Vegetables · Vegetable
Onions
Allium cepa · also called Bulb onion, common onion, cebolla (Spanish)
Onions are the bulbs of a plant in the allium family, eaten raw, cooked or pickled mostly for flavour. They are low in calories and give some fibre, vitamin C and vitamin B6. Small drops in blood pressure were seen with quercetin supplements, but no trial shows that eating onions treats or prevents a disease. The main cautions are irritable bowel syndrome (onions are very high in fructans), rare allergy, and that onions are toxic to dogs, cats and horses.
General information, not medical advice. Last substantive update · Not medically reviewed. A named clinician has not signed off on this page. How this page is made
About Onions
Onions are a low-calorie flavouring vegetable. They are a food, not a proven remedy: most health findings are for quercetin supplements, onion peel, capsules or powders, or for allium vegetables in general.
What it is
Onion (Allium cepa) is the bulb of a plant in the amaryllis family. People eat it raw, cooked, pickled, frozen, canned or dried.
USDA data for raw onion: 40 kcal per 100 g. One cup of chopped onion (160 g) has about 64 kcal, 2.7 g fibre, 6.8 g sugars, 11.8 mg vitamin C, 0.19 mg vitamin B6, 30 micrograms folate (DFE) and 234 mg potassium. One medium onion (110 g) has about 44 kcal and 1.9 g fibre. Raw onion has almost no vitamin K or vitamin A.
What the research shows
The clearest finding is for quercetin supplements: three meta-analyses found a small drop in blood pressure. Onion is a source of quercetin, but those trials did not test onions.
Trials of onion peel extract, a steamed onion capsule and onion powder gave mixed results on body fat. We found no trial of eating whole onions with a clear health result.
Observational studies found no clear link between allium vegetables and overall cancer risk, and a mixed link with low mood. Observational studies cannot prove cause.
Food versus supplements and extracts
The blood pressure results are for quercetin supplements, and the body fat results are for onion peel, capsules and powder. A result for a capsule or extract does not carry over to the onion on your plate. Cooking changes the compounds in allium vegetables in ways that depend on method, time and temperature.
Amounts
This page gives no amounts to eat. Ask a doctor, pharmacist or dietitian what is right for you, especially if you have IBS, take medicines or use supplements.
At a glance
Traditional uses history, not proof
- Eaten as a vegetable and seasoning, raw, cooked or pickled
- Long use as food is not proof that it treats any condition
Studied for research
- Blood pressure, low certainty (trials of quercetin supplements, not onions)
- Body fat and weight, mixed certainty (onion peel, capsules and powder)
- Cholesterol, low certainty (onion supplements and a garlic plus onion extract)
- Blood sugar, insufficient evidence
- Cancer risk, mixed certainty (allium vegetables as a group, observational)
- Low mood, very low certainty (two cohorts, observational)
Main compounds
- Quercetin
- Fructans
- Organosulfur compounds
- Vitamin C
- Vitamin B6
- Fibre
- Potassium
Common forms
- Raw, cooked, pickled, frozen or canned onion as food
- Dried onion flakes and onion powder as seasoning
- Onion peel extract, steamed onion capsules and quercetin supplements (used in trials, not the same as the food)
Research links: Fructan content in common foods (2024) · ASPCA: Onion (toxic plants) · Quercetin and blood pressure: meta-analysis (2020) · Research Library · Genes & Biomarkers Library
What it is and its history
- Accepted scientific name
- Allium cepa
- Naming authority
- L. (GBIF accepted)
- Common names
- Onion (English); Bulb onion (English); Common onion (English); Cebolla (Spanish)
- Family or group
- Amaryllidaceae (the amaryllis family), as given by GBIF. The ASPCA page lists Liliaceae.
- Classification
- Plantae > Tracheophyta > Liliopsida > Asparagales > Amaryllidaceae > Allium > Allium cepa
- Native range
- Not verified from a source read.
- Habitat
- Not verified from a source read.
- What it looks like
- Roots: Not verified.; Stems: Not verified.; Leaves: Not verified. The bulb is the food. USDA gives household portions of 1 cup chopped as 160 g, 1 medium onion as 110 g, 1 large onion as 150 g and 10 rings as 60 g.; Flowers: Not verified.; Fruit: Not verified.
- Cultivation
- Not verified from a source read.
- Harvest and processing
- Eaten raw, cooked, pickled, frozen or canned, dried as flakes or powder, or used as peel extract or quercetin capsules in trials (existing nutrient summary). The effect of cooking on nutrients was not read from a source.
- Look-alikes and adulterants
- Name: Other allium vegetables; Note: Garlic, leeks, shallots, chives and spring onions are related plants. Many studies pool allium vegetables, so their results are not onion-only.; Name: Green onion root (Allium fistulosum); Note: A different species used in some trials. It is not Allium cepa and is not covered here.
- Sustainability and conservation
- Not assessed.
Photographs
What is inside, and how it may work
What it contains and how those compounds act in cells and animals. Laboratory and animal mechanisms are not proven effects in people.
Main compounds
| Compound | Family | Where it is concentrated | Activity studied | Level of evidence |
|---|---|---|---|---|
| Quercetin PubChem 5280343 | flavonol | Bulb (and peel) | A plant pigment tested as a supplement for blood pressure; the supplement trials lowered it by about 2 to 3 mmHg. 14 8 13 | Human data |
| Fructans (short-chain) | carbohydrate (FODMAP) | Bulb 1.97 g per 100 g fresh onion (2024 measurement of Slovenian foods) | Ferment quickly in the gut and can worsen IBS symptoms. 6 11 | Human data |
| Dipropyl disulfide (N-propyl disulfide) PubChem 12377 | organosulfur compound | Bulb | Named by the ASPCA as the toxic principle of onion for pets. 20 | Animal research |
Mechanisms, receptors and pathways
| Target or pathway | Gene or receptor | What was found | Level |
|---|---|---|---|
| Fructan fermentation in the gut | Not specified | Short-chain fructans have a rapid fermentation rate and can worsen symptoms in people with IBS. 6 | Human data |
| Red blood cell damage in dogs | Glucose-6-phosphate dehydrogenase (in dog red cells) | In dogs fed cooked onions, red cell glutathione and G6PD fell and oxidative damage rose, with Heinz bodies and hemolytic anemia. 15 | Animal research |
| Oxidative stress and inflammation (NF-kB, Nrf2) | NF-kB and Nrf2 | Allium compounds are described as modulating these pathways in lab studies. Limited clinical validation. 1 | Laboratory research |
Absorption, metabolism and elimination
- Absorption
- The 2025 review names low bioavailability of allium compounds as a barrier. Numbers were not read.
- Distribution
- Not found.
- Metabolism
- Not found.
- Elimination
- Not found.
- Bioavailability
- Not established for onion compounds in the sources read.
Isolated compounds versus whole extracts
The blood pressure results come from quercetin supplements and the body fat results from onion peel, capsules and powder, so none tell us about eating onions. A garlic plus onion extract cannot be credited to onion alone. Food, powder, peel and extract are different products. Preparation and cooking change the compounds in allium vegetables in ways that depend on method, time and temperature.
Nutritional composition
Nutrition from USDA FoodData Central (Onions, raw, FDC 170000, SR Legacy, last updated 4/1/2019, read 2026-10-26). Per 100 g: 40 kcal, 1.1 g protein, 0.1 g fat, 9.34 g carbohydrate, 1.7 g fibre, 4.24 g sugars, about 89 g water, 23 mg calcium, 0.21 mg iron, 10 mg magnesium, 29 mg phosphorus, 146 mg potassium, 4 mg sodium, 7.4 mg vitamin C, 0.12 mg vitamin B6, 19 micrograms folate (DFE), 0.4 micrograms vitamin K, 0.129 mg manganese, 0.039 mg copper, 4 micrograms lutein plus zeaxanthin, no vitamin A (RAE). One cup of chopped raw onion (160 g): about 64 kcal, 1.8 g protein, 14.9 g carbohydrate, 2.7 g fibre, 6.8 g sugars, 37 mg calcium, 234 mg potassium, 11.8 mg vitamin C, 0.19 mg vitamin B6, 30 micrograms folate (DFE). One medium onion (110 g): about 44 kcal, 10.3 g carbohydrate, 1.9 g fibre, 161 mg potassium, 8.1 mg vitamin C. Per-serving figures are our own arithmetic from the per-100 g values. Figures vary with variety, ripeness and storage. The quercetin content of onions and cooked onion values were not read from a source.
