Home Ingredients Onion Extract (Allium cepa, Quercetin-Rich)
Cardiovascular

Onion Extract (Allium cepa, Quercetin-Rich)

Allium cepa
Evidence Level
Limited
8 Clinical Trials
7 Documented Benefits
2/5 Evidence Score

Onion extract is made from the onion bulb (Allium cepa) or, more often, its dry outer skin, a rich source of the flavonoid quercetin. In the bulb, quercetin is mostly bound to glucose, and in human absorption studies this form was taken up better than pure quercetin. An onion skin extract also gave 4.8 times higher blood quercetin exposure than quercetin dihydrate. Supplement trials have used extracts or quercetin-rich onion powder supplying about 50 to 162 mg of quercetin a day. In a 70-person crossover trial, onion skin extract did not change blood pressure in the whole group; 24-hour systolic pressure fell about 4 mmHg only in the subgroup with high blood pressure. Korean trials of onion peel extract reported better artery dilation and lower body fat within the extract groups. In placebo comparisons, blood sugar, insulin and inflammation markers did not change.

Studied Dose Onion skin or peel extract supplying 100 to 162 mg of quercetin a day for 6 to 12 weeks (for example 170 mg of peel extract with 50 mg quercetin, twice daily); quercetin-rich onion powder (9 g a day) supplying about 50 to 60 mg of quercetin for 12 to 24 weeks.
Active Compound Quercetin from onion: mainly quercetin glucosides in the bulb, and mostly free quercetin (aglycone) in dry onion skin extracts. Trial extracts were standardized to quercetin, about 29% in one onion peel extract and about 41% in one onion skin extract.

Benefits

Blood pressure: change seen only in a higher-reading subgroup

In a crossover trial of 70 overweight adults with high-normal or high blood pressure, 162 mg a day of quercetin from onion skin extract for 6 weeks did not change 24-hour or office blood pressure in the whole group. In the subgroup with hypertension, 24-hour systolic pressure fell 3.9 mmHg more than on placebo. A single 54 mg dose had no effect, and a Japanese onion powder trial found no change in clinic or home readings.

Supported by Trial 1, Trial 6

Artery dilation (endothelial function)

In 72 healthy overweight adults, onion peel extract supplying 100 mg of quercetin a day for 12 weeks raised flow-mediated dilation, a measure of how well an artery widens, from 12.5% to 15.2% within the extract group, with no change on placebo; circulating endothelial progenitor cells also rose. A German trial at 162 mg a day found no change in a different endothelial test.

Supported by Trial 3, Trial 1

Blood lipid levels in healthy male smokers

In 92 healthy male smokers, onion peel extract capsules with 100 mg of quercetin a day for 10 weeks lowered total and LDL cholesterol within the extract group but not the placebo group, and HDL cholesterol rose more than on placebo. Blood pressure and glucose also fell within the extract group. Blood lipids did not change in the 6-week German trial at 162 mg a day.

Supported by Trial 4, Trial 1

Body fat in overweight adults

In 72 overweight Korean adults, onion peel extract with 100 mg of quercetin a day for 12 weeks lowered body weight and body fat percentage within the extract group but not on placebo. In 70 Japanese adults, quercetin-rich onion powder did not change visceral fat area versus placebo. A Korean steamed onion powder lowered body fat versus placebo in 70 overweight adults.

Supported by Trial 5, Trial 6

Quercetin absorption from onion compared with pure quercetin

In 9 volunteers with an ileostomy, about 52% of the quercetin glucosides in fried onions was absorbed, versus 24% of pure quercetin. In 12 healthy adults, an onion skin extract gave about 4.8 times higher blood quercetin exposure than a similar dose of quercetin dihydrate. These studies measured absorption only, not health outcomes.

