Active constituents
Two sets of compounds make up the majority of onion’s known active
constituents—sulfur compounds, such as allyl propyl disulphide (APDS), and flavonoids, such as quercetin. Each of these groups of compounds has
multiple medicinal actions.
The sulfur compounds form a strongly scented oil, particularly the compound known as
thioproanal-s-oxide or lacrimatory factor. It is responsible for the tearing many people
suffer while cutting onions.5 Onion and onion oil constituents have been repeatedly
shown to kill various microbes in the test tube.6 7 Studies have not
been conducted in humans to determine whether onion is a useful antimicrobial agent.
APDS has been shown to block the breakdown of insulin by the liver and possibly to
stimulate insulin production by the pancreas, thus increasing the amount of insulin and
reducing sugar levels in the blood.8 Several uncontrolled human studies9
10 and at least one double-blind clinical trial11 have shown that large
amounts of onion can lower blood sugar levels in people with diabetes. Onion does not reduce blood sugar levels in
healthy nondiabetic people.12
Sulfur compounds in onion oil have also been shown to be anti-inflammatory, both by
inhibiting formation of thromboxanes and by inhibiting the action of platelet-activating
factor (PAF).13 14 Not all studies have confirmed that these actions
occur in humans.15 The anti-inflammatory effect is strong enough that subcutaneous
onion injections and topical onion applications inhibit skin reactions to intensely
inflammatory compounds in people with or without
eczema, according to the results of one double-blind study.16 Human studies
have not been performed to determine whether onion would be useful in people with asthma or
cough, though the anti-inflammatory action cited above suggests it might be. These
actions, coupled with an ability to reduce the stickiness of platelets17 and,
overall, to decrease the thickness of the blood,18 have led to interest in onion as
a way to prevent or possibly reduce
atherosclerosis.
Human studies have proven mixed as to whether onion is helpful for people with
atherosclerosis.19 Intake of quercetin in the diet, primarily from onion, tea, and
apples, has been linked to a decreased risk of having a heart attack.20 High intake
of quercetin and other flavonoids from onion and other foods has been shown to decrease risk
of atherosclerosis in an epidemiologic study in the United States, although the result was not
considered statistically significant.21 One open clinical trial showed that a crude
onion extract could lower blood pressure in some people with hypertension.22 On the whole, it is unclear
whether or not onion supplements, as opposed to onions eaten as food, have a beneficial effect
on heart disease.
In a preliminary study of healthy male volunteers, administration of 50 grams of raw or
boiled onion prevented the rise in serum cholesterol induced by consumption of a high-fat
meal.23
The evidence on cancer prevention with
onion suggests a benefit for some but not necessarily for all types of cancer. Onion
consumption at a level of at least half an onion a day was associated with a 50% decline in
stomach cancer risk in one study.24 Higher onion intake was also correlated with
lower risk of breast cancer in a French epidemiological study.25 No protective
effect against colorectal cancer was seen from higher onion intake.26
1. Onstad D. Whole Foods Companion: A Guide for Adventurous Cooks,
Curious Shoppers & Lovers of Natural Foods. White River Junction, VT: Chelsea Green
Publishing Co, 1996:202–6.
2. Onstad D. Whole Foods Companion: A Guide for Adventurous Cooks,
Curious Shoppers & Lovers of Natural Foods. White River Junction, VT: Chelsea Green
Publishing Co, 1996:202–6.
3. Felter HW, Lloyd JU. King’s American Dispensatory, 18th
ed, vol 1. Portland, OR: Eclectic Medical Publications, 1898, 1983:146.
4. Onstad D. Whole Foods Companion: A Guide for Adventurous Cooks,
Curious Shoppers & Lovers of Natural Foods. White River Junction, VT: Chelsea Green
Publishing Co, 1996:202–6.
5. Brodnitz MH, Pascale JV. Thiopropanal S-oxide: A lachrymatory [sic]
factor in onions. J Agric Food Chem 1971;19:269–72.
6. Zohri AN, Abdel-Gawad K, Saber S. Antibacterial, antidermatophytic and
antitoxigenic activities of onion (Allium cepa L.) oil. Microbiol Res
1995;150:167–72.
7. Kim JH. Anti-bacterial action of onion (Allium cepa L.)
extracts against oral pathogenic bacteria. J Nihon Univ Sch Dent
1997;39:136–41.
8. Sharma KK, Gupta RK, Gupta S, Samuel KC. Antihyperglycemic effect of
onion: Effect on fasting blood sugar and induced hyperglycemia in man. Indian J Med
Res 1977;65:422–9.
9. Jain RC, Sachdev KN. A note on hypoglycemic action of onion in
diabetes. Curr Med Pract 1971;15:901–2.
10. Mathew PT, Augusti KT. Hypoglycaemic effect of onion, Allium
cepa Linn, on diabetes mellitus, a preliminary report. Indian J Physiol
Pharmacol 1975;19:231–7.
11. Tjokroprawiro A, Pikir BS, Budhiarta AA, et al. Metabolic effects of
onion and green beans on diabetic patients. Tohoku J Exp Med
1983;141(suppl):671–6.
12. Sharma KK, Gupta RK, Gupta S, Samuel KC. Antihyperglycemic effect of
onion: Effect on fasting blood sugar and induced hyperglycemia in man. Indian J Med
Res 1977;65:422–9.
13. Dorsch W, Ettl M, Hein G, et al. Anti-asthmatic effects of onions:
Inhibition of platelet-activating factor-induced bronchial construction by onion oils. Int
Arch Allergy Appl Immunol 1987;82:535–6.
14. Dorsch W, Wagner H, Bayer T, et al. Antiasthmatic effects of onions:
Alk(en)ylsulfinothic acid alk(en)ylesters inhibit histamine release, leukotriene and
thromboxane biosynthesis in vitro and counteract PAF- and allergen-induced bronchial
obstruction in vivo. Biochem Pharmacol 1988;37:4479–85.
15. Srivastava KC. Effect of onion and ginger consumption on platelet
thromboxane production in humans. Prostaglandins Leukot Essent Fatty Acids
1989;35:183–5.
16. Dorsch W, Ring J. Suppression of immediate and late anti-IgE-induced
skin reactions by topically applied alcohol/onion extract. Allergy
1984;39:43–9.
17. Chen JH, Chen HI, Tsai SJ, Jen CJ. Chronic consumption of raw but not
boiled Welsh onion juice inhibits rat platelet function. J Nutr
2000;130:34–7.
18. Kendler BS. Garlic (Allium sativum) and onion (Allium
cepa): A review of their relationship to cardiovascular disease. Prev Med
1987;16:670–85 [review].
19. Kleijnen J, Knipschild P, ter Riet G. Garlic, onions and
cardiovascular risk factors: A review of the evidence from human experiments with emphasis on
commercially available preparations. Br J Clin Pharmacol 1989;28:535–44
[review].
20. Hertog MGL, Feskens EJ, Hooman PC, et al. Dietary antioxidant
flavonoids and the risk of coronary heart disease: The Zutphen Elderly Study. Lancet
1993;342:1007–11.
21. Rimm ER, Katan MB, Ascherio A, et al. Relation between intake of
flavonols and risk for coronary heart disease in male health professionals. Ann Intern
Med 1996;125:384–9.
22. Louria DB, McAnally JF, Lasser N, et al. Onion extract in treatment
of hypertension and hyperlipidemia: A preliminary communication. Curr Ther Res
1985;37:127–131.
23. Sharma KK, Chowdhury NK, Sharma AL, Misra MB. Studies on
hypocholestraemic activity of onion. I. Effect on serum cholesterol in alimentary lipaemia in
man. Indian J Nutr Diet 1975;12:288–91.
24. Dorant E, van der Brandt PA, Goldbohm RA, Sturmans F. Consumption of
onions and a reduced risk of stomach carcinoma. Gastroenterology
1996;110:12–20.
25. Challier B, Perarnau JM, Viel JF. Garlic, onion and cereal fibre as
protective factors for breast cancer: A French case-control study. Eur J Epidemiol
1998;14:737–47.
26. Dorant E, van den Brandt PA, Goldbohm RA. A prospective cohort study
on the relationship between onion and leek consumption, garlic supplement use and the risk of
colorectal carcinoma in the Netherlands. Carcinogenesis 1996;17:477–84.
27. Tjokroprawiro A, Pikir BS, Budhiarta AA, et al. Metabolic effects of
onion and green beans on diabetic patients. Tohoku J Exp Med
1983;141(suppl):671–6.
28. Jain RC, Sachdev KN. A note on hypoglycemic action of onion in
diabetes. Curr Med Pract 1971;15:901–2.
29. Bordia T, Mohammed N, Thomson M, Ali M. An evaluation of garlic and
onion as antithrombotic agents. Prostaglandins Leukot Essent Fatty Acids
1996;54:183–6.
30. Tjokroprawiro A, Pikir BS, Budhiarta AA, et al. Metabolic effects of
onion and green beans on diabetic patients. Tohoku J Exp Med
1983;141(suppl):671–6.
31. Chen JH, Chen HI, Tsai SJ, Jen CJ. Chronic consumption of raw but not
boiled Welsh onion juice inhibits rat platelet function. J Nutr
2000;130:34–7.
32. Ali M, Bordia T, Mustafa T. Effect of raw versus boiled aqueous
extract of garlic and onion on platelet aggregation. Prostaglandins Leukot Essent Fatty
Acids 1999;60:43–7.
33. Louria DB, McAnally JF, Lasser N, et al. Onion extract in treatment
of hypertension and hyperlipidemia: A preliminary communication. Curr Ther Res
1985;37:127–131.
34. Allen ML, Mellow MH, Robinson MG, Orr WC. The effect of raw onions on
acid reflux and reflux symptoms. Am J Gastroenterol 1990;85:377–80.
35. Valdivieso R, Subiza J, Varela-Losada S, et al. Bronchial asthma,
rhinoconjunctivitis, and contact dermatitis caused by onion. J Allergy Clin Immunol
1994;94:928–30.
36. Wuensch KL. Exposure to onion taste in mother’s milk leads to
enhanced preference for onion diet among weanling rats. J Gen Psychol 1978;99(2d
Half):163–7.