Onion (Allium cepa)
Synonyms / Common Names / Related Terms
Allium cepa, Allium cepa L., allium vegetables, botanicals, Liliaceae (family), onion extract, onion juice, pickling onions, white onion.
Combination product example: Contractubex gel (cepae extract from onions, heparin, and allantoin).
Mechanism of Action
- Constituents: Onion has been found to contain quercetin, fructose, quercetin-3-glucoside, isorhamnetin-4-glucoside, xylose, galactose, glucose, mannose, organosulfur compounds, allylsulfides, flavonoids, flavenols, S-alk(en)yl cysteine sulfoxides, cycloalliin, selenium, thiosulfinates, and sulfur and seleno compounds.7,22,37,9,5,3,38,15,10,4,24,39,6,38,1
- Antibiotic effects: Allium plants, which include onion, exhibit antibiotic activity against both Gram-positive and Gram-negative bacteria.2
- Anticancer effects: Numerous in vitro, animal, and epidemiological studies indicate that onion or onion extract prevents cancer including gastrointestinal cancer, ovarian cancer, and skin cancer.2,10,32,8,5,33,34 The most common current theory is that the metabolites of organosulfur compounds, specifically S-alk(en)yl cysteine sulfoxide, found in these plants inhibit mutatgenesis, induce phase II detoxification enzymes, influence cell arrest and apoptosis, scavenge free radicals, and inhibit DNA adduct formation.9,29,11,10,8,5 Although the research seems promising, results are still preliminary and high quality clinical trials are needed to define onion's cancer preventative activity.
- Antidiabetic effects: Onion (Allium cepa) has been experimentally documented to possess antidiabetic potential.29 Tjokroprawiro et al. conducted a crossover comparative study in 20 cooperative diabetic outpatients to assess the effects of a diet including onions or green beans on diabetic symptoms (hypercholesterolemia, serum glucose levels).27 Ten of the patients consumed a specific diet (68% cal carbohydrate, 20% cal fat, 12% cal protein) plus 3 X 20g fresh onion daily, or plus 3 X 200g green beans daily in the first week, and the diet alone in the second week; the other half was assigned the other way around. The onion group had a significant decrease in blood sugar level (4.37mg%, p<0.05), however, no blood lipid levels-changes occurred in these diets.
- Antihypercholesterolemic effects: In a clinical study of alimentary hyperlipidemia, onion and onion essential oil prevented fat-induced increases in serum cholesterol and plasma fibrinogen and decreases in coagulation time and fibrinolytic activity.25,13 Onion and garlic juices can help to prevent the rise of serum cholesterol. The main active constituents seemed to be sulfur-containing compounds, mainly allyl propyl disulphide and diallyl disulphide.
- Antihyperglycemic effects: One early clinical study assessed the antihyperglycemic effect of onion on fasting blood sugar and induced hyperglycemia.26
- Antimutagenic effects: In a study of six healthy non-obese normocholesterolemic females (age 20-44 years), a meal of fried onions or a meal of fried onions and fresh cherry tomatoes increased resistance of lymphocyte DNA to DNA strand breakage.39
- Antioxidant effects: A randomized crossover study of 36 healthy human subjects on either a low-flavonol diet or a high flavonol diet (low-flavonol diet plus one 150g onion cake (89.7mg quercetin) who also consumed one 300mL cup of black tea (1.4mg quercetin) daily did not find significant differences in oxidative damage to leukocytes DNA bases.40
- Cardiovascular effects: In a clinical study of subjects with arterial hypertension (WHO class I), an onion-olive oil maceration product significantly decreased systolic blood pressure; there was also a trend towards a decrease in diastolic blood pressur.28 In a clinical study in healthy subjects, an onion-olive oil maceration product non-significantly decreased arterial blood pressure.23
- Cicatrizant effects: Several clinical trials have been conducted to define onion extract's role in scar healing in adults and children, specifically due to injuries incurred during laser tattoo removal or surgery.16,17,18,19,20,21 The overall results were positive in the studies for scar appearance, type, size, and color, but not all differences between the verum groups and placebo groups were significant. In addition, most of the studies used the combination product Contractubex® gel (cepae extract from onions, heparin, and allantoin), which prevents the determination of onion's effects alone on cictrization. As the results are promising, more research should be conducted.
- Hemostatic effects: In pharmacologic and in vitro studies, onion and onion extract, alone and in combination with other products, have shown hemostatic effects including inhibited platelet aggregation, reduced plasma viscosity, decreased hematocrit, and increased fibribolytic activity.22,23,24 In alimentary hyperlipidemia, onion also had a protective effect against increased plasma fibrinogen and it decreased coagulation time and fibrinolytic activity.25 Onion's quercetin content is often hypothesized to stimulate these effects. However, two in vivo clinical studies indicate that 70-200g of raw onion daily for seven days does not significantly affect platelet aggregation, thromboxane B2 production, factor VII, or other hemostatic variables.30,31 Most of the studies showing anticoagulant properties for onion used either processed onion (e.g. onion soup, onion capsules), whereas the studies finding no such effects used raw onion. More studies are needed to define the relationship between onion preparation and its anticoagulant effects.