Supported by Trial 8

Memory and mood test scores in healthy older adults

In 70 healthy Japanese adults aged 60 to 79, 24 weeks of quercetin-rich onion powder (about 50 mg of quercetin a day) improved Mini-Mental State Examination scores and emotional function scores compared with quercetin-free onion powder, while a tablet-based total cognitive score did not differ. An earlier 50-person trial by the same group found no overall change in these scores.

Supported by Trial 7

Blood sugar, insulin and inflammation markers

In placebo-controlled comparisons these markers did not change. At 162 mg of quercetin a day for 6 weeks, glucose, insulin, leptin, adiponectin, CRP and TNF-alpha did not differ from placebo. In 37 overweight women, 12 weeks of onion peel extract did not change leptin, TNF-alpha or IL-4 versus placebo. Glucose fell within the extract group in one trial of male smokers.

Supported by Trial 2, Trial 4

Mechanism of action

1

Quercetin glucosides are taken up in the small intestine

In ileostomy volunteers, quercetin bound to glucose (as in onion) was absorbed about twice as well as pure quercetin. In a crossover study, an onion supplement behaved like pure quercetin-4'-glucoside, peaking in blood within about 40 minutes, while the rutin form found in tea and buckwheat peaked after 4 to 7 hours. Only conjugated quercetin, not free quercetin, was found in blood.

2

Blood pressure mechanism not identified in trials

In the 6-week onion skin extract trial, endothelin-1, adhesion molecules, ADMA, ACE activity, endothelial function and markers of oxidation and inflammation did not change, so the small blood pressure fall in the subgroup with hypertension had no measured explanation. The authors state the mechanism remains unclear.

3

Effects on drug absorption (animal data)

Quercetin has been reported to affect CYP drug-metabolizing enzymes and the P-glycoprotein drug transporter. In rats, onion given by mouth with cyclosporin sharply cut cyclosporin absorption from the gut, while cyclosporin given by injection was unaffected, which points to an interaction at the gut wall.

Clinical trials

1
Onion Skin Extract and 24-Hour Blood Pressure: Crossover RCT
PubMed

Randomized, double-blind, placebo-controlled crossover trial: 162 mg a day of quercetin from onion skin extract powder (three capsules of 54 mg) or placebo for 6 weeks each, with a 6-week washout. Funded by the German Research Foundation. (Brüll et al. 2015, Br J Nutr)

70 overweight-to-obese adults in Germany with pre-hypertension or stage 1 hypertension.

In the whole group, 24-hour ambulatory and office blood pressure did not change versus placebo. In the subgroup with hypertension, 24-hour systolic pressure fell by 3.6 mmHg (treatment difference versus placebo 3.9 mmHg, P = 0.049); day-time and night-time changes were not significant between treatments. Endothelial function, vasoactive markers, lipids, glucose, oxidation and inflammation markers did not change.

2
Onion Skin Extract, Leptin, Glucose and Safety Markers: Crossover RCT
PubMed

Second report from the same randomized, double-blind, placebo-controlled crossover trial (162 mg a day of quercetin from onion skin extract for 6 weeks). (Brüll et al. 2017, Eur J Nutr)

68 of the 70 overweight-to-obese adults randomized in the 6-week crossover trial above (2 dropped out for personal reasons).

Compared with placebo, quercetin did not change leptin, adiponectin, glucose, insulin or HOMA-IR, and neither treatment changed CRP or TNF-alpha. Liver and kidney function tests, blood counts and electrolytes did not differ from placebo, and the authors judged the dose safe over 6 weeks.

3
Onion Peel Extract and Flow-Mediated Dilation: 12-Week RCT
PubMed

Randomized, double-blind, placebo-controlled trial of onion peel extract soft capsules (50 mg quercetin twice daily, 100 mg a day) for 12 weeks. (Choi et al. 2015, Nutrition)

72 healthy overweight and obese adults in Korea (36 extract, 36 placebo).