2 mixed findings2 low certainty1 very low certainty2 not enough evidence
What was looked at
The 7 benefits and uses checked for Onions, each with how strong the evidence is. Tap one to open it.
- Blood pressure (quercetin supplements)Low certainty
- Body fat and weight (onion peel, capsules and powder)Mixed findings
- Cholesterol and blood fats (onion supplements)Low certainty
- Blood sugarNot enough evidence
- Cancer risk (allium vegetables as a group)Mixed findings
- Low mood (depressive symptoms)Very low certainty
- Antioxidant and anti-inflammatory effectsNot enough evidence
Lab markers tracked in the studies
Systolic blood pressure after quercetin supplementsDiastolic blood pressure after quercetin supplementsVisceral (belly) fat areaLDL cholesterol after onion supplementsBlood glucose after quercetin supplementsOverall cancer risk and high allium vegetable intake
What is Onions good for?
The main studied uses and how strong the human evidence is for each. Most results are for quercetin supplements, onion peel, capsules or powders, or allium vegetables as a group, not for eating onions.
Benefits, one by one
Each card is a research question, not a promise. Tap a goal to filter. Open a card to see the studies, doses and limits.
Blood pressure (quercetin supplements)
Low certaintyOnions contain quercetin, a plant pigment. Three meta-analyses of trials found that quercetin supplements lowered blood pressure by a few points. These trials tested supplements, not onions on the plate, so they do not show the same effect from eating onions.
3 studiesQuercetin supplements or quercetin-enriched extracts (not onions).
Studies, doses and limits
What the trials found
Serban 2016 pooled 7 randomised trials (587 adults): systolic pressure fell 3.04 mm Hg and diastolic 2.63 mm Hg. Huang 2020 pooled 17 trials (896 people): systolic fell 3.09 mmHg and diastolic 2.86 mmHg. Popiolek-Kalisz 2022 pooled 10 trials (841 people): systolic fell 2.38 mmHg overall.
Results
Quercetin supplements lowered blood pressure by about 2 to 3 mmHg in the pooled trials. No trial of eating onions for blood pressure was read in these reviews.
Effect sizes as reported
Systolic: -3.04 (CI -5.75 to -0.33), -3.09 (CI -4.59 to -1.59), -2.38 mmHg (CI -3.80 to -0.96). Diastolic: -2.63, -2.86 mmHg (and -3.14 in the (pre)hypertensive subgroup).
Why this rating
Low for onions as food. The three meta-analyses agree on a small drop, but all tested quercetin supplements or extracts, no GRADE rating was given in the abstracts, and a 2 to 3 mmHg change is small. Evidence for eating onions themselves is insufficient.
Form and dose studied
Quercetin supplements or quercetin-enriched extracts (not onions).
Who was studied
Adults, including people with normal and raised blood pressure.
Limits
Supplements are not the food. Amount of quercetin in a serving of onion was not read from a source, so no link between a trial dose and a meal is made.
Why researchers looked
Onion is a well-known quercetin food, and quercetin has been tested for blood pressure.
Traditional use
Onions are eaten as a vegetable, raw, cooked or pickled, mostly for flavour. Long use as food is not proof that it treats any condition.
Proposed mechanism Human data
Quercetin is a plant flavonol with antioxidant effects in the lab. How it lowers blood pressure in people was not explained in the abstracts read.
A proposed mechanism does not show that the effect happens in people.
People who take blood pressure medicines should ask a clinician or pharmacist before starting quercetin supplements.
Body fat and weight (onion peel, capsules and powder)
Mixed findingsA 2023 meta-analysis of 5 trials in people with obesity found lower weight, BMI, waist size and triglycerides with onion products, mostly onion peel. A trial of onion powder in healthy overweight adults found no overall change in belly fat. These are not tests of eating whole onions.
3 studiesOnion peel extract, a steamed onion capsule and onion powder (not who…
Studies, doses and limits
What the trials found
Chung 2023: 5 trials in people with obesity (4 used onion peel, 1 used onion); body weight, BMI, waist circumference and triglycerides were significantly lower than placebo, and the authors say the effect was greatest with onion peel. Jeong 2020: 70 overweight adults took a steamed onion capsule for 12 weeks; body fat percentage and fat mass fell compared with placebo. Nishimura 2019: 70 healthy adults took onion powder for 12 weeks; no significant difference in visceral fat area overall.
Results
Results are mixed. Onion peel and a steamed onion capsule lowered body fat measures in trials. Onion powder did not change belly fat overall, with a lower-HDL subgroup showing a difference. No trial of eating whole onions for body fat was read.
Effect sizes as reported
Chung: numbers not in the abstract. Jeong: body fat percentage P=0.028, fat mass P=0.022 versus placebo. Nishimura: visceral fat area, no significant difference overall.
Why this rating
Mixed: two of three sources report lower body fat, one reports no overall change. All use peel, capsules or powder, groups are small (70 people in each single trial) and numbers were not given in the abstracts. For eating onions, evidence is insufficient.
Form and dose studied
Onion peel extract, a steamed onion capsule and onion powder (not whole onions).
Who was studied
Adults with obesity or overweight.
Limits
Onion peel is not the part people normally eat. Short trials of 12 weeks. Single-site trials.
Why researchers looked
Researchers have tested onion products, including peel (not usually eaten), for weight and body fat.
Traditional use
Onions are eaten as a vegetable, raw, cooked or pickled, mostly for flavour. Long use as food is not proof that it treats any condition.
Proposed mechanism Human data
The abstracts read do not explain how onion products might change body fat.
A proposed mechanism does not show that the effect happens in people.
No harm was named in the abstracts. Anyone with a weight concern should ask a clinician.
Cholesterol and blood fats (onion supplements)
Low certaintyA 2023 meta-analysis of 14 trials of onion supplements found improvements in LDL, total and HDL cholesterol. A small trial of a garlic plus onion extract also lowered LDL. These are supplements and extracts, not onions as food.
3 studiesOnion supplements of varied form and a garlic plus onion extract (not…
Studies, doses and limits
What the trials found
Hejazi 2023 pooled 14 randomised trials of onion supplementation: LDL cholesterol, total cholesterol and HDL cholesterol improved, with more change in trials over 12 weeks. Vezza 2024: an 8-week randomised trial of a garlic plus onion extract in adults with slightly raised cholesterol lowered LDL and total cholesterol. Huang 2020 (quercetin supplements): blood lipids did not change significantly overall.
Results
Onion supplements lowered LDL and total cholesterol in the pooled trials, while the separate quercetin meta-analysis did not find an overall change in lipids. No trial of eating onions for cholesterol was read.
Effect sizes as reported
Effect sizes were not in the abstracts for Hejazi or Vezza.
Why this rating
Low: supplement and extract trials only, numbers not read, variable products, one trial combines garlic with onion, and the quercetin meta-analysis did not agree.
Form and dose studied
Onion supplements of varied form and a garlic plus onion extract (not onions as food).
Who was studied
Adults, including people with slightly raised cholesterol.
Limits
Products differ between trials. Funding of the pooled trials was not read. The garlic plus onion trial cannot be credited to onion alone.
Why researchers looked
Onion products have been tested for blood cholesterol and related measures.
Traditional use
Onions are eaten as a vegetable, raw, cooked or pickled, mostly for flavour. Long use as food is not proof that it treats any condition.
Proposed mechanism Human data
The abstracts read do not explain how onion products might change blood fats.
A proposed mechanism does not show that the effect happens in people.
People on cholesterol medicines should ask a clinician before using supplements.
