Chopping an onion starts a chain of reactions in the cells your knife breaks. The enzyme alliinase works on a sulfur compound in the bulb. A second enzyme, lachrymatory factor synthase, finishes the job by making the gas that stings your eyes.
When researchers in New Zealand silenced the gene for that second enzyme in onion plants, its activity fell by up to 1,544-fold. Japanese breeders have also grown tearless, non-pungent onions from seeds irradiated with neon ions, without adding any genes.
Onions are one of the richest sources of dietary flavonoids, a large family of plant compounds. Quercetin compounds are the main ones in every variety. On this site, 100 g of onion has 20.3 mg of quercetin. It also has 5 mg of isorhamnetin, a close relative that is measured in only 12 foods in our database.
Nine volunteers absorbed quercetin from onions about three times as well as from apples. It also leaves the blood slowly, with a half-life of 28 hours.
Color is a weaker guide than most people expect. Published measurements put red onions at 415 to 1,917 mg of flavonols per kilogram, and yellow onions at 270 to 1,187, so the ranges overlap. In a lab test of ten US varieties, a yellow one had the most flavonoids, while sweet Vidalia ranked last for antioxidant activity.
At 40 calories and 1.7 g of fiber per 100 g, onion adds a lot of flavor for very little energy.
Health benefits of onions
The top findings from 30 studies, strongest evidence first
What the stars mean
- Strong. Several randomized trials, pooled in a meta-analysis, point the same way, or regulators (FDA or EFSA) have approved the claim.
- Moderate. Good randomized trials support it, but there are fewer of them, or their results vary.
- Limited. Small, short or single trials, or mixed results. Promising, but it needs confirmation.
Population and lab studies get no stars: they show a link or a mechanism, not proof.
How we rate evidence →- ★★Improved LDL and blood pressure with extractsA pooled analysis of 14 trials found onion extracts and powders improved LDL, systolic blood pressure and body fat percentage. These were not onions as eaten.
- ★★Lower blood pressure with quercetin pillsIn 7 trials, quercetin pills, the main flavonol in onions, lowered blood pressure by 3.0/2.6 mm Hg (top and bottom number). A 100 g serving of onion has far less quercetin than those pill doses.
- ★Slightly lower LDL in polycystic ovary syndromeIn 54 women with polycystic ovary syndrome, 80 to 120 g of raw red onion a day for 8 weeks lowered LDL and total cholesterol. The comparison group, eating 20 to 30 g, also improved, so the gap is small.
- ★Lower fasting blood sugar during chemotherapyIn 56 breast cancer patients, 100 to 160 g a day of fresh yellow onion for 8 weeks lowered fasting blood sugar and insulin. The comparison group ate 30 to 40 g a day of small onions.
- ★Less body fat with onion capsules70 overweight people took capsules of steamed onion or a placebo before meals for 12 weeks. Body fat percentage and belly fat fell on the onion capsules.
All 30 studies, strongest evidence first
Onion supplements improved body fat percentage, LDL cholesterol and systolic blood pressure
This meta-analysis, which pools the results of many studies, covered 14 randomized trials of onion extracts and powders, not onions as eaten. Total cholesterol, HDL (“good”) cholesterol, adiponectin and the liver enzyme AST also improved. Weight, waist size and BMI (weight relative to height) improved only in trials longer than 12 weeks or using more than 300 mg a day. The abstract gives no effect sizes.
Onions’ main flavonol, quercetin, lowered blood pressure as pills
In 7 placebo-controlled trials (587 people), quercetin pills, not onions, lowered systolic pressure by 3.0 mm Hg and diastolic by 2.6 mm Hg. Doses of 500 mg a day or more lowered systolic pressure by 4.45 mm Hg, and lower doses had no clear effect. Even so, the difference between the two dose groups could be chance. A 100 g serving of onion has about 20 mg of quercetin on this site, far below those doses.