Flow-mediated dilation rose from 12.5% to 15.2% in the extract group (P = 0.002 versus baseline), with no change on placebo; nitroglycerin-mediated dilation did not change in either group. Endothelial progenitor cell counts rose in the extract group. The abstract reports changes within each group rather than a direct comparison with placebo.

4
Onion Peel Extract in Male Smokers: 10-Week RCT
PubMed

Randomized, double-blind, placebo-controlled parallel trial of quercetin-rich capsules made from onion peel extract (four capsules a day supplying 100 mg of quercetin dihydrate plus 128 mg of other flavonoids of unknown composition) for 10 weeks. (Lee et al. 2011, Nutr Res Pract)

92 healthy male smokers in Korea (49 quercetin, 43 placebo).

Total and LDL cholesterol fell within the quercetin group but not the placebo group. HDL cholesterol rose in both groups, more with quercetin. Systolic and diastolic blood pressure and glucose fell within the quercetin group. IL-6 and sVCAM-1 did not change. Most results are within-group changes, and no adverse effects were reported.

5
Onion Peel Extract and Body Fat: 12-Week RCT
PubMed

Randomized, double-blind, placebo-controlled parallel trial of onion peel extract capsules supplying 100 mg of quercetin a day (170 mg extract with 50 mg quercetin per capsule) for 12 weeks, funded by a Korean government agri-food research grant. (Lee et al. 2016, Nutr Res Pract)

72 overweight and obese Korean adults (11 men, 61 women).

Body weight and body fat percentage measured by DXA fell in the extract group (P = 0.02) but not in the placebo group. Blood glucose and leptin fell in both groups, and resting energy expenditure rose in both groups. The abstract reports within-group changes.

6
Quercetin-Rich Onion Powder and Visceral Fat: 12-Week RCT
PubMed

Randomized, double-blind, placebo-controlled parallel trial: 9 g a day of quercetin-rich onion powder (about 60 mg quercetin) or quercetin-free onion powder for 12 weeks, funded by a Japanese public research program. (Nishimura et al. 2019, Nutrients)

70 healthy Japanese adults with a BMI of 23 to under 30.

The main outcome, visceral fat area on CT, did not differ between groups. In an exploratory subgroup with HDL cholesterol of 40 to under 74 mg/dL, visceral fat area was lower with the quercetin-rich onion. Clinic and home blood pressure did not improve, and the liver enzyme ALT was lower than on placebo.

7
Quercetin-Rich Onion Powder and Cognitive Scores: 24-Week RCT
PubMed

Randomized, double-blind, placebo-controlled parallel trial of quercetin-rich onion powder (about 50 mg quercetin a day) versus quercetin-free onion powder for 24 weeks, funded by a Japanese public research grant. (Nishihira et al. 2021, J Clin Biochem Nutr)

70 healthy Japanese adults aged 60 to 79.

Mini-Mental State Examination scores improved more with the quercetin-rich onion than with placebo after 24 weeks. Emotional function scores on a tablet-based assessment also improved, but its total cognitive score did not differ between groups, and a caregiver-burden scale favored the active food. The authors note an earlier 50-person trial of the same design found no overall MMSE difference. No serious adverse events or abnormal laboratory changes were reported.

8
Onion Skin Extract vs Quercetin Dihydrate: Absorption Crossover
PubMed

Randomized, single-blind, diet-controlled crossover study comparing one dose of onion skin extract (about 163 mg quercetin) with quercetin dihydrate (134 mg quercetin) in hard capsules. (Burak et al. 2017, Eur J Nutr)

12 healthy adults aged 21 to 33 (6 men, 6 women).

Blood quercetin exposure (area under the curve) was 4.8 times higher and peak level 5.4 times higher after the onion skin extract; time to peak did not differ. Peak levels of the metabolites isorhamnetin and tamarixetin were 3.8 and 4.4 times higher. A single-dose absorption study with no health outcomes.

