Onion
Scientific Name(s): Allium cepa L. Family: Liliaceae (lilies)
Common Name(s): Onion
Clinical Overview
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Uses of Onion
Onion is used as an antimicrobial, cardiovascular-supportive, hypoglycemic, antioxidant/anticancer, and asthma-protective agent. However, few clinical trials are available to support the use of onion for any indication. In folk medicine, onion has been used for asthma, bronchitis, whooping cough, and similar ailments. Other uses include the treatment of stingray wounds, warts, acne, appetite loss, urinary tract disorders, and indigestion. Onion skin dye has been used as an egg and cloth coloring.
Onion Dosing
Fresh onion bulbs are used at daily doses of 50 g; dried onion is used at a dose of 20 g/day for dyspepsia. 1
Contraindications
Contraindications have not yet been identified.
Pregnancy/Lactation
Generally recognized as safe for use as food. Avoid dosages above those found in foods because safety and efficacy are unproven.
Onion Interactions
None well documented.
Onion Adverse Reactions
The Complete German Commission E Monographs lists no contraindications, side effects, or interactions.
Toxicology
The reputed toxicity of large doses of onion has been unresolved.
Botany
The onion plant is a perennial herb growing to about 1.2 m, with 4 to 6 hollow, cylindrical leaves. On top of the long stalk, greenish-white flowers are present in the form of solitary umbels growing up 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. The onion is one of the leading vegetable crops in the world. 2 , 3 , 4 , 5
History
The onion is believed to have been domesticated in central Asia. 5 Onions were used as early as 5,000 years ago in Egypt, as depicted on ancient monuments; ancient Greek and Roman records also refer to the onion. During the Middle Ages, onions were consumed throughout Europe. They later were thought to guard against evil spirits and the plague, probably because of their strong odor. Onion skin dye has been used for egg and cloth coloring for many years in the Middle East and Europe. Columbus was said to have brought the onion to America. Folk healers used the onion to prevent infection. The combination of onions and garlic cooked in milk is a European folk remedy used to clear congestion. Onions also are used in homeopathic medicine. 2 , 3 , 6 , 7
Chemistry
Onions contain 89% water, 1.5% protein, and vitamins, including B 1 , B 2 , and C, along with potassium. 2 , 3 Polysaccharides such as fructosans, saccharose, and others are also present, as are peptides, flavonoids, and essential oil. 4 , 5 Onion contains alliin and similar sulfur compounds, including allylalliin and methyl and propyl compounds of cysteine sulfoxide. 4 , 5 , 7 Sulfur and other compounds of A. cepa have been analyzed. 8 The flavor components of onion have been evaluated by thin-layer chromatography. 9 Prostaglandins also have been identified in onion, 10 and onion cell wall analyses have been performed. 11 , 12 The chemical analysis of onion seed oil is available. 13
Onion Uses and Pharmacology
The main properties of onion include antimicrobial activity, cardiovascular support, hypoglycemic action, antioxidant/anticancer effects, and asthma protection.
Antimicrobial effectsIn vitro
Onion has shown antibacterial, 4 antiparasitic, 14 and antifungal actions. 15 , 16 Salmonella typhimurium mutagenicity was reduced in hamburger when onions were added. 17 Growth of oral pathogenic bacteria, including Streptococcus mutans , Streptococcus subrinus , Porphyromonas gingivalis , and Prevotella intermedia , the main causes of dental caries and periodontitis, was prevented by onion extracts. 18 Onion juice or oil also have inhibited growth of other gram-positive bacteria and the gram-negative bacterium Klebsiella pneumoniae . 15 , 16 , 19 Antifungal actions of onion include inhibition of yeasts, 15 Microsporum canis , Microsporum gypseum , Trichophyton simii , Trichophyton mentagrophytes , Chrysosporium queenslandicum , Aspergillus flavus , and Penicillium rubrum . 16 One source identifies the thiosulfinate principle in the onion as a main antimicrobial agent. 5
Cardiovascular diseaseCertain onion genotypes containing higher contents of sulfur in the bulb correlated with greater antiplatelet activity. 20 Thiosulfinates dimethyl- and diphenylthiosulfinate, for example, are known to retard thrombocyte biosynthesis. 5 , 21 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. 22 Synthesis of thromboxanes and prostaglandins in vitro has been shown with onions, as well as with garlic and other Liliaceae family members. 23
Animal dataThe hypolipidemic effects of sulfur-containing principles in onion, including S-methyl cysteine sulfoxide and allylpropyl disulfide, have been demonstrated in several studies in rats and rabbits. 24 , 25 , 26 , 27 , 28 Examples include onion's protective effects against diet-induced atherosclerosis 27 and its marked action in controlling lipids 28 and triglycerides. 26 Cardiovascular disease risk factors also involve blood coagulability. Several reports confirm the onion's inhibitory effects on platelet formation. Raw, but not cooked, onion demonstrated antithrombotic effects in rats. 29 , 30 Dose-dependent inhibitory effects on platelet aggregation with raw onion also were seen in rabbits. 31 Boiling onion may cause decomposition of the antithrombotic ingredient. 29
Clinical dataOne report evaluates onion's hemostatic effects in humans, 32 but certain lipid-reducing and blood pressure-lowering effects in humans have not yet been clinically proven. 5 Onion's benefits relating to cardiovascular disease have been reviewed. 6 , 33
DiabetesAnimal data
Studies from 1965 to 1975 report antidiabetic activity, “hypoglycemic principles,” and blood sugar level reduction in diabetic rabbits. 34 , 35 , 36 More recent reports confirm many of these claims, finding similar outcomes. Onion decreased the hyperglycemic peak in rabbits. 37 In addition, onion amino acid s-methyl cysteine sulfoxide contributed to antidiabetic effects in affected rats, controlling blood glucose in addition to other diabetic effects comparable to insulin. 28 , 38
Clinical dataAlthough more research is needed on the use of onion as a treatment for diabetes in humans, many articles describe its benefits in improving glucose levels. 39
CancerOnion has proven to be an antioxidant and may be beneficial in certain cancers. The organosulfur compounds contained in onion exert chemopreventive effects on chemical carcinogenesis. The constituent diallyl disulfide possesses inhibitory properties against colon and renal cancers. 40
Animal dataOil of onion is an effective antioxidant against nicotine-induced damage in rats. 41
Clinical dataPeople consuming diets high in allium vegetables, including onion, suffer from fewer incidences of stomach cancer. 2 , 42 Onion's protective factors for breast cancer have been evaluated in a case-control study in France. 43 Another report compares the antioxidant activity of onion polyphenols with those of other fruits and vegetables. 44 The quercetin component in onion, however, was found to be absorbed by humans from dietary sources but provided no direct protective effect during low-density lipoprotein oxidation. 45
Miscellaneous usesRespiratory problems
Folk medicine has used the onion for treatment of asthma, whooping cough, bronchitis, and similar ailments. 5 , 39 The onion is used in homeopathic medicine. 6 Onion juice administration protected guinea pigs from asthma attacks. An ethanol extract of onion reduced allergy-induced, bronchial constriction in certain patients. 5 The thiosulfinates present in the onion are said to inhibit bronchoconstriction, but definite efficacy remains unproven in this area. 21
Other usesOnions have been used in the treatment of stingray wounds, 46 warts, acne, 3 appetite loss, 2 , 4 urinary tract disorders, 6 and indigestion. 2 Onion cell extract was ineffective in treating postsurgical scarring. 47 General reviews of therapeutic uses of onion are available. 48 , 49
Dosage
The Complete German Commission E Monographs lists the average daily dose as 50 g of fresh onion, the juice from 50 g of fresh onion, or 20 g dried onion. A maximum of diphenylamine 35 mg/day is recommended if onion preparations are used over several months. 4
Pregnancy/Lactation
Generally recognized as safe when used as food. Avoid dosages above those found in foods because safety and efficacy are unproven.
Interactions
None well documented.
Adverse Reactions
Certain sulfur compounds (eg, propanethial-S-oxide) escape from onion in vapor form and hydrolyze to sulfuric acid when cut, causing the familiar eye irritation and lacrimation. 2 , 7 Corneal swelling from onion exposure has been reported. 50 Using a sharp knife minimizes the crushing of onion tissue and liberation of volatiles, and cutting an onion under running water reduces lacrimation. Ingestion of onion seems relatively safe, as the Complete German Commission E Monographs lists no contraindications, side effects, or interactions from the plant. 4 Onion can be taken frequently in low doses without any side effects, as seen with rat experimentation. 29 With large intake, the stomach may be affected, and frequent contact with onion rarely causes allergic reaction. 5 Onion seeds have been reported as occupational allergens. 51 Onion toxicity is only associated with high intake.
Toxicology
A review of literature discussing large amounts of onion bulb ingestion leaves toxicity questions unresolved. 33 Low doses of onion (50 mg/kg) given to rats had little effect on the lung and liver tissues. 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. 52 Eighty-five young cattle were allowed 1,000 kg/day of onion, negatively affecting approximately 26%, with 1 fatality. New illnesses continued to occur for 5 days after the withdrawal of onion, including lack of appetite, tachycardia, staggering, and collapse, all probably because of adverse red blood cell effects. 53
Food colorant extracted from the tan onion bulb covering had no acute or subacute toxic effects in mice. 54
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52. Thomson M, Alnaqeeb MA, Bordia T, Al-Hassan JM, Afzal M, Ali M. Effects of aqueous extract of onion on the liver and lung of rats. J Ethnopharmacol . 1998;61:91-99.
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54. Kojima T, Tanaka T, Mori H, Kato Y, Nakamura M. Acute and subacute toxicity tests of onion coat, natural colorant extracted from onion ( Allium cepa L.), in (C57BL/6 × C3H)F1 mice. J Toxicol Environ Health . 1993;38:89-101.
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