Raw red onion lowered LDL and total cholesterol in polycystic ovary syndrome
In this randomized trial, where chance decides who gets what, 54 overweight or obese women with polycystic ovary syndrome took part. They ate 80 to 120 g of raw red onion a day for 8 weeks. The comparison group ate only 20 to 30 g a day, and its cholesterol fell too, so the gap between the groups is small. Fasting blood sugar, triglycerides, HDL cholesterol and lipoprotein(a) did not change, and the authors call for more research.
Yellow onion lowered fasting blood sugar and insulin during breast cancer chemotherapy
This randomized, triple-blind trial included 56 patients treated with doxorubicin, a drug linked to insulin resistance in animal studies. For 8 weeks they ate 100 to 160 g a day of fresh yellow onion. The comparison group ate 30 to 40 g a day of small onions. Insulin resistance fell and insulin sensitivity improved (23 patients per group analyzed), but the result applies to this setting and needs repeating.
Onion capsules reduced body fat percentage and belly fat in overweight adults
This trial gave 70 overweight people capsules of steamed Jeju onion (ONIRO) or a placebo (a dummy capsule) before meals for 12 weeks. Fat mass fell on DEXA scans. CT scans showed less fat around the organs (visceral) and under the skin (subcutaneous) at the L1 level. Triglycerides and C-peptide were lower, while lean body mass did not change.
Onion cut the blood sugar peak by 44% in lactose-intolerant adults
This pilot study tested an onion meal, amount not given in the abstract, in 12 lactose-intolerant and 12 lactose-tolerant adults during a glucose tolerance test. In the lactose-tolerant group, the peak fell 19%. The researchers expected lactose-intolerant people to keep more intact quercetin glucosides, which can block glucose uptake. The total blood sugar rise (area under the curve) fell 54.5% and 42.1%, a gap that could be chance.
High-quercetin onion soup reduced platelet clumping in blood taken afterward
In this double-blind crossover pilot study, people ate soup with 69 mg of quercetin or 5 mg, on separate occasions. Blood quercetin peaked at 2.59 µmol/L (micromoles per liter) after the high-quercetin soup. Clumping triggered by collagen fell, and a key platelet signaling step (Syk phosphorylation) was lower. Platelets were tested in blood samples, so it did not measure clots or heart disease.
Onion skin quercetin lowered 24-hour systolic pressure only in hypertension
In this placebo-controlled crossover trial, 70 overweight or obese people with pre-hypertension or stage I hypertension took onion skin extract. It gave 162 mg of quercetin a day for 6 weeks. 24-hour and office pressure did not clearly change overall, but systolic fell 3.6 mm Hg in the hypertension group (3.9 mm Hg below placebo). Markers of vessel function, inflammation, lipids and glucose did not change, and onion skin is not the part people eat.
Quercetin did not change post-meal blood pressure or blood fats in hypertension
In this randomized crossover trial, 22 overweight or obese adults with hypertension took a single 54 mg dose of quercetin from onion skin extract. It came with a heavy meal of 4,754 kJ (kilojoules) and 61.6 g of fat. Blood pressure, vessel function, triglycerides, insulin and inflammation markers did not differ between quercetin and placebo. The meal itself did not impair vessel function in these patients.
Quercetin-rich onion powder did not change visceral fat in healthy adults
In this placebo-controlled trial, 70 healthy Japanese adults (BMI 23 to 30) took 9 g of onion powder a day for 12 weeks. CT scans measured visceral fat, the fat around the organs. It was lower only in the subgroup with low HDL cholesterol. The liver enzyme ALT was lower than on placebo.
Onion juice shifted blood markers; just three women had bone scans
In this small trial, 24 healthy middle-aged and postmenopausal people drank 100 mL of onion juice or a placebo a day for 8 weeks. Blood levels of alkaline phosphatase, free radicals and antioxidants changed. Bone mineral density was measured in just three postmenopausal women and was mildly better. A separate cell experiment found onion extract slowed osteoclast (bone-removing cell) development, which does not show an effect in people.