Side effects and drug interactions

Common Potential side effects

In a 6-week trial at 162 mg of quercetin a day from onion skin extract, liver and kidney function tests, blood counts and electrolytes did not differ from placebo.
No adverse effects were reported in a 10-week trial at 100 mg of quercetin a day from onion peel extract, and no serious adverse events or abnormal laboratory changes were seen over 24 weeks of quercetin-rich onion powder.
The trials lasted 6 to 24 weeks; longer-term use of concentrated onion extracts has not been studied.
Avoid onion products if you have had an allergic reaction to onion.

Important Drug interactions

Cyclosporine: in rats, onion given by mouth with cyclosporin cut its peak blood level by 60% and its total exposure by 68%, and the authors advise avoiding quercetin-rich foods or herbs with cyclosporin. Do not use onion extract with cyclosporine without your prescribing doctor's advice.
Other prescription medicines: quercetin affects drug-metabolizing enzymes (CYP) and the P-glycoprotein transporter in laboratory studies, so ask a pharmacist before combining a concentrated onion or quercetin product with medicines that have a narrow safe range.
Blood pressure medicines: in one trial, onion skin extract lowered 24-hour systolic pressure by a few mmHg in a subgroup with hypertension; tell your doctor if you combine them.

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Frequently asked questions about Onion Extract (Allium cepa, Quercetin-Rich)

Is onion extract the same as a quercetin supplement?

No. Onion extracts supply quercetin from onion at lower doses: trials used about 50 to 162 mg of quercetin a day, while many pure quercetin trials used 500 mg a day or more. In human studies, quercetin from onion or onion skin extract reached higher blood or urine levels than pure quercetin; in one, onion soup with 47 mg of quercetin gave urinary quercetin similar to a 544 mg tablet. See our separate quercetin page for trials of the pure compound.

How much onion extract was used in the studies?

Onion peel or skin extracts were dosed by their quercetin content, 100 to 162 mg of quercetin a day for 6 to 12 weeks, for example 170 mg capsules of peel extract with 50 mg of quercetin taken twice daily. Japanese trials used 9 g a day of quercetin-rich onion powder supplying about 50 to 60 mg of quercetin. Check the label for the quercetin content.

What did the blood pressure studies show?

In the largest controlled trial, 162 mg a day of quercetin from onion skin extract for 6 weeks did not change blood pressure in the whole group of 70 overweight adults; only the subgroup with hypertension saw 24-hour systolic pressure fall by about 4 mmHg. A single dose with a meal had no effect. Do not use onion extract in place of blood pressure care from your doctor.

Is onion extract safe to take?

In trials lasting 6 to 24 weeks, onion extracts and quercetin-rich onion powder were well tolerated, with no changes in liver, kidney or blood count tests versus placebo in one 6-week trial. Longer use has not been studied. Ask your doctor first if you take cyclosporine or other prescription medicines, since onion reduced cyclosporin absorption in rats.

What is Onion Extract?

Onion extract is made from the onion bulb (Allium cepa) or, more often, its dry outer skin, a rich source of the flavonoid quercetin. In the bulb, quercetin is mostly bound to glucose, and in human absorption studies this form was taken up better than pure quercetin. An onion skin extract also gave 4.

What is Onion Extract used for?

Onion Extract is researched primarily for Cardiovascular. In a crossover trial of 70 overweight adults with high-normal or high blood pressure, 162 mg a day of quercetin from onion skin extract for 6 weeks did not change 24-hour or office blood pressure in the whole group.

What is the recommended dosage of Onion Extract?

The clinically studied dose is Onion skin or peel extract supplying 100 to 162 mg of quercetin a day for 6 to 12 weeks (for example 170 mg of peel extract with 50 mg quercetin, twice daily); quercetin-rich onion powder (9 g a day) supplying about 50 to 60 mg of quercetin for 12 to 24 weeks. Always follow the product label and check with a healthcare provider for personal advice.

Is Onion Extract safe, and does it have side effects?

