers is possible due to publication and recall bias, and insufficient evidence exists for breast, lung, and other cancers.Bianchini 2001, Challier 1998, Griffiths 2002, Rose 2005 In the case of GI-related cancers, proposed mechanisms of action for the Allium species include inhibition of H. pylori and other bacterial activity, as well as general decreases in endogenous production of carcinogenic N-nitroso compounds.Bianchini 2001, Griffiths 2002
Prospective clinical trials are lacking.Nicastro 2015
In a study of women with breast cancer undergoing doxorubicin-based chemotherapy, reductions in tumor markers and hepatic enzymes were demonstrated with 100 to 160 g/day of fresh yellow onion for 8 weeks versus a low-onion (30 to 40 g/day) comparator group.Jafarpour 2015
Quercetin, onion extract, and onion soup have been evaluated in vitro and in vivo for effects on platelet aggregation in limited studies with mixed results.Griffiths 2002, Hubbard 2006, Hubbard 2004, WHO 1999
Animal data and in vitro data
Hypolipidemic effects of onion sulfur compounds, including S-methyl cysteine sulfoxide and allyl propyl disulfide, have been demonstrated in rats and rabbits, and include effects on diet-induced atherosclerosis, hypolipidemic action, and inhibitory effects on platelet formation. Raw, but not cooked, onion demonstrated antithrombotic effects in rats.Bordia 1996, Chen 2000, Kumari 1995, Lata 1991, Tang 2017, Wilcox 1984
Studies evaluating certain onion genotypes show that those containing higher sulfur content 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 against platelet aggregation; thus, a greater inhibition of thromboxane synthesis was reported.Goldman 1996, Griffiths 2002, Miller 1998
Trials have generally been conducted in small populations, and limited substantial evidence supporting claims of cardiovascular benefits exists.
In a clinical trial evaluating raw red onion consumption on metabolic features in overweight or obese women with polycystic ovary syndrome, total cholesterol and low-density lipoprotein cholesterol decreased compared with the control group. Other measures were unaffected.Ebrahimi 2014
In a study on endothelial function (n=72), onion peel extract (quercetin 100 mg/day administered for 12 weeks) improved flow-mediated dilation and circulating endothelial progenitor cells versus placebo.Choi 2015
In vitro data
Inhibition of human skin fibroblasts and keloidal fibroblasts has been demonstrated in vitro.WHO 1999
Limited clinical trials evaluating the effects of onion extract gels on scarring have demonstrated equivocal results. In certain populations, reductions in redness, itching, burning, and pain associated with surgical procedures have been demonstrated,Draelos 2008, Ho 2006, Jackson 1999, Koc 2008 while in other populations, there was no difference over standard petroleum-based emollient therapy.Chung 2006, Koc 2008
Quality clinical trials are lacking; and limited trials have reported equivocal results. Some studies show decreases in blood glucose levels in healthy volunteers and non-insulin–dependent diabetic subjects with aqueous onion extract and raw dietary onion.Akash 2014, Griffiths 2002, WHO 1999
Glucose metabolism was not affected by supplemental quercetin (162 mg/day over 6 weeks from onion skin extract powder) in one clinical trial (N=70) evaluating the effect of quercetin on cardiovascular biomarkers.Brull 2015 In a similar study by the same researchers, postprandial metabolic and vascular responses induced by a high-fat and high-carbohydrate meal were not attenuated by ingestion of quercetin.Brull 2017
The levels of fasting blood sugar were unaffected in a trial evaluating effects of raw red onion consumption on metabolic features in overweight or obese women with polycystic ovary syndrome.Ebrahimi 2014
Findings from a clinical study suggest quercetin supplementation (500 mg/day for 4 weeks) reduces elevated plasma uric acid concentrations in healthy males.Shi 2016
In one study, quercetin, but not raw onion, decreased blood pressure (BP) in hypertensive subjects.Edwards 2007
In a subgroup analysis of hypertensive subjects using supplemental quercetin 162 mg/day over 6 weeks (from onion skin extract powder), 24-hour systolic BP was reduced by −3.6 mm Hg (P=0.022) compared with placebo (mean treatment difference, −3.9 mm Hg; P=0.049). Hypertension was unaffected in the total group analysis (N=70 overweight to obese patients with prehypertension and stage I hypertension), and other biomarkers of cardiovascular disease and glucose metabolism were not affected by quercetin.Brull 2015
In a similar study by the same researchers, postprandial metabolic and vascular responses induced by a high-fat and high-carbohydrate meal were also not attenuated by ingestion of quercetin.Brull 2017
In vitro data
Studies evaluating inhibitory effects on the differentiation of osteoclasts in the cell line indicate that onion extract effectively inhibits osteoclastogenesis and differentiation.Law 2016
A study evaluating the impact of onion juice intake on bone mineral density and bone loss in corroboration with antioxidant effects in middle-aged, postmenopausal healthy subjects showed a positive modulatory effect on bone loss and bone mineral density due to improvement in antioxidant activity.Law 2016
Pharmacokinetic studies in rodents and healthy participants suggest greater bioavailability of quercetin from onion powder over apple peel,Lee 2012 of fresh red onion over dietary supplementation (aglycone quercetin),Shi 2015 of onion peel powder over onion peel extract,Kashino 2015 and of onion skin extract over pure quercetin dihydrate powder–filled hard capsules.Burak 2017
Based on a study in healthy adult males, it has been suggested that supplemental quercetin 166 mg would be comparable to about 10 mg of quercetin aglycone equivalent from red onion (in soup).Shi 2015
Onion peel extract (quercetin 100 mg/day for 12 weeks) has been used in a study evaluating effects on endothelial function.Choi 2015
Quercetin 500 mg/day for 4 weeks (bioavailable amount of quercetin present in approximately 100 g of red onions) has been studied for treatment of hyperuricemia.Shi 2016
Supplemental quercetin 162 mg/day over 6 weeks (from onion skin extract powder) has been studied for treatment of hypertension.Brull 2015
Pregnancy / Lactation
Onion has GRAS status when used as food. Avoid amounts greater than those found in foods because safety has not been established.
None well documented. No potentiation of antiplatelet effects has been reported.
Ingestion of onion and onion extract appears to be relatively safe.Blumenthal 1998, WHO 1999 Excessive consumption of onion has been associated with GI upset (burning sensation and diarrhea), flatulence, and changes in intestinal flora.Suleria 2015
Onion seeds have been reported as occupational allergens, but frequent contact with onion rarely causes allergic reaction.Navarro 1995 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.Chan 1972 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.WHO 1999 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.Thomson 1998 Poisoning in cattle fed large quantities of onion has been reported.Verhoeff 1985 Food colorant extracted from the tan onion bulb covering had no acute or subacute toxic effects in mice.Kojima 1993
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