Onion quercetin was absorbed about three times better than apple quercetin
Nine volunteers ate a single large dose of onions, apples or pure quercetin rutinoside (the form in tea). Their blood was measured over 36 hours. Apples and the rutinoside each gave about 30% of the onion level. Onion quercetin peaked within 0.7 hours and cleared slowly (a half-life of 28 hours), so repeated eating builds up blood levels.
Red onion soup roughly matched a 544 mg quercetin tablet in urine
In this crossover study, 6 healthy men had both the tablet and a soup made with 100 g of red onion (47 mg of quercetin). The 24-hour urine amounts were similar: 1.69 against 1.17 µmol, a difference that could be chance. So the authors estimate that 166 mg of supplement matches about 10 mg from onion. It measured excretion only, in 6 men.
Top-third onion eaters had 31% lower odds of stomach cancer
This pooled analysis (the Stomach cancer Pooling Project) used individual data from 17 studies: 6,097 people with stomach cancer and 13,017 without. Total allium vegetables gave 0.71 times the odds, and garlic 0.83. The link held in Asian studies (0.50) but not in European ones (0.96), and results varied so much between studies that the authors urge caution.
Case-control studies tied the most onion to 45% lower stomach cancer risk
This meta-analysis pooled 13 case-control studies, but four cohort studies found no link for allium vegetables (a pooled risk of 1.02). In their own Italian study (230 cases, 547 controls), two or more onion portions a week gave 0.59 times the odds, possibly by chance. Case-control studies rely on people recalling past diet, so the authors ask for large cohorts.
Allium vegetables were not linked to overall cancer risk in 22 studies
This meta-analysis covered allium vegetables, a group that includes onions and garlic. High intake gave 0.97 times the risk, with no dose-response trend (eating more did not change the risk step by step). The authors say the finding needs further validation. Single cancer types can differ from the overall result, as other studies here show.
Allium vegetables were not linked to colorectal cancer risk in prospective studies
This meta-analysis covered 8 prospective studies, which follow people forward in time, with 5,458 colorectal cancer cases. Higher allium vegetable intake gave 1.06 times the risk. Women showed a borderline higher risk of colon cancer (1.23 times), which the authors treat as marginal.
Top onion intake went with 28% lower mouth and throat cancer risk
This meta-analysis pooled 21 case-control and 4 cohort studies of squamous cell cancers of the mouth, throat and esophagus. Onion gave 0.72 times the risk and garlic 0.74, but allium vegetables as a whole gave 0.79, which could be chance. The link was unclear in cohort studies, so the result rests mostly on case-control studies. There was no link for adenocarcinoma, a different type of esophageal cancer.
Top onion eaters in Italy and Switzerland: far lower esophageal cancer odds
These case-control studies ran in Italy and Switzerland. The highest onion group had 0.12 times the odds of esophageal cancer and 0.16 times for mouth and throat cancer. Odds were also lower for laryngeal (0.17), ovarian (0.27), prostate (0.29), colorectal (0.44), kidney (0.62) and breast cancer (0.75). Case-control studies depend on people recalling past diet.
Half an onion a day or more: half the stomach cancer rate
This Dutch cohort followed 120,852 men and women aged 55 to 69 for 3.3 years, with 139 stomach cancer cases. The comparison was with people who ate no onions. The drop was seen in the non-cardia part of the stomach, and leeks and garlic supplements showed no link. Few cases and short follow-up make the estimate imprecise, and the range was wide.
Per 5 g of allium daily, artery-disease deaths were 18% less likely
This cohort followed 1,226 Australian women aged 70 and older for 15 years, with 238 deaths from atherosclerotic vascular disease (hardened, narrowed arteries). The hazard ratio, a measure of relative risk, was 0.82, and it held after adjusting for other vegetables and heart risk factors. It covers allium vegetables as a group, onions among them.