Side effects and cautions reported for Onion Extract include: In a 6-week trial at 162 mg of quercetin a day from onion skin extract, liver and kidney function tests, blood counts and electrolytes did not differ from placebo. It may also interact with some medications. Onion Extract is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Onion Extract interact with any medications?

Possible interactions include: Cyclosporine: in rats, onion given by mouth with cyclosporin cut its peak blood level by 60% and its total exposure by 68%, and the authors advise avoiding quercetin-rich foods or herbs with cyclosporin. If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Onion Extract?

NutraSmarts rates the evidence for Onion Extract as Limited (2 out of 5). It is backed by 8 clinical trials and 15 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(15 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Brüll V, Burak C, Stoffel-Wagner B, Wolffram S, Nickenig G, Müller C, Langguth P, Alteheld B, Fimmers R, Naaf S, Zimmermann BF, Stehle P, Egert S. Effects of a quercetin-rich onion skin extract on 24 h ambulatory blood pressure and endothelial function in overweight-to-obese patients with (pre-)hypertension: a randomised double-blinded placebo-controlled cross-over trial. Br J Nutr. 2015;114(8):1263-77. doi: 10.1017/S0007114515002950.PubMedUsed to support: Randomized double-blind placebo-controlled crossover trial in 70 overweight-to-obese adults with pre-hypertension or stage 1 hypertension: 162 mg a day of quercetin from onion skin extract for 6 weeks did not change 24-hour or office blood pressure in the whole group; in the subgroup with hypertension, 24-hour systolic pressure fell 3.6 mmHg (treatment difference 3.9 mmHg, P = 0.049). Endothelial function, vasoactive, lipid, glucose, oxidation and inflammation markers did not change. Funded by the German Research Foundation.
  2. Brüll V, Burak C, Stoffel-Wagner B, Wolffram S, Nickenig G, Müller C, Langguth P, Alteheld B, Fimmers R, Stehle P, Egert S. No effects of quercetin from onion skin extract on serum leptin and adiponectin concentrations in overweight-to-obese patients with (pre-)hypertension: a randomized double-blinded, placebo-controlled crossover trial. Eur J Nutr. 2017;56(7):2265-2275. doi: 10.1007/s00394-016-1267-0.PubMedUsed to support: Same crossover trial (68 analyzed): 162 mg a day of quercetin from onion skin extract for 6 weeks did not change leptin, adiponectin, glucose, insulin, HOMA-IR, CRP or TNF-alpha versus placebo, and liver, kidney, blood count and electrolyte measures did not differ from placebo.
  3. Brüll V, Burak C, Stoffel-Wagner B, Wolffram S, Nickenig G, Müller C, Langguth P, Alteheld B, Fimmers R, Stehle P, Egert S. Acute intake of quercetin from onion skin extract does not influence postprandial blood pressure and endothelial function in overweight-to-obese adults with hypertension: a randomized, double-blind, placebo-controlled, crossover trial. Eur J Nutr. 2017;56(3):1347-1357. doi: 10.1007/s00394-016-1185-1.PubMedUsed to support: Randomized double-blind crossover meal study in 22 overweight-to-obese adults with hypertension: a single 54 mg dose of quercetin from onion skin extract taken with a high-fat, high-carbohydrate meal did not change blood pressure, endothelial function (reactive hyperemia index), lipids, glucose or insulin over 4 hours versus placebo.