- Osteoclastic effects: In animal studies, ingestion of onion significantly inhibited bone resorption.35,36,14
- In a clinical trial of 10 type 2 diabetic patients (five male, five female) ingesting a low-flavonol diet or a high-flavonol diet (low-flavonol diet plus 1500mL tea daily and 400g fried white onion in olive oil with and without tomato ketchup and herbs), plasma flavonol concentration (r=0.750, p=0.001), 24-hour urine concentration (r=0.847, p=0.001), and 24-hour urine excretion (r=0.728, p≤0.001) were all highly significantly related to dietary intake and gave similar estimates of intake.37 The authors reported that fasting plasma flavonols concentrations on habitual diets ranged from 0 to 43.7ng/mL mean.
- In a double-blind randomized cross-over pilot study, the subjects ingested an onion soup containing either a high or a low amount of quercetin22 Plasma quercetin concentrations were significantly higher after ingestion of high-quercetin soup (69mg total quercetin per serving) than after ingestion of low-quercetin soup (5mg total quercetin per serving) and peaked at 2.59 (sem 0.42) mcM/L.
- After six healthy non-obese normocholesterolemic females (age 20-44 years) ingested a meal of fried onions, flavonoid glucosides (quercetin-3-glucoside and isorhamnetin-4-glucoside) were significantly elevated.39 A significant decrease in the level of urinary 8-hydroxy-2'-deoxyguanosine was evident at four hours following ingestion of the onion meal. After a washout period and ingestion of a combined meal of cherry tomatoes and fried onions, only quercetin was detected in plasma. There was no significant change in the excretion of urinary malondialdehyde following either meal.
- Beatty et al. conducted a randomized crossover study to assess the effect of dietary intake of flavonols (predominantly quercetin) on oxidative DNA damage in 36 healthy human subjects (sixteen men, twenty women).40 The subjects consumed either a low-flavonol or a high-flavonol diet for 14 days, then switched to the other diet after a 14-day washout period. Subjects consumed one 150g onion (Allium cepa) cake (containing 89.7mg quercetin) and one 300mL cup of black tea (containing 1.4mg quercetin) daily during the high-flavonol dietary treatment. No significant differences in intake of macronutrients, assessed micronutrients, or plasma vitamin C were found between the high-flavonol and low-flavonol dietary treatments. The plasma quercetin concentration was significantly higher after the high-flavonol dietary treatment period (228.5 (SEM 34.7) nM/L) than after the low-flavonol dietary treatment period (less than the limit of detection, i.e. <66.2nM/L).
- To determine the influence of the sugar moiety or matrix on the absorption of quercetin, two isolated quercetin glycosides and two plant extracts were administered to 12 healthy volunteers in a four-way crossover study.41 Each subject received an onion supplement or quercetin-4'-O-glucoside (both equivalent to 100mg quercetin), as well as quercetin-3-O-rutinoside and buckwheat tea (both equivalent to 200mg quercetin). In human plasma, only quercetin glucuronides, but no free quercetin, could be detected. There was no significant difference in the bioavailability and pharmacokinetic parameters between the onion supplement and quercetin-4'-O-glucoside. Peak plasma concentrations were 2.3 ± 1.5mcg/mL and 2.1 ± 1.6mcg/mL (mean ± SD) and were reached after 0.7 ± 0.2 hours and 0.7 ± 0.3 hours, respectively. After administration of buckwheat tea and rutin, however, peak plasma levels were (despite the higher dose) only 0.6 ± 0.7mcg/mL and 0.3 ± 0.3mcg/mL, respectively. Peak concentrations were reached 4.3 ± 1.8 hours after administration of buckwheat tea and 7.0 ± 2.9 hours after ingestion of rutin. The terminal elimination half-life was about 11 hours for all treatments.
- In ileostomy patients, the intake of quercetin monoglucoside and quercetin diglucoside from an onion meal in four patients ranged from 10.9 to 51.6mg.42 No quercetin monoglucoside or quercetin diglucoside was detected in the ileostomy fluid. In contrast, the amounts of the aglycone quercetin were substantial, 2.9-11.3mg, which was 19.5-35.2% of total quercetin glucosides ingested implying absorption of 64.5-80.7%.
- In 15 subjects consuming fried onions (129g daily) for three days (13mg quercetin and no kaempferol, daily), the excretion of unmodified quercetin was 1.1%.12 CVs for quercetin were 30% in plasma and 42% in urine.