Women over 50 eating onions daily had 5% higher bone density
This US survey (NHANES 2003 to 2004) covered 507 non-Hispanic white women past menopause or going through it. They were compared with women who ate onions once a month or less, adjusting for age, BMI, calcium, vitamin D and other factors. The authors project a lower hip fracture risk, but fractures were not measured. Diet and bone density were measured at one time, so it cannot show cause.
Frequent onion eaters had 22 to 27% lower risk of depressive symptoms
This Chinese cohort in Tianjin followed 7,739 adults through 2018 and measured depressive symptoms. In the UK Biobank (169,806 people), lower intake levels also went with lower risk (0.79 and 0.81 times), but the highest did not (0.97). The two cohorts defined depression differently: a symptom scale in one, hospital records in the other.
Western Yellow had the most flavonoids of 10 onion varieties
This lab analysis tested 10 US varieties plus shallots. Western Yellow had 69.2 mg of flavonoids per 100 g, 11 times as much as Western White. Antioxidant activity ranked shallots first, then Western Yellow, New York Bold and Northern Red, with sweet Vidalia last. Extracts of New York Bold and Western Yellow also slowed colon cancer cell growth in a dish, which says nothing yet about people.
Red and yellow onions have overlapping ranges of flavonols
This review gathers published measurements of onion flavonoids. Red onions held 415 to 1,917 mg of flavonols per kilogram, and yellow onions 270 to 1,187. Quercetin compounds are the main flavonols in every cultivar, and red onions also have anthocyanins (the red pigments) at about 10% of their total flavonoids. This is plant chemistry, not a study in people.
Home peeling kept 79% of quercetin glucoside but 27% of anthocyanins
This lab analysis measured Tropea red onions layer by layer. Storing them for 6 weeks under home-like conditions cut anthocyanins to 64 to 73% and antioxidant activity to 29 to 36%. Cold storage kept anthocyanins better. This is a measurement of the vegetable, not a study in people.
Boiling onions sent about 30% of their quercetin into the water
This lab study compared boiling, frying in oil and butter, and microwaving. Frying did not change flavonoid intake, and microwaving without water kept flavonoids and vitamin C better. Adding ferrous ions (a form of iron) sped up the loss of flavonoids. Seasonings such as glutamic acid may break down quercetin glycosides.
Chopping barely changed quercetin; boiling for 60 minutes cut flavonols by 20.6%
This lab test looked at chopping, maceration and boiling on onion bulbs, and on asparagus. Boiling also changed the antioxidant capacity of the tissue, while chopping did not. These are measurements of cooked tissue, not a study in people.
Silencing one gene cut tear-enzyme activity by up to 1,544-fold
This lab study used genetically engineered onion plants (six plants). Wounded bulbs made far less of the tear-inducing compound, but more of other sulfur compounds that had been trace or undetected. The authors discuss what this means for health and flavor, which was not tested here.
Neon-irradiated seeds grew onions with no tears or pungency for 20 tasters
This plant-breeding study from House Foods in Japan used no gene transfer. The selected bulbs made about 7.5-fold less of the tear-inducing compound, because the alliinase enzyme was almost absent (under 1% of normal mRNA). Sensory panelists tested the bulbs; no health effects were studied.
Who should be careful with onions
Most people can eat onions freely. Take a little care if one of these applies:
- Heartburn and reflux. In a small test, a slice of raw onion on a hamburger increased reflux and heartburn episodes in 16 people with heartburn. It did not in 16 people without heartburn. The trial tested raw onion only, so cooked onion is untested.
- Irritable bowel syndrome. Monash University lists onion among the vegetables particularly rich in fructans. Fructans are a carbohydrate that can trigger symptoms in some people with IBS. They dissolve into soups and stocks, so lifting the pieces out does not help. Monash suggests the green parts of spring onions instead.
- Allergy. Allergic reactions to eating onion are rare and mostly reported as single cases, including one man with anaphylaxis (a severe allergic reaction) after cooked onion. Handling onions can cause contact dermatitis (a skin rash), rhinoconjunctivitis (a runny nose and itchy eyes) and asthma in some people.
How to choose, store and eat onions
- Chopped or sliced: chopping left onion quercetin glucosides virtually unchanged in a lab test, so cutting ahead costs little.
- Fried or microwaved: frying did not change flavonoid intake. Microwaving without water kept flavonoids and vitamin C better than boiling, which sent about 30% of the quercetin into the cooking water.
- Boiled: boiling for 60 minutes cut flavonols by 20.6%, so use the liquid in a soup or sauce. The cholesterol trial used 80 to 120 g of raw red onion a day, close to one medium onion (110 g).
Choose onions that are firm and dry, without cuts or soft spots. Keep them in a dry, cool place such as a pantry for up to 4 weeks. Wrap cut onion tightly and use it within 2 to 3 days.
Residues are no reason to skip onions. In USDA (US Department of Agriculture) tests of 177 samples in 2023, none had a residue above the EPA tolerance, the legal limit.
Sources
- Allen ML, et al. The effect of raw onions on acid reflux and reflux symptoms. Am J Gastroenterol. 1990.
- High and low FODMAP foods. Monash University FODMAP Diet.
- Cooking with onion and garlic: myths and facts. Monash FODMAP blog. 2015.
- Albanesi M, et al. Immunological characterization of onion (Allium cepa) allergy. Postepy Dermatol Alergol. 2019.
- Makris DP, Rossiter JT. Domestic processing of onion bulbs (Allium cepa) and asparagus spears (Asparagus officinalis): effect on flavonol content and antioxidant status. J Agric Food Chem. 2001.
- Ioku K, et al. Various cooking methods and the flavonoid content in onion. J Nutr Sci Vitaminol (Tokyo). 2001.
- Ebrahimi-Mamaghani M, Saghafi-Asl M, Pirouzpanah S, Asghari-Jafarabadi M. Effects of raw red onion consumption on metabolic features in overweight or obese women with polycystic ovary syndrome: a randomized controlled clinical trial. J Obstet Gynaecol Res. 2014.
- Onions. Eat.Move.Save., University of Illinois Extension.
- Onion. Wikipedia.
- Pesticide Data Program annual summary, calendar year 2023. USDA Agricultural Marketing Service.
History of onions
The bulb onion is known only from cultivation, so nobody has found its truly wild form. Its origin has been argued over for a long time. A 2026 genome study of 122 onion accessions strongly supports Allium vavilovii, a wild relative from Turkmenistan, as the wild ancestor. People have grown and bred onions for at least 7,000 years.
Some of the oldest recipes in the world are Babylonian clay tablets from about 1730 BCE, now at Yale. Some of their stews name onion alongside leek and garlic. The onion is also mentioned in the Hebrew Bible. Onion remains have turned up at ancient sites in Israel, from Chalcolithic Nahal Mishmar to Bronze Age Jericho.
Ancient Egyptians saw the onion’s round shape and concentric rings as a symbol of eternal life. Onion traces were found in the eye sockets of Ramesses IV.
In the first century AD, Pliny the Elder wrote that Romans believed onions could improve eye problems and aid sleep. They also thought onions could heal everything from toothaches to dog bites. Onions appear in the Roman recipes collected under the name Apicius, too.
The English word comes from Latin ūniō, which meant onion, by way of Old French oignon. The Latin word for the plant itself, cepa, gave the species its name, Allium cepa, and also lives on in Spanish cebolla and Italian cipolla. The genus name Allium is the classical Latin word for garlic.
Why onions make people cry stayed a puzzle for a long time. In 2002, a Japanese team reported in Nature the enzyme that finishes the reaction. Later work traced the full chain from the sulfur compound in the bulb to the gas that stings the eyes.
English colonists’ diaries list the bulb onion among the first crops the Pilgrims planted in North America. Between 1883 and 1939, inventors in the United States patented 97 machines meant to make onion growing more efficient.
Sources
- Lei P, et al. A near-complete genome assembly of onion resolves its genomic architecture, origin, and evolution. Plant Commun. 2026.
- Onion. Wikipedia.
- Yale culinary tablets. Wikipedia.
- onion. Wiktionary.
- Imai S, et al. Plant biochemistry: an onion enzyme that makes the eyes water. Nature. 2002.
- Kato M, et al. Production and characterization of tearless and non-pungent onion. Sci Rep. 2016.
- Eady CC, et al. Silencing onion lachrymatory factor synthase causes a significant change in the sulfur secondary metabolite profile. Plant Physiol. 2008.
Frequently asked questions about onions
Are onions good for you?
For most people, yes, as a flavorful, low-calorie food. Raw onion has 40 calories, 1.7 g of fiber and 7.4 mg of vitamin C per 100 g, plus 20.3 mg of quercetin.
A meta-analysis of 14 trials found that onion supplements improved LDL cholesterol, systolic blood pressure and body fat. Pooled studies find no overall link between allium vegetables and cancer (0.97 times the risk). None of the findings here reaches strong evidence.
Which is healthier, red or yellow onions?
Neither clearly wins. Published measurements overlap: red onions held 415 to 1,917 mg of flavonols per kilogram and yellow onions 270 to 1,187.
In a lab test of ten US varieties, a yellow one had the most flavonoids (69.2 mg per 100 g). That was 11 times as much as a white one. Red onions add anthocyanins, but after home-style peeling one red variety kept only 27% of them, against 79% of its quercetin glucoside.
Why do onions make you cry?
Chopping releases a gas that stings the nerves in your eyes. Cut cells release the enzyme alliinase, which starts breaking down a sulfur compound in the bulb. A second enzyme, lachrymatory factor synthase, makes the tear-inducing compound. Your tear glands then produce tears to flush it out.
How can you stop onions making you cry?
Illinois Extension says slicing the onion in half and rinsing it under running water removes some of the sulfur compounds behind the tears. Cutting it under water and leaving the root end intact, where sulfur compounds are more concentrated, also reduces irritation. No trial here tested these tips.
Breeders have made onions with no tears and no pungency at all, in a Japanese program tested by 20 panelists.
Are raw or cooked onions healthier?
Both have a case, and how you cook matters more. Boiling moved about 30% of onion quercetin into the cooking water, and boiling for 60 minutes cut flavonols by 20.6%. Frying did not change flavonoid intake, and microwaving without water kept flavonoids and vitamin C.
Raw onion is the form used in the cholesterol trial (80 to 120 g a day for 8 weeks). No trial here compares raw and cooked onion in people.
Do onions cause gas or heartburn?
They can in some people. In a small test, raw onion increased reflux and heartburn episodes in 16 people with heartburn, but not in 16 people without it. Monash University lists onion among the vegetables particularly rich in fructans, which can trigger symptoms in people with irritable bowel syndrome. No trial here measured gas directly.
Do onions lower blood sugar or cholesterol?
Possibly a little, but the evidence is thin. Raw red onion (80 to 120 g a day) lowered LDL and total cholesterol in 54 women with polycystic ovary syndrome. The comparison group’s cholesterol fell too.
Fresh yellow onion lowered fasting glucose and insulin in 56 breast cancer patients. Both trials were small and lasted 8 weeks. If you take medicine for diabetes or cholesterol, keep taking it and check with your care team before changing your diet.
Are onions good for weight loss?
Not on their own. Onions have 40 calories and 1.7 g of fiber per 100 g and are 89% water, so they add flavor and volume cheaply.
Trials of onion products gave mixed results for body fat. A steamed onion capsule reduced it in 70 overweight adults over 12 weeks. But 9 g a day of onion powder did not change visceral fat overall. Those were concentrated products, not a plate of onions.