  4. Choi EY, Lee H, Woo JS, Jang HH, Hwang SJ, Kim HS, Kim WS, Kim YS, Choue R, Cha YJ, Yim JE, Kim W. Effect of onion peel extract on endothelial function and endothelial progenitor cells in overweight and obese individuals. Nutrition. 2015;31(9):1131-5. doi: 10.1016/j.nut.2015.04.020.PubMedUsed to support: Randomized double-blind placebo-controlled trial in 72 healthy overweight and obese adults: onion peel extract with 100 mg quercetin a day for 12 weeks raised flow-mediated dilation from 12.5% to 15.2% within the extract group, with no change on placebo, and raised circulating endothelial progenitor cells. Within-group comparisons in the abstract.
  5. Lee KH, Park E, Lee HJ, Kim MO, Cha YJ, Kim JM, Lee H, Shin MJ. Effects of daily quercetin-rich supplementation on cardiometabolic risks in male smokers. Nutr Res Pract. 2011;5(1):28-33. doi: 10.4162/nrp.2011.5.1.28.PubMedUsed to support: Randomized double-blind placebo-controlled trial in 92 healthy male smokers: quercetin-rich onion peel extract capsules (100 mg quercetin a day) for 10 weeks lowered total and LDL cholesterol, blood pressure and glucose within the quercetin group, and raised HDL cholesterol more than placebo. IL-6 and sVCAM-1 did not change.
  6. Lee JS, Cha YJ, Lee KH, Yim JE. Onion peel extract reduces the percentage of body fat in overweight and obese subjects: a 12-week, randomized, double-blind, placebo-controlled study. Nutr Res Pract. 2016;10(2):175-81. doi: 10.4162/nrp.2016.10.2.175.PubMedUsed to support: Randomized double-blind placebo-controlled trial in 72 overweight and obese Korean adults: onion peel extract with 100 mg quercetin a day for 12 weeks lowered body weight and DXA body fat percentage within the extract group, not in the placebo group; glucose and leptin fell in both groups. Funded by a Korean government agri-food research grant. A published erratum (PMID 27231523) corrects the first author's name to Ji-Sook Park; PubMed still lists the author as Lee JS.
  7. Kim KA, Yim JE. The Effect of Onion Peel Extract on Inflammatory Mediators in Korean Overweight and Obese Women. Clin Nutr Res. 2016;5(4):261-269. doi: 10.7762/cnr.2016.5.4.261.PubMedUsed to support: Randomized double-blind placebo-controlled trial in 37 overweight and obese women: onion peel extract with 100 mg quercetin a day for 12 weeks did not change ALT, AST, leptin, visfatin, TNF-alpha or IL-4 versus placebo; body fat percentage fell in both groups. Funded by Changwon National University.
  8. Nishimura M, Muro T, Kobori M, Nishihira J. Effect of Daily Ingestion of Quercetin-Rich Onion Powder for 12 Weeks on Visceral Fat: A Randomised, Double-Blind, Placebo-Controlled, Parallel-Group Study. Nutrients. 2019;12(1):91. doi: 10.3390/nu12010091.PubMedUsed to support: Randomized double-blind placebo-controlled trial in 70 healthy Japanese adults with a BMI of 23 to under 30: 9 g a day of quercetin-rich onion powder for 12 weeks did not change visceral fat area versus quercetin-free onion powder; visceral fat was lower in an exploratory subgroup with HDL cholesterol of 40 to under 74 mg/dL, clinic and home blood pressure did not improve, and ALT was lower than on placebo. Funded by a Japanese public research program.
  9. Jeong S, Chae J, Lee G, Shin G, Kwon YI, Oh JB, Shin DY, Lee JH. Effect of Steamed Onion (ONIRO) Consumption on Body Fat and Metabolic Profiles in Overweight Subjects: A 12-Week Randomized, Double-Blind, Placebo-Controlled Clinical Trial. J Am Coll Nutr. 2020;39(3):206-215. doi: 10.1080/07315724.2019.1635052.PubMedUsed to support: Randomized double-blind placebo-controlled trial in 70 overweight adults: capsules of Jeju steamed onion (ONIRO) taken before meals for 12 weeks reduced DXA body fat percentage and fat mass versus placebo, reduced CT visceral and subcutaneous fat areas, and lowered triglycerides and C-peptide, with no change in lean mass. The trial registry (NCT03645382) lists 56 participants actually enrolled and a baked barley powder placebo.
  10. Nishihira J, Nishimura M, Kurimoto M, Kagami-Katsuyama H, Hattori H, Nakagawa T, Muro T, Kobori M. The effect of 24-week continuous intake of quercetin-rich onion on age-related cognitive decline in healthy elderly people: a randomized, double-blind, placebo-controlled, parallel-group comparative clinical trial. J Clin Biochem Nutr. 2021;69(2):203-215. doi: 10.3164/jcbn.21-17.PubMedUsed to support: Randomized double-blind placebo-controlled trial in 70 healthy Japanese adults aged 60 to 79: 24 weeks of quercetin-rich onion powder (about 50 mg quercetin a day) improved Mini-Mental State Examination and emotional function scores versus quercetin-free onion powder. Funded by a Japanese public research grant.
  11. Burak C, Brüll V, Langguth P, Zimmermann BF, Stoffel-Wagner B, Sausen U, Stehle P, Wolffram S, Egert S. Higher plasma quercetin levels following oral administration of an onion skin extract compared with pure quercetin dihydrate in humans. Eur J Nutr. 2017;56(1):343-353. doi: 10.1007/s00394-015-1084-x.PubMedUsed to support: Randomized single-blind crossover study in 12 healthy adults: one dose of onion skin extract (about 163 mg quercetin, mostly as free quercetin) gave 4.8 times higher blood quercetin exposure and 5.4 times higher peak level than quercetin dihydrate.
  12. Hollman PC, de Vries JH, van Leeuwen SD, Mengelers MJ, Katan MB. Absorption of dietary quercetin glycosides and quercetin in healthy ileostomy volunteers. Am J Clin Nutr. 1995;62(6):1276-82. doi: 10.1093/ajcn/62.6.1276.PubMedUsed to support: Study in 9 healthy ileostomy volunteers: absorption was 52% for quercetin glucosides from fried onions, 17% for quercetin rutinoside and 24% for pure quercetin aglycone; the authors conclude that conjugation with glucose enhances absorption.
  13. Graefe EU, Wittig J, Mueller S, Riethling AK, Uehleke B, Drewelow B, Pforte H, Jacobasch G, Derendorf H, Veit M. Pharmacokinetics and bioavailability of quercetin glycosides in humans. J Clin Pharmacol. 2001;41(5):492-9. doi: 10.1177/00912700122010366.PubMedUsed to support: Four-way crossover in 12 healthy volunteers: an onion supplement and pure quercetin-4'-glucoside (each 100 mg quercetin) gave similar blood levels, peaking at about 0.7 hours, while rutin and buckwheat tea (200 mg quercetin) gave lower peaks after 4 to 7 hours; only quercetin glucuronides, no free quercetin, were found in plasma, with a half-life of about 11 hours.
  14. Shi Y, Williamson G. Comparison of the urinary excretion of quercetin glycosides from red onion and aglycone from dietary supplements in healthy subjects: a randomized, single-blinded, cross-over study. Food Funct. 2015;6(5):1443-8. doi: 10.1039/c5fo00155b.PubMedUsed to support: Randomized crossover study in 6 healthy men: onion soup from 100 g of red onion (47 mg quercetin) gave 24-hour urinary quercetin excretion similar to a 544 mg quercetin dihydrate tablet; the authors estimate about 166 mg of supplement quercetin is comparable to about 10 mg of quercetin from onion.
  15. Yang CY, Chao PD, Hou YC, Tsai SY, Wen KC, Hsiu SL. Marked decrease of cyclosporin bioavailability caused by coadministration of ginkgo and onion in rats. Food Chem Toxicol. 2006;44(9):1572-8. doi: 10.1016/j.fct.2006.04.008.PubMedUsed to support: Rat study: oral ginkgo and onion given with oral cyclosporin cut cyclosporin peak level by 62% and 60% and total exposure by 51% and 68%, with no effect on intravenous cyclosporin, pointing to an interaction at the gut absorption site. Animal data only.