Blood sugar
Not enough evidenceNo trial of eating onions for blood sugar was read. A meta-analysis of quercetin supplements found no significant change in blood glucose.
3 studiesQuercetin supplements and onion supplements (not onions as food).
Studies, doses and limits
What the trials found
Huang 2020 pooled 17 randomised trials of quercetin supplements (896 people): glucose concentrations did not change significantly. Hejazi 2023 searched for blood sugar measures in 14 onion supplement trials; its abstract lists LDL, cholesterol, body fat, systolic pressure, adiponectin and AST as improved, and does not list blood sugar measures.
Results
No benefit for blood sugar was shown in the sources read. USDA lists 9.34 g of carbohydrate and 4.24 g of sugars per 100 g raw onion.
Effect sizes as reported
Glucose: not significantly changed (Huang 2020). Numbers not in the abstract.
Why this rating
Insufficient: no trial of eating onions was read, and the supplement meta-analysis found no significant glucose change.
Form and dose studied
Quercetin supplements and onion supplements (not onions as food).
Who was studied
Adults in trials of supplements.
Limits
Supplements, not food. Abstract-level reading only.
Why researchers looked
Onion is promoted as a food for blood sugar, so researchers have looked at glucose in trials of onion products and quercetin.
Traditional use
Onions are eaten as a vegetable, raw, cooked or pickled, mostly for flavour. Long use as food is not proof that it treats any condition.
Proposed mechanism Human data
Not covered in the abstracts read.
A proposed mechanism does not show that the effect happens in people.
People who take diabetes medicines should follow their care team's advice.
Cancer risk (allium vegetables as a group)
Mixed findingsA meta-analysis of 22 observational studies found no significant link between eating more allium vegetables (onion, garlic and relatives) and overall cancer risk. An older meta-analysis linked high intake to lower stomach cancer risk but warned about confounding.
3 studiesAllium vegetables as food, pooled.
Studies, doses and limits
What the trials found
Zhang 2022 pooled 22 studies of allium vegetables: RR 0.97 (95% CI 0.92-1.03) for overall cancer. Zhou 2011 pooled 19 case-control and 2 cohort studies (543,220 people): highest versus lowest allium vegetable intake, odds ratio 0.54 (95% CI 0.43-0.65) for stomach cancer, with similar results for onion.
Results
No overall link with cancer. A link with lower stomach cancer risk in mainly case-control studies, which the authors say may be confounded.
Effect sizes as reported
Overall cancer RR 0.97 (CI 0.92-1.03). Stomach cancer OR 0.54 (CI 0.43-0.65).
Why this rating
Mixed and observational: one newer meta-analysis found no link with overall cancer, one older one found lower stomach cancer risk using weak study designs. Studies pooled several allium vegetables.
Form and dose studied
Allium vegetables as food, pooled.
Who was studied
Adults in observational studies.
Limits
Observational studies cannot show cause. Onion was not separated from garlic and other alliums in the overall result.
Why researchers looked
Lab studies of allium compounds suggested a cancer-protective effect, so researchers checked people's eating patterns.
Traditional use
Onions are eaten as a vegetable, raw, cooked or pickled, mostly for flavour. Long use as food is not proof that it treats any condition.
Proposed mechanism Laboratory research
Reviews describe organosulfur compounds and flavonoids acting on oxidative stress and inflammation in lab models. The 2025 review says clinical validation is limited.
A proposed mechanism does not show that the effect happens in people.
None specific.
Low mood (depressive symptoms)
Very low certaintyIn two large groups followed for years, eating more onions went with fewer depressive symptoms in one group and a weaker, non-significant trend in the other.
1 studiesOnions as food, reported by questionnaire or recall.
Studies, doses and limits
What the trials found
Yang 2023 followed 7,739 adults in Tianjin and 169,806 adults in the UK Biobank. Tianjin: hazard ratios for higher versus lowest intake 0.78, 0.73 and 0.77 (p for trend 0.01). UK Biobank: 0.79, 0.81 and 0.97 (p for trend 0.07).
Results
A lower risk in Tianjin. In the UK Biobank the highest intake group did not show a lower risk.
Effect sizes as reported
Tianjin HRs 0.73 to 0.78; UK Biobank HRs 0.79, 0.81, 0.97.
Why this rating
Very low: observational data, intake self-reported, two cohorts measured depression differently, and the second cohort's trend was not significant.
Form and dose studied
Onions as food, reported by questionnaire or recall.
Who was studied
Adults in China and the UK.
Limits
Cannot show cause; people who eat more onions may differ in other ways.
Why researchers looked
Onion compounds showed antidepressant-like effects in animals, so researchers looked at people's onion intake.
Traditional use
Onions are eaten as a vegetable, raw, cooked or pickled, mostly for flavour. Long use as food is not proof that it treats any condition.
Proposed mechanism Human data
Not established in the abstract read.
A proposed mechanism does not show that the effect happens in people.
Low mood needs a clinician's advice, not a food change.
Antioxidant and anti-inflammatory effects
Not enough evidenceReviews describe antioxidant and anti-inflammatory effects of onion compounds in cells and animals. The reviewers themselves note low absorption and limited testing in people.
1 studiesIsolated compounds and extracts in lab studies.
Studies, doses and limits
What the trials found
A 2025 review of allium vegetables names low bioavailability and limited clinical validation as barriers to using these findings in people.
Results
Lab and animal findings only. They do not show a benefit from eating onions.
Why this rating
Insufficient: lab and animal evidence only, as the review itself states.
Form and dose studied
Isolated compounds and extracts in lab studies.
Who was studied
Cells and animals.
Limits
A narrative review of several allium vegetables. Not a human trial.
Why researchers looked
Onions contain organosulfur compounds and flavonoids that react with oxidising molecules in the lab.
Traditional use
Onions are eaten as a vegetable, raw, cooked or pickled, mostly for flavour. Long use as food is not proof that it treats any condition.
Proposed mechanism Laboratory research
Allium compounds are described as acting on oxidative stress and inflammation pathways (NF-kB and Nrf2) in lab studies.
A proposed mechanism does not show that the effect happens in people.
None specific.
Connections to the rest of health
How Onions connects to the rest of our health knowledge graph. Every link says what kind of connection it is. A safety link (for example to the liver) is a warning, not a benefit. A study that merely mentions a topic never creates a benefit link.
Explore every connection: goals, body systems, organs, symptoms, genes
A. Wellness goals
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Blood Pressure Support | Low certainty | Blood pressure (quercetin supplements): Onions contain quercetin, a plant pigment. Three meta-analyses of trials found that quercetin supplements lowered blood pressure by a few points. These trials tested supplements, not onions on the plate, so they do not show the same effect from eating onions. | 14 8 13 | |
| Metabolic Health | Mixed findings | Body fat and weight (onion peel, capsules and powder): A 2023 meta-analysis of 5 trials in people with obesity found lower weight, BMI, waist size and triglycerides with onion products, mostly onion peel. A trial of onion powder in healthy overweight adults found no overall change in belly fat. These are not tests of eating whole onions. | 4 9 12 | |
| Cholesterol Balance | Low certainty | Cholesterol and blood fats (onion supplements): A 2023 meta-analysis of 14 trials of onion supplements found improvements in LDL, total and HDL cholesterol. A small trial of a garlic plus onion extract also lowered LDL. These are supplements and extracts, not onions as food. | 7 16 8 | |
| Blood Sugar Balance | Biological association | Not enough evidence | Blood sugar: No trial of eating onions for blood sugar was read. A meta-analysis of quercetin supplements found no significant change in blood glucose. | 8 7 24 |
B. Body systems
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Cardiovascular System | Biological association | Low certainty | Studied for: Blood pressure (quercetin supplements) Studied for: Cholesterol and blood fats (onion supplements) | 14 8 13 7 16 |
| Metabolism Weight | Biological association | Mixed findings | Studied for: Body fat and weight (onion peel, capsules and powder) Studied for: Blood sugar | 4 9 12 8 7 24 |
| Immune System | Biological association | Mixed findings | Studied for: Cancer risk (allium vegetables as a group) Studied for: Antioxidant and anti-inflammatory effects | 18 19 1 |
| Digestive System | Biological association | Mixed findings | Studied for: Cancer risk (allium vegetables as a group) | 18 19 1 |
| Nervous System | Biological association | Very low certainty | Studied for: Low mood (depressive symptoms) | 17 |
E. Health conditions
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Irritable bowel syndrome | Adverse effect or risk | Documented risk | Onion has the most fructans of 40 foods measured; fructans can worsen IBS symptoms in some people. Preparation or compound: Fructans in onions |
6 11 |
| Food allergy | Adverse effect or risk | Case report | Rare but severe reactions are reported, including to cooked onion. Rare but severe reactions to onion, including cooked onion, are reported. Preparation or compound: Onion |
2 |
I. Biomarkers
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Systolic blood pressure after quercetin supplements | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 14 8 13 |
| Diastolic blood pressure after quercetin supplements | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 14 8 13 |
| Visceral (belly) fat area | Biological association | Mixed findings | Measured in trials: mixed. A lab change is not the same as a health benefit. | 12 9 |
| LDL cholesterol after onion supplements | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 7 16 |
| Blood glucose after quercetin supplements | Biological association | Not enough evidence | Measured in trials: no-change. A lab change is not the same as a health benefit. | 8 |
| Overall cancer risk and high allium vegetable intake | Biological association | Low certainty | Measured in trials: no-change. A lab change is not the same as a health benefit. | 18 19 |
K. Biological pathways
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Fructans (FODMAPs) | Preliminary mechanism | Human | Onion is the highest-fructan food in a 40-food measurement; relevant to IBS. | 6 11 |
| Quercetin | Preliminary mechanism | Human | Onion is a source of the flavonol quercetin, tested as a supplement for blood pressure. | 14 8 |
L. Active compounds
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Quercetin | Biological association | Human | A plant pigment tested as a supplement for blood pressure; the supplement trials lowered it by about 2 to 3 mmHg. Preparation or compound: Bulb (and peel) |
14 8 13 |
| Fructans (short-chain) | Biological association | Human | Ferment quickly in the gut and can worsen IBS symptoms. Preparation or compound: Bulb |
6 11 |
| Dipropyl disulfide (N-propyl disulfide) | Biological association | Animal | Named by the ASPCA as the toxic principle of onion for pets. Preparation or compound: Bulb |
20 |
Each connection is stored with the ingredient, the connected record, the kind of connection, the evidence status, the supporting studies, the preparation or compound involved, and a review date (last reviewed October 26, 2026).
The evidence
The research centre for Onions. Every record here was checked against PubMed or the publisher's page; we never list a study we have not verified.
Evidence by outcome
Bars show how many studies we found for each outcome, not how well it works. People are added up across trials only (reviews are not added, to avoid double counting).
How we rate evidence
High certainty
Several well-run trials or reviews agree; further research is unlikely to change the picture much.
Moderate certainty
Good trials point the same way, but some doubt remains.
Low certainty
Small, short or inconsistent trials. The finding could change with better research.
Very low certainty
Very little or very weak human data. Treat as a possibility only.
Mixed findings
Trials or reviews disagree with each other.
Not enough evidence
Not enough human research to say either way.
Evidence against
Good research found no benefit, or found harm.
We take the reviewers' own risk-of-bias and GRADE statements where they exist, and otherwise judge from study size, design, consistency, directness and whether a product matches what is sold. Many publications do not make evidence high. Method note: We started from our existing onion summary, re-checked nutrition at USDA FoodData Central, read the NHS, ASPCA and FDA pages and searched PubMed for reviews, trials and case reports. Only abstracts were read for studies, and one allergy record had no abstract. Items we could not verify are flagged in the research gaps.
Study library: every study we checked 24
| Study | Design | People | Product, dose, length | Result |
|---|---|---|---|---|
| Ahmed HGMD 2025 Beyond seasoning nutrients bioactive ingredients and healthcare effects of Allium vegetables. Frontiers in Nutrition · PMID 41425624 · DOI |
Narrative review lab/animal and review |
Review of garlic, onion, leek and chive | Allium vegetables and their compounds Bulbs and other parts Trial detail, not advice |
The review describes organosulfur compounds and flavonoids in allium vegetables acting on oxidative stress and inflammation pathways (NF-kB and Nrf2). It names low bioavailability, differences between species and limited clinical validation as limits. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Antioxidant, anti-inflammatory and anticancer activity; barriers to use Limitations: A narrative review of mostly laboratory findings. It covers several allium vegetables, not onion alone. Funding and conflicts: The authors declare no commercial or financial relationships. Read: abstract |
| Albanesi 2019 Immunological characterization of onion (Allium cepa) allergy. Postepy Dermatologii i Alergologii · PMID 30858787 · DOI |
Case report |
1 One 35-year-old man with anaphylaxis after eating cooked onion |
Cooked onion (allergy testing) Bulb Trial detail, not advice Single episode |
Skin test and ImmunoCAP confirmed an IgE-dependent reaction to onion. Peach extract only slightly reduced onion-specific IgE (about 30 percent). The authors say cooked onion can cause severe allergic reactions, suggesting some heat-stable allergen parts. Their introduction notes that handling onion can cause contact dermatitis, rhinoconjunctivitis and asthma, while reports of reactions to eating onion are sporadic. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Allergy tests (skin test, ImmunoCAP, cross-reactivity with peach) Limitations: A single case, so it says nothing about how common onion allergy is. Funding and conflicts: The authors declare no conflict of interest. Read: abstract |
| Almogren 2013 Garlic and onion sensitization among Saudi patients screened for food allergy: a hospital based study. African Health Sciences · PMID 24250308 · DOI |
Cross sectional |
108 108 patients in Saudi Arabia with suspected food allergy |
Blood test for garlic and onion IgE antibodies Trial detail, not advice Records 2008 to 2009 |
15 of 108 patients (13.8 percent) had garlic and onion IgE antibodies; 13 had onion IgE. Of these 15, 12 had both. The authors say this shows sensitization and allergenic potential. 15 of 108 (13.8%) with garlic or onion IgE; 13 with onion IgE (Not applicable) Methods, limits and fundingOutcomes: Garlic and onion specific IgE Limitations: Sensitization (a blood marker) is not the same as a reaction. These were people already suspected of food allergy, so it does not show how common onion allergy is in the general public. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Chung 2023 Antiobesity effects of onion (Allium cepa) in subjects with obesity: Systematic review and meta-analysis. Food Science & Nutrition · PMID 37576046 · DOI |
Systematic review meta analysis |
People with obesity in 5 clinical trials (38 studies screened) | Onion peel in 4 trials and onion in 1 trial Onion peel (4 trials) or onion (1 trial) Trial detail, not advice Not given in the abstract · vs Placebo |
Body weight, BMI, waist circumference and triglycerides were significantly lower in the onion groups than with placebo. The authors say the effect was particularly pronounced with onion peel. Risk of bias was generally low. Weighted mean differences are not given in the abstract. (Not given in the abstract) Methods, limits and fundingOutcomes: Body weight, BMI, waist circumference, triglycerides Limitations: Only 5 trials, and 4 of them used onion peel, which is not the part people usually eat. Numbers were not read (abstract only). Funding and conflicts: The authors declare no conflicts of interest. Read: abstract |
| Enrique 2007 Involvement of lipid transfer protein in onion allergy. Annals of Allergy, Asthma & Immunology · PMID 17304894 · DOI |
Case report |
Letter on onion allergy (PubMed lists it as a case report) | Not applicable Trial detail, not advice |
Only the title and publication type were read (the PubMed record has no abstract). The title reports that a lipid transfer protein is involved in onion allergy. Not available (Not available) Methods, limits and fundingOutcomes: Not available Limitations: No abstract to read, so no claim is made beyond the title. Funding and conflicts: Not stated. Read: title-only |
| Ferjancic 2024 Fructan content in most commonly consumed Slovenian foods and estimation of daily fructan consumption. Frontiers in Nutrition · PMID 39155934 · DOI |
Laboratory measurement food analysis |
40 of the most commonly eaten foods in Slovenia | Fructan content measured in foods Fresh food Trial detail, not advice |
Fructan content was relatively low overall (0.1 to 1.97 g per 100 g). The highest was in onion (1.97 g per 100 g), then wheat flour (0.75 g per 100 g). Short-chain fructans ferment fast and can worsen symptoms in people with irritable bowel syndrome (IBS). Onion 1.97 g fructan per 100 g fresh weight (highest of 40 foods) (Not applicable) Methods, limits and fundingOutcomes: Fructan content per 100 g fresh weight; estimated daily fructan intake Limitations: A food measurement in Slovenian foods, not a trial in people with IBS. It does not say how any one person will react. Funding and conflicts: The authors declare no commercial or financial relationships. Read: abstract |
| Hejazi 2023 Onion supplementation and health metabolic parameters: A systematic review and meta-analysis of randomized controlled trials. Clinical Nutrition ESPEN · PMID 38056991 · DOI |
Systematic review meta analysis |
14 randomised trials of onion supplementation (participant number not given in the abstract) | Onion supplementation (forms vary) Onion products, mostly not the plain vegetable Trial detail, not advice Trials searched up to October 2022 · vs Placebo or control |
In the crude analysis, onion supplementation significantly improved body fat percentage, LDL cholesterol, total cholesterol, HDL cholesterol, systolic blood pressure, adiponectin and AST (a liver enzyme). Longer trials (over 12 weeks) and higher supplement doses showed more changes. The abstract does not list blood sugar measures among the improved ones. Effect sizes are not given in the abstract. (Not given in the abstract) Methods, limits and fundingOutcomes: Body measures, blood lipids, blood sugar measures, liver enzymes, blood pressure, adiponectin, leptin Limitations: Supplements of varied type. Effect sizes were not read (abstract only). Quality of the 14 trials was not stated in the abstract. Funding and conflicts: The authors declare no competing interests. Read: abstract |
| Huang 2020 Effect of quercetin supplementation on plasma lipid profiles, blood pressure, and glucose levels: a systematic review and meta-analysis. Nutrition Reviews · PMID 31940027 · DOI |
Systematic review meta analysis |
896 896 people in 17 randomised trials |
Quercetin or standardised quercetin-enriched extract Supplement or extract, not onions Trial detail, not advice Trials searched up to July 2018 · vs Placebo or control |
Quercetin lowered systolic and diastolic blood pressure. Blood lipids and glucose did not change significantly overall. In some subgroups (parallel-design trials of 8 weeks or longer) HDL cholesterol and triglycerides changed. Systolic BP WMD -3.09 mmHg; diastolic BP WMD -2.86 mmHg; glucose and lipids not significantly changed overall (Systolic 95% CI -4.59 to -1.59, P=0.0001; diastolic 95% CI -5.09 to -0.63, P=0.01) Methods, limits and fundingOutcomes: Blood lipids, blood pressure, blood glucose Limitations: Supplements and extracts, not onions as food. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Jeong 2020 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. Journal of the American College of Nutrition · PMID 31368861 · DOI |
Controlled trial |
70 70 overweight adults in Korea |
Jeju steamed onion product (ONIRO) in capsules Steamed onion capsule, not whole onions Trial detail, not advice 12 weeks · vs Placebo capsule |
Compared with placebo, the steamed onion capsule reduced percentage body fat and fat mass (DEXA) with no significant effect on lean mass, reduced fat areas on CT scans, and lowered triglycerides and C-peptide. Body fat percentage P=0.028; fat mass P=0.022; visceral fat area at L1 P=0.039 (P values only in the abstract) Methods, limits and fundingOutcomes: Body fat by DEXA and CT, blood fats, C-peptide, hormones Limitations: Capsule of a branded steamed onion product, one site, 70 people. Not a trial of eating onions. Effect sizes were not given in the abstract. Funding and conflicts: Funding is not stated in the abstract. Registration: NCT03645382 Read: abstract |
| Katircioglu 2024 Effects of different preparation and cooking processes on the bioactive molecules of Allium vegetables. Frontiers in Nutrition · PMID 38962447 · DOI |
Narrative review review |
Review of allium vegetables | Preparation and cooking of allium vegetables Bulbs and other parts Trial detail, not advice |
Preparation and cooking change the bioactive compounds in allium vegetables, and the result depends on method, time, temperature and the vegetable. The authors say more research is needed to find the best preparation for each allium vegetable. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Changes in bioactive compounds with preparation and cooking Limitations: A review of several allium vegetables. It does not give a rule for onions. Funding and conflicts: The authors declare no commercial or financial relationships. Read: abstract |
| Khalighi Sikaroudi 2024 Effects of a low FODMAP diet on the symptom management of patients with irritable bowel syndrome: a systematic umbrella review with the meta-analysis of clinical trials. Food & Function · PMID 38711328 · DOI |
Umbrella review meta analysis |
1646 People with IBS in 24 randomised trials (1,646 participants) within 11 meta-analyses |
Low FODMAP diet Whole diet Trial detail, not advice Under and over 4 weeks · vs Usual or other diet |
A low FODMAP diet improved total IBS symptoms (RR 1.42), stool consistency and stool frequency, and quality of life, but not anxiety or depression. Some studies suggested it may increase fecal pH and dysbiosis and reduce Bifidobacterium. The authors rate the certainty of evidence as low. Clinical improvement on IBS-SSS: RR 1.42 (95% CI 1.02-1.97, P=0.04) Methods, limits and fundingOutcomes: IBS symptoms, quality of life, gut bacteria, stool short-chain fatty acids Limitations: Low certainty of evidence per the authors. It is a diet study, not a study of onions alone. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Nishimura 2019 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 · PMID 31905615 · DOI |
Controlled trial |
70 70 healthy Japanese adults with BMI 23 to under 30 |
Quercetin-rich onion powder Onion powder (a processed food ingredient) Trial detail, not advice 12 weeks · vs Placebo |
There was no significant difference in visceral fat area between the groups overall. In people with lower HDL cholesterol, visceral fat area was significantly lower with onion powder, and ALT (a liver enzyme) was lower in the onion group than placebo. Visceral fat area: no significant difference overall (Not given in the abstract) Methods, limits and fundingOutcomes: Visceral fat area by CT; blood tests; liver enzymes Limitations: The main result was null. The lower-HDL result is a subgroup finding. It is a powder, not a whole onion. Funding and conflicts: The authors declare no conflict of interest. Read: abstract |
| Popiolek-Kalisz 2022 The Effects of Quercetin Supplementation on Blood Pressure - Meta-Analysis. Current Problems in Cardiology · PMID 35948195 · DOI |
Meta-analysis |
841 841 participants in 10 randomised trials, including normotensive and (pre)hypertensive adults |
Quercetin supplements Supplement, not onions Trial detail, not advice Trials searched up to May 2022 · vs Placebo or control |
Quercetin supplements lowered systolic pressure in the mixed population and in the normotensive subgroup, and lowered diastolic pressure in the (pre)hypertensive subgroup. Systolic BP (all) MD -2.38 mmHg; systolic (normotensive) MD -1.82 mmHg; diastolic ((pre)hypertensive) MD -3.14 mmHg (Systolic (all) 95% CI -3.80 to -0.96; systolic (normotensive) -2.43 to -1.20; diastolic ((pre)hypertensive) -4.44 to -1.84) Methods, limits and fundingOutcomes: Systolic and diastolic blood pressure Limitations: Supplements, not onions as food. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Serban 2016 Effects of Quercetin on Blood Pressure: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Journal of the American Heart Association · PMID 27405810 · DOI |
Systematic review meta analysis |
587 587 adults in 7 placebo-controlled randomised trials (9 treatment arms) |
Quercetin supplements Supplement, not onions Trial detail, not advice Trials searched up to January 2015 · vs Placebo |
Quercetin supplements lowered systolic and diastolic pressure compared with placebo. The effect was seen in trials of higher quercetin doses and not in lower-dose trials, but the indirect comparison did not show a significant difference between doses. The authors say the clinical relevance needs more study. Systolic BP WMD -3.04 mm Hg; diastolic BP WMD -2.63 mm Hg (Systolic 95% CI -5.75 to -0.33, P=0.028; diastolic 95% CI -3.26 to -2.01, P<0.001) Methods, limits and fundingOutcomes: Systolic and diastolic blood pressure Limitations: Quercetin supplements, not onions as food. Trial doses are not advice. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Tang 2008 An experimental study of hemolysis induced by onion (Allium cepa) poisoning in dogs. Journal of Veterinary Pharmacology and Therapeutics · PMID 18307506 · DOI |
Animal experiment dog (animal) |
6 Six adult, clinically normal Pekingese dogs |
Cooked onions fed to dogs Cooked onion Trial detail for dogs only: 30 g per kg body weight per day 2 days of feeding, blood followed to day 24 |
Red blood cell counts, hemoglobin and hematocrit fell, significantly by day 5, with a rise in Heinz bodies and bilirubin. The authors confirm onion poisoning in dogs causes hemolytic anemia (red blood cells breaking down). Red blood cells, hemoglobin and hematocrit decreased from day 1, significantly by day 5 (P<0.01) (P values only) Methods, limits and fundingOutcomes: Red blood cells, hemoglobin, Heinz bodies, bilirubin and red cell chemistry Limitations: Six dogs, no control group. This is about dogs, not people. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Vezza 2024 LDL-Cholesterol-Lowering Effects of a Dietary Supplement Containing Onion and Garlic Extract Used in Healthy Volunteers. Nutrients · PMID 39203947 · DOI |
Controlled trial |
Adults with slightly raised cholesterol (number not given in the abstract) | Standardised garlic plus onion extract formulation Extract of garlic and onion (a combination product) Trial detail, not advice 8 weeks · vs Placebo |
LDL cholesterol and total cholesterol were significantly lower with the extract. Blood pressure and oxidative and inflammatory markers also improved, according to the authors. Not given in the abstract (Not given in the abstract) Methods, limits and fundingOutcomes: LDL and total cholesterol, blood pressure, oxidative and inflammatory markers Limitations: A combination of garlic and onion extracts, so the effect cannot be given to onion alone. Numbers were not read. The paper's conflict statement says the funders had no role in design, analysis or publication, so it was funded. Funding and conflicts: Funded; the funders had no role in design, analysis or the decision to publish (per the paper's statement). Read: abstract |
| Yang 2023 The longitudinal association between onion consumption and risk of depressive symptoms: results from the TCLSIH Cohort study and the UK Biobank. Food & Function · PMID 36477763 · DOI |
Cohort study |
177545 7,739 adults in the Tianjin TCLSIH cohort and 169,806 adults in the UK Biobank |
Habitual onion eating, reported by questionnaire (Tianjin) or 24-hour recall (UK Biobank) Onions as food Trial detail, not advice Up to 2018 · vs Lowest intake group |
In the Tianjin cohort, higher onion intake went with fewer depressive symptoms (p for trend 0.01). In the UK Biobank, the two middle intake groups showed lower risk, but the highest group did not (HR 0.97) and the trend was not significant (p 0.07). Tianjin HRs 0.78, 0.73, 0.77; UK Biobank HRs 0.79, 0.81, 0.97 (Tianjin 95% CIs 0.65-0.94, 0.61-0.87, 0.64-0.92; UK Biobank 0.68-0.93, 0.69-0.94, 0.85-1.12) Methods, limits and fundingOutcomes: Depressive symptoms (self-rating scale in Tianjin; hospital records and deaths in UK Biobank) Limitations: Observational; intake came from questionnaires and recalls; cannot show onions cause the difference. The two cohorts measured depression differently. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Zhang 2022 Allium Vegetables, Garlic Supplements, and Risk of Cancer: A Systematic Review and Meta-Analysis. Frontiers in Nutrition · PMID 35402472 · DOI |
Systematic review meta analysis |
22 studies (25 reports) of allium vegetables; 10 studies of garlic supplements | Allium vegetables as food (onion, garlic and others, pooled) Bulbs and other allium vegetables Trial detail, not advice Observational follow-up · vs Lowest intake group |
High allium vegetable intake showed no significant link with overall cancer risk. Garlic supplements also showed no significant link. The authors found no evidence that higher intake reduced cancer risk and say the finding needs further validation. Allium vegetables and cancer risk RR 0.97 (95% CI 0.92-1.03 (fixed-effects model)) Methods, limits and fundingOutcomes: Cancer risk Limitations: Pooled allium vegetables, not onions alone. Observational studies cannot show cause. Funding and conflicts: The authors declare no commercial or financial relationships. Registration: PROSPERO CRD42021246947 Read: abstract |
| Zhou 2011 Consumption of large amounts of Allium vegetables reduces risk for gastric cancer in a meta-analysis. Gastroenterology · PMID 21473867 · DOI |
Meta analysis observational |
543220 543,220 people in 19 case-control and 2 cohort studies |
Allium vegetables as food, including onion Bulbs and other allium vegetables Trial detail, not advice Observational · vs Lowest intake group |
The highest versus lowest intake of allium vegetables was linked to lower gastric cancer risk. Onion analyses gave similar results. The authors say confounding and exposure misclassification mean more studies are needed to establish the link. Highest versus lowest intake, odds ratio 0.54 (95% CI 0.43-0.65) Methods, limits and fundingOutcomes: Stomach (gastric) cancer risk Limitations: Mostly case-control studies, which are weak for diet questions. Allium vegetables pooled. Funding and conflicts: Not stated in the abstract. Read: abstract |
| s-aspca-onion ASPCA: Toxic and Non-toxic Plants, Onion · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-fda-produce FDA: Selecting and Serving Produce Safely · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-gbif-allium-cepa GBIF Backbone: Allium cepa L. (usageKey 2857697) · Source |
Reference database n/a |
Methods, limits and fundingRead: full-text | ||
| s-nhs-ibs NHS: Further help and support for IBS · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-usda-fdc-170000 USDA FoodData Central: Onions, raw (FDC 170000) · Source |
Reference database n/a |
Methods, limits and fundingRead: full-text |
What we do not know yet
- No trial of eating whole onions was found for blood pressure, cholesterol, body fat, blood sugar or mood.
- The blood pressure trials tested quercetin supplements, and the quercetin content of a serving of onion was not read from a source.
- Onion peel, capsules and powders were tested for body fat; effect sizes were not in the abstracts.
- Allergy was found only in a case report, a sensitization study and a title-only record; how common it is was not found.
- Pregnancy, breastfeeding, children, pesticide residues, heavy metals, onion-linked outbreaks and the effect of cooking were not researched.
How new research is added
New studies are found by a saved PubMed search and placed in a review queue before the page changes.
Lab markers
What changed in blood or saliva tests during trials. A changed number is not the same as feeling or being healthier, and we give no "normal ranges".
Metabolic 2
LDL cholesterol after onion supplements
Lowered in trials
The 'bad' cholesterol in blood, measured in trials of onion products.
Low certaintyResults and what it means
Why it was measured
High LDL cholesterol is a risk factor for heart disease.
Statistical vs. clinical meaning
Supplements and a combination extract. Not a result for eating onions.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 7 | LDL cholesterol significantly improved in the crude analysis of onion supplement trials (numbers not in the abstract) | 14 randomised trials | Onion supplementation (varied forms) |
| 16 | Significant reductions in LDL and total cholesterol with garlic plus onion extract versus control | Adults with slightly raised cholesterol | Garlic plus onion extract, 8 weeks |
Blood glucose after quercetin supplements
No clear change in trials
The amount of sugar in the blood, measured in trials of quercetin supplements.
Not enough evidenceResults and what it means
Why it was measured
Trials of plant products often check glucose.
Statistical vs. clinical meaning
No significant change. Not a test of onions.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 8 | Glucose concentrations did not change significantly | 896 people in 17 trials (glucose analysis) | Quercetin or quercetin-enriched extract (not onions) |
Other 4
Systolic blood pressure after quercetin supplements
Lowered in trials
The top blood pressure number, measured during trials of quercetin supplements.
Low certaintyResults and what it means
Why it was measured
Quercetin is found in onions and has been tested as a supplement for blood pressure.
Statistical vs. clinical meaning
A small drop of about 2 to 3 mmHg in trials of supplements. Not a result for eating onions.
Safety implication
People taking blood pressure medicines should ask a clinician before supplements.
Diastolic blood pressure after quercetin supplements
Lowered in trials
The bottom blood pressure number, measured during trials of quercetin supplements.
Low certaintyResults and what it means
Why it was measured
Reported alongside systolic pressure in the same trials.
Statistical vs. clinical meaning
A small drop, in supplement trials only.
Safety implication
None specific.
Visceral (belly) fat area
Mixed results in trials
The area of fat around the organs in the belly, measured by a CT scan.
Mixed findingsResults and what it means
Why it was measured
Belly fat is linked with metabolic problems, so trials of onion products measure it.
Statistical vs. clinical meaning
Two small trials disagreed. Neither tested whole onions.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 12 | No significant difference between onion powder and placebo overall; lower in a lower-HDL subgroup | 70 healthy adults with BMI 23 to under 30 | Quercetin-rich onion powder, 12 weeks |
| 9 | Visceral fat area at L1 significantly lower with steamed onion capsule than placebo (P=0.039) | 70 overweight adults | Steamed onion capsule, 12 weeks |
Overall cancer risk and high allium vegetable intake
No clear change in trials
How many people in observational studies develop cancer, compared across how many allium vegetables they eat.
Low certaintyResults and what it means
Why it was measured
These are the outcomes that matter, but the studies only observe eating patterns.
Statistical vs. clinical meaning
No clear link. Allium vegetables pooled, not onions alone.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 18 | RR 0.97 (95% CI 0.92-1.03), no significant association | 22 studies (25 reports) | Allium vegetables as food, pooled |
Safety
What studies and official sources say about side effects, medicines and who should check first. Each point shows what kind of evidence it comes from.
Main points to know 3
- Irritable bowel syndrome: onion is the highest-fructan food in a 40-food measurement. A 2024 study measured fructans in 40 common foods and found the most in onion (1.97 g per 100 g fresh). Short-chain fructans ferment quickly and can worsen symptoms in people with irritable bowel syndrome. A low FODMAP diet improved IBS symptoms overall in a 2024 umbrella review of trials, with low certainty. The NHS says a dietitian may recommend this diet. Ask a dietitian instead of cutting foods out alone. Lab 6 11 23
- Pets: onion is toxic to dogs, cats and horses. The ASPCA lists onion as toxic to dogs, cats and horses. Signs include vomiting, breakdown of red blood cells, blood in urine, weakness, high heart rate and panting. In an experiment, six dogs fed cooked onions for 2 days developed hemolytic anemia. If a pet ate onion, call a veterinarian or ASPCA Poison Control at (888) 426-4435 (a fee may apply). Authoritative guidance 20 15
- Allergy: rare after eating, but severe cases are reported. A 2019 report describes anaphylaxis after cooked onion in a 35-year-old man, suggesting some allergen parts survive heat. Handling onion can cause skin rash, runny eyes or nose and asthma. For trouble breathing or swelling of the face or throat, call emergency services. Case report 2 3
Side effects 5
| Effect | How often | Severity and recovery | Evidence |
|---|---|---|---|
| Worse IBS symptoms in some people (fructans are a type of FODMAP) The fructan study measured foods, not people. Whether a given person reacts, and to how much onion, is individual. | Not established | not stated Not stated | Lab 6 11 |
| Anaphylaxis after eating cooked onion A single case. It cannot tell us how common onion allergy is. | One case described | severe Not stated in the abstract | Case report 2 |
| Contact dermatitis, rhinoconjunctivitis and asthma from handling onion Stated in the introduction of the 2019 report as known reactions to handling onion; frequency was not given. | Not established | not stated Not stated | Case report 2 |
| Onion and garlic IgE antibodies (sensitization) Sensitization is a blood marker, not proof of a reaction. These were people already suspected of food allergy. | 15 of 108 patients (13.8%) with suspected food allergy had garlic or onion IgE; 13 had onion IgE | not stated Not applicable | Observational 3 |
| Possible link between onion allergy and lipid transfer protein (title only) Only the title was read; the record has no abstract. | Not established | not stated Not stated | Case report 5 |
Liver
No liver injury from eating onions was found in the sources read. A liver-safety source (such as LiverTox) was not searched for this food page. One trial of onion powder reported a lower ALT (a liver enzyme) than placebo, which is not a safety signal. 12
How often. No frequency was found.
Monitoring. No agency monitoring advice was found.
This is a safety connection. It is not evidence that Onions supports liver health.
Medicines and supplements 2
| With | Why it matters | Importance | Evidence | What to do |
|---|---|---|---|---|
| Blood pressure medicines (when taking quercetin or onion supplements) prescription | Quercetin supplements lowered blood pressure by about 2 to 3 mmHg in trials, so an added effect with blood pressure medicines is possible in theory. No interaction study was read. | Not studied. This concerns supplements, not onions eaten as food. | Theoretical Not enough evidence 8 13 | Ask a pharmacist or clinician before using quercetin or onion extract supplements, especially if you take blood pressure medicines. |
| Medicines in general (onions eaten as food) prescription | No source read described a medicine interaction with onions as food. | No claim either way is made. | None found Not enough evidence | A pharmacist can check any medicine or supplement you take. |
Who should check first 3
| Situation | Advice | Evidence |
|---|---|---|
| Irritable bowel syndrome that flares with onion | Ask a dietitian about a low FODMAP plan instead of cutting foods out alone. | Lab 6 23 |
| Known onion allergy or a past reaction to cooked onion | Avoid onion and ask an allergy clinician about testing. Call emergency services for trouble breathing or swelling of the face or throat. | Case report 2 |
| Feeding onion to dogs, cats or horses | Do not feed onion to them (ASPCA). | Authoritative guidance 20 |
Overdose, product quality and regulators
Summary. No toxicity from onions eaten as ordinary food in people was found in the sources read. Onion is toxic to dogs, cats and horses.
Animal. Six dogs fed cooked onions for 2 days had falling red blood cell counts and hemoglobin, with Heinz bodies, and the authors confirm onion causes hemolytic anemia in dogs. The ASPCA names N-propyl disulfide as the toxic principle for pets.
Human doses studied. Trials of onion powder, steamed onion capsules and onion peel extracts ran 8 to 12 weeks or longer; the abstracts read name no harm.
Overdose response. Not found for people.
Genotoxicity. Not researched.
Fresh produce can carry germs. The FDA gives handling advice. Pesticide residues, heavy metals and recalls linked to onions were not researched.
Washing. The FDA says to wash all produce under running water, not to use soap, detergent or commercial produce wash, and to dry it with a clean towel. Washing can reduce bacteria but will not eliminate them. Cooking produce thoroughly kills harmful bacteria. 21
Storage and cut onion. The FDA says to refrigerate all produce purchased pre-cut or packaged and to keep perishable produce at 40 degrees F or below. 21
Raw onion sprouts. The FDA says people who are more vulnerable to foodborne illness should avoid eating raw or lightly cooked sprouts of any kind, including onion sprouts. 21
Cutting boards. The FDA says to wash boards, dishes and utensils with soap and hot water between preparing raw meat, poultry and seafood and preparing produce that will not be cooked. 21
| Body | Action | Date |
|---|---|---|
| NHS (UK) (United Kingdom) | IBS page: a GP may refer you to a dietitian, who may recommend a low FODMAP diet that avoids some fruit and vegetables. Onion is not named on the page. source | 2025-03-17 |
| ASPCA (US animal welfare charity) (United States) | Lists onion (Allium cepa) as toxic to dogs, cats and horses. source | not shown |
| FDA (US) (United States) | Produce safety advice: wash under running water without soap, refrigerate cut produce, vulnerable people avoid raw sprouts including onion sprouts. source | 2024-03-05 |
When to get medical help
- Trouble breathing, swelling of the lips, face or throat, or widespread hives after eating onion
- A pet that has eaten onion and shows vomiting, weakness, blood in the urine or a fast heart rate: call a veterinarian or ASPCA Poison Control at (888) 426-4435 (a fee may apply)
- Vomiting, diarrhea, belly pain, fever or flu-like symptoms after eating raw or cut produce: contact your healthcare provider (FDA)
Stop using the product and contact a clinician or emergency service. In the US, Poison Control is 1-800-222-1222.
General information only, not medical advice. Ask a clinician or pharmacist before use, especially with medicines, in pregnancy or with a health condition. See the Natural Remedy Safety guide.
Frequently asked questions
What are onions good for?
Do onions lower blood pressure?
Can onions help you lose weight?
Do onions help with cholesterol?
Are onions good for blood sugar or diabetes?
Do onions prevent cancer?
Can onions make IBS worse?
Can you be allergic to onions?
Are onions safe for dogs and cats?
Are onions safe in pregnancy and for children?
The sources we read did not single out onions eaten in meals, so we make no claim. The FDA says pregnant women and children are especially vulnerable to foodborne illness, so wash produce, keep cut onion cold and avoid raw sprouts. Ask your care provider.
Is onion peel or quercetin the same as eating onions?
Does cooking change onions?
Yes, cooking changes the compounds in allium vegetables, depending on method, time and temperature. We did not read a source on how it changes the vitamins or fructans in onions, so we make no claim.
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Editorial transparency
- Written by
- AlternativeMed editorial team. Research compiled with AI assistance, then checked against the sources listed.
- Medical review
- None. This page has not been reviewed by a physician or pharmacist.
- First published
- 2026-10-26
- Last substantive update
- 2026-10-26
- How we research
- We started from our existing onion summary, re-checked nutrition at USDA FoodData Central, read the NHS, ASPCA and FDA pages and searched PubMed for reviews, trials and case reports. Only abstracts were read for studies, and one allergy record had no abstract. Items we could not verify are flagged in the research gaps.
- How we rate evidence
- See "How we rate evidence" in the Evidence tab.
- Independence
- No product is promoted on this page. No affiliate links are shown.
Sources 24
Each source was opened and checked. Where we read only the abstract, it says so. The small numbers in the text point here.
- Ahmed HGMD, Yang J, Zhu T, et al.. Beyond seasoning nutrients bioactive ingredients and healthcare effects of Allium vegetables.. Frontiers in Nutrition. 2025. PMID 41425624. DOI 10.3389/fnut.2025.1597788. (abstract)
- Albanesi M, Pasculli C, Giliberti L, et al.. Immunological characterization of onion (Allium cepa) allergy.. Postepy Dermatologii i Alergologii. 2019. PMID 30858787. DOI 10.5114/ada.2019.82829. (abstract)
- Almogren A, Shakoor Z, Adam MH. Garlic and onion sensitization among Saudi patients screened for food allergy: a hospital based study.. African Health Sciences. 2013. PMID 24250308. DOI 10.4314/ahs.v13i3.24. (abstract)
- Chung MY, Hwang JT, Park SH. Antiobesity effects of onion (Allium cepa) in subjects with obesity: Systematic review and meta-analysis.. Food Science & Nutrition. 2023. PMID 37576046. DOI 10.1002/fsn3.3426. (abstract)
- Enrique E, Malek T, De Mateo JA, et al.. Involvement of lipid transfer protein in onion allergy.. Annals of Allergy, Asthma & Immunology. 2007. PMID 17304894. DOI 10.1016/S1081-1206(10)60700-4. (title-only)
- Ferjancic B, Korosec M, Kocevar Baloh A, et al.. Fructan content in most commonly consumed Slovenian foods and estimation of daily fructan consumption.. Frontiers in Nutrition. 2024. PMID 39155934. DOI 10.3389/fnut.2024.1446771. (abstract)
- Hejazi N, Ghalandari H, Nouri M, Askarpour M. Onion supplementation and health metabolic parameters: A systematic review and meta-analysis of randomized controlled trials.. Clinical Nutrition ESPEN. 2023. PMID 38056991. DOI 10.1016/j.clnesp.2023.08.032. (abstract)
- Huang H, Liao D, Dong Y, Pu R. Effect of quercetin supplementation on plasma lipid profiles, blood pressure, and glucose levels: a systematic review and meta-analysis.. Nutrition Reviews. 2020. PMID 31940027. DOI 10.1093/nutrit/nuz071. (abstract)
- Jeong S, Chae J, Lee G, et al.. 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.. Journal of the American College of Nutrition. 2020. PMID 31368861. DOI 10.1080/07315724.2019.1635052. (abstract)
- Katircioglu B, Navruz-Varli S. Effects of different preparation and cooking processes on the bioactive molecules of Allium vegetables.. Frontiers in Nutrition. 2024. PMID 38962447. DOI 10.3389/fnut.2024.1350534. (abstract)
- Khalighi Sikaroudi M, Soltani S, Ghoreishy SM, et al.. Effects of a low FODMAP diet on the symptom management of patients with irritable bowel syndrome: a systematic umbrella review with the meta-analysis of clinical trials.. Food & Function. 2024. PMID 38711328. DOI 10.1039/d3fo03717g. (abstract)
- 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. PMID 31905615. DOI 10.3390/nu12010091. (abstract)
- Popiolek-Kalisz J, Fornal E. The Effects of Quercetin Supplementation on Blood Pressure - Meta-Analysis.. Current Problems in Cardiology. 2022. PMID 35948195. DOI 10.1016/j.cpcardiol.2022.101350. (abstract)
- Serban MC, Sahebkar A, Zanchetti A, et al.. Effects of Quercetin on Blood Pressure: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.. Journal of the American Heart Association. 2016. PMID 27405810. DOI 10.1161/JAHA.115.002713. (abstract)
- Tang X, Xia Z, Yu J. An experimental study of hemolysis induced by onion (Allium cepa) poisoning in dogs.. Journal of Veterinary Pharmacology and Therapeutics. 2008. PMID 18307506. DOI 10.1111/j.1365-2885.2007.00930.x. (abstract)
- Vezza T, Guillamon E, Garcia-Garcia J, et al.. LDL-Cholesterol-Lowering Effects of a Dietary Supplement Containing Onion and Garlic Extract Used in Healthy Volunteers.. Nutrients. 2024. PMID 39203947. DOI 10.3390/nu16162811. (abstract)
- Yang H, Gu Y, Zhang B, et al.. The longitudinal association between onion consumption and risk of depressive symptoms: results from the TCLSIH Cohort study and the UK Biobank.. Food & Function. 2023. PMID 36477763. DOI 10.1039/d2fo01640k. (abstract)
- Zhang Q, Zhao Q, Shen Y, Zhao F, Zhu Y. Allium Vegetables, Garlic Supplements, and Risk of Cancer: A Systematic Review and Meta-Analysis.. Frontiers in Nutrition. 2022. PMID 35402472. DOI 10.3389/fnut.2021.746944. (abstract)
- Zhou Y, Zhuang W, Hu W, et al.. Consumption of large amounts of Allium vegetables reduces risk for gastric cancer in a meta-analysis.. Gastroenterology. 2011. PMID 21473867. DOI 10.1053/j.gastro.2011.03.057. (abstract)
- . ASPCA: Toxic and Non-toxic Plants, Onion. . . (full-text)
- . FDA: Selecting and Serving Produce Safely. . . (full-text)
- . GBIF Backbone: Allium cepa L. (usageKey 2857697). . . (full-text)
- . NHS: Further help and support for IBS. . . (full-text)
- . USDA FoodData Central: Onions, raw (FDC 170000). . . (full-text)