- In 18 healthy volunteers, 220g onions daily providing 114mg quercetin raised mean plasma quercetin concentrations to 1.5mcM/L.30
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- Augusti, K. T. Therapeutic values of onion (Allium cepa L.) and garlic (Allium sativum L.). Indian J. Exp. Biol. 1996;34(7):634-640. 8979497
- Sengupta, A., Ghosh, S., and Bhattacharjee, S. Allium vegetables in cancer prevention: an overview. Asian Pac. J Cancer. Prev. 2004;5(3):237-245. 15373701
- Baraboi, V. A. and Shestakova, E. N. [Selenium: the biological role and antioxidant activity]. Ukr. Biokhim. Zh. 2004;76(1):23-32. 15909414
- Arnault, I. and Auger, J. Seleno-compounds in garlic and onion. J. Chromatogr., A 4-21-2006;1112(1-2):23-30. 16480995
- Wargovich, M. J. New dietary anticarcinogens and prevention of gastrointestinal cancer. Dis. Colon Rectum 1988;31(1):72-75. 3284725
- Rose, P., Whiteman, M., Moore, P. K., and Zhu, Y. Z. Bioactive S-alk(en)yl cysteine sulfoxide metabolites in the genus Allium: the chemistry of potential therapeutic agents. Nat. Prod. Rep. 2005;22(3):351-368. 16010345
- Fukushima, S., Takada, N., Hori, T., and Wanibuchi, H. Cancer prevention by organosulfur compounds from garlic and onion. J. Cell Biochem. Suppl. 1997;27:100-105. 9591199
- Heber, D. Vegetables, fruits and phytoestrogens in the prevention of diseases. J. Postgrad. Med. 2004;50(2):145-149. 15235216
- de Vries, J. H., Hollman, P. C., Meyboom, S., Buysman, M. N., Zock, P. L., van Staveren, W. A., and Katan, M. B. Plasma concentrations and urinary excretion of the antioxidant flavonols quercetin and kaempferol as biomarkers for dietary intake. Am. J. Clin. Nutr. 1998;68(1):60-65. 9665097
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- Chung, V. Q., Kelley, L., Marra, D., and Jiang, S. B. Onion extract gel versus petrolatum emollient on new surgical scars: prospective double-blinded study. Dermatol. Surg. 2006;32(2):193-197. 16442038
- Ho, W. S., Ying, S. Y., Chan, P. C., and Chan, H. H. Use of onion extract, heparin, allantoin gel in prevention of scarring in chinese patients having laser removal of tattoos: a prospective randomized controlled trial. Dermatol. Surg. 2006;32(7):891-896. 16875470
- Willital, G. H. and Heine, H. Efficacy of Contractubex gel in the treatment of fresh scars after thoracic surgery in children and adolescents. Int. J. Clin. Pharmacol. Res. 1994;14(5-6):193-202. 7672876
- Jackson, B. A. and Shelton, A. J. Pilot study evaluating topical onion extract as treatment for postsurgical scars. Dermatol. Surg. 1999;25(4):267-269. 10417579
- Maragakis, M., Willital, G. H., Michel, G., and Gortelmeyer, R. Possibilities of scar treatment after thoracic surgery. Drugs Exp. Clin. Res. 1995;21(5):199-206. 8846750
- Zurada, J. M., Kriegel, D., and Davis, I. C. Topical treatments for hypertrophic scars. J. Am. Acad. Dermatol. 2006;55(6):1024-1031. 17097399
- Hubbard, G. P., Wolffram, S., de Vos, R., Bovy, A., Gibbins, J. M., and Lovegrove, J. A. Ingestion of onion soup high in quercetin inhibits platelet aggregation and essential components of the collagen-stimulated platelet activation pathway in man: a pilot study. Br. J. Nutr. 2006;96(3):482-488. 16925853
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- Tjokroprawiro, A., Pikir, B. S., Budhiarta, A. A., Pranawa, Soewondo, H., Donosepoetro, M., Budhianto, F. X., Wibowo, J. A., Tanuwidjaja, S. J., Pangemanan, M., and . Metabolic effects of onion and green beans on diabetic patients. Tohoku J. Exp. Med. 1983;141 Suppl:671-676. 6393443
- Mayer, B., Kalus, U., Grigorov, A., Pindur, G., Jung, F., Radtke, H., Bachmann, K., Mrowietz, C., Koscielny, J., Wenzel, E., and Kiesewetter, H. Effects of an onion-olive oil maceration product containing essential ingredients of the Mediterranean diet on blood pressure and blood fluidity. Arzneimittelforschung. 2001;51(2):104-111. 11258039
- Srinivasan, K. Plant foods in the management of diabetes mellitus: spices as beneficial antidiabetic food adjuncts. Int. J. Food Sci. Nutr. 2005;56(6):399-414. 16361181
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- Muhlbauer, R. C., Lozano, A., and Reinli, A. Onion and a mixture of vegetables, salads, and herbs affect bone resorption in the rat by a mechanism independent of their base excess. J. Bone Miner. Res. 2002;17(7):1230-1236. 12096836
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- Beatty, E. R., O'Reilly, J. D., England, T. G., McAnlis, G. T., Young, I. S., Geissler, C. A., Sanders, T. A., and Wiseman, H. Effect of dietary quercetin on oxidative DNA damage in healthy human subjects. Br. J. Nutr. 2000;84(6):919-925. 11177210
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- Walle, T., Otake, Y., Walle, U. K., and Wilson, F. A. Quercetin glucosides are completely hydrolyzed in ileostomy patients before absorption. J. Nutr. 2000;130(11):2658-2661. 11053503