Onion

Onion (Allium cepa L.): A Comprehensive Reference

1. Identity and Botanical Classification

Onion (Allium cepa L.) is a member of the family Amaryllidaceae and one of the most widely cultivated species of the genus Allium. It is a culinary and medicinal herb that ranks as the third most important horticultural crop globally after potato and tomato, widely consumed throughout the world mainly for its distinctive flavor.

The onion plant is a perennial herb growing to about 1.2 m in height, with 4 to 6 hollow, cylindrical leaves. On top of the long stalks, greenish-white flowers in the form of solitary umbels grow to 2.5 cm wide. The seeds of the plant are black and angular.

The underground bulb, which is used medicinally and consumed as food, is comprised of fleshy leaf sheaths forming a thin-skinned capsule and varies greatly in size (2 to 20 cm). The shape (flattened, spherical, or pear-shaped) and color of the bulb depend on the variety. Onion is believed to have been domesticated in central and western Asia.

Common Forms and Preparations

• Fresh bulb (raw or cooked): The most common dietary form, consumed globally as a vegetable and culinary base ingredient.
• Onion juice: Expressed from fresh bulbs, used in traditional medicine and studied in clinical trials.
• Onion extract (aqueous or ethanolic): Onion peel extract, standardized to quercetin 100 mg/day, has been used for 12-week clinical trials evaluating endothelial function.
• Onion powder/dried onion: Produced by dehydration; used in foods and studied as a supplement form. Different drying methods, such as microwave drying, air drying, and freeze drying, have been evaluated for influence on antimicrobial activity, with freeze-dried onion bulbs showing stronger antimicrobial properties.
• Onion skin extract: Onion peels, often discarded as waste, are a concentrated source of dietary fibre, phenolics, and flavonoids, and have been studied for benefits such as cholesterol reduction and improved gut health.
• Quercetin standardized capsules/tablets: Isolated quercetin from onion sources, used at defined doses in interventional trials.

2. Traditional and Historical Use

Origins and Early Cultivation

The cultivation history of onions can be traced back to Central Asia and the Mediterranean region around 3000 BC. Onions grew in Chinese gardens as early as 5,000 years ago and are referenced in some of the oldest Vedic writings from India. In Egypt, onions can be traced back to 3500 BC, while there is evidence that the Sumerians were growing onions as early as 2500 BC.

Ancient Egypt

In Egypt, onions were an object of worship. The onion symbolized eternity to the Egyptians, who buried onions along with their Pharaohs. The Egyptians saw eternal life in the anatomy of the onion because of its circle-within-a-circle structure. Paintings of onions appear on the inner walls of the pyramids and in the tombs of both the Old Kingdom and the New Kingdom.

Classical Greece and Rome

Dioscorides, a Greek physician in the first century AD, noted several medicinal uses of onions. The Greeks used onions to fortify athletes for the Olympic Games; before competition, athletes would consume pounds of onions, drink onion juice, and rub onions on their bodies. The ancient Greek doctor Hippocrates once recommended using onions to treat respiratory diseases, while the Roman scholar Pliny recorded 62 medicinal methods of onions in his Natural History.

The Romans ate onions regularly and carried them on journeys to their provinces in England and Germany. Pliny the Elder wrote of Pompeii's onions and cabbages.

India (Ayurvedic Tradition)

In India, as early as the sixth century BC, the famous medical treatise Charaka Sanhita celebrates the onion as medicine — a diuretic, good for digestion, the heart, the eyes, and the joints. Ayurvedic and Traditional Chinese Medicine employed onions for respiratory and digestive health.

Medieval Europe and Beyond

Onions were consumed throughout Europe during the Middle Ages and were later thought to guard against evil spirits and the plague, probably because of their strong odor. Folk healers traditionally used onions to prevent infections, and an onion and garlic concoction cooked in milk was used as a European folk remedy for congestion. Christopher Columbus is said to have introduced the plant to North America on his 1492 expedition.

Range of Traditional Indications

Onion has been consumed for centuries in various indigenous cultures for the management of several ailments including microbial infections, respiratory, gastrointestinal, skin and cardiovascular disorders, diabetes, renal colic, rheumatism, sexual impotence, menstrual pain, and headache. As a remedy, onions have been historically used for a wide variety of ailments including hair loss, mouth sores, vision problems, asthma, and snake bites.

3. Key Constituents and Active Compounds

Over 400 compounds of diverse chemical classes have been identified in onion. The principal bioactive categories are described below.

Flavonoids

The most abundant flavonoids found in onions are quercetins, namely quercetin-4′-monoglucoside and quercetin-3,4′-diglucoside, which account for more than 85% of the total flavonoid content. Diallyl monosulfide, diallyl disulfide, diallyl trisulfide, and diallyl tetrasulfide are the major organosulfur compounds, whereas quercetin, kaempferol, anthocyanin, and luteolin are considered the main flavonoids.

In addition to flavonoids, onions — especially the red varieties — are a rich source of anthocyanins. The most frequently reported anthocyanins in red onions are cyanidin derivatives, although minor amounts of peonidin, petunidin, and delphinidin derivatives have also been identified.

Organosulfur Compounds

Flavonoids such as quercetin and kaempferol, alk(en)yl cysteine sulfoxides including S-methyl cysteine sulfoxide and S-propyl cysteine sulfoxide, cycloalliin, thiosulfinates, and sulfides are the main compounds existing in the plant. Onion has plentiful chemical compounds such as allicin, quercetin, fisetin, and other sulphurous compounds including diallyl disulphide and diallyl trisulphide.

Species belonging to the genus Allium have been widely used for human food and traditional medicine, and their beneficial health effects, as well as the specific aroma, are associated with their bioactive chemical compounds, such as sulfur compounds and flavonoids.

Other Notable Compounds

Onions contain 89% water, 1.5% protein, and vitamins B1, B2, and C, along with potassium and selenium. Polysaccharides such as fructosans, saccharose, and others are also present, as are peptides, flavonoids (mostly quercetin), and essential oil. Ascorbic acid and fructooligosaccharides are also regarded as bioactive compounds.

One of the components found in onion is fisetin (3,3′,4′,7-tetrahydroxyflavone) at concentrations of 2–160 µg/g. Fisetin is a bioflavonoid with antioxidant, anti-inflammatory, anticancer, antihyperlipidaemic, and neuroprotective properties.

Quercetin Content and Bioavailability

Onions contain 20–30 mg of quercetin per 100 g, making them a rich dietary source. Average daily dietary intake in the US is approximately 25–50 mg. Pharmacokinetic studies suggest greater bioavailability of quercetin from onion powder over apple peel, of fresh red onion over dietary supplementation with aglycone quercetin, and of onion skin extract over pure quercetin dihydrate powder-filled hard capsules.

4. Mechanisms of Action

Antioxidant Activity

The beneficial effect on health is attributed to high contents of biologically active phytomolecules, such as phenolic compounds, especially flavonoids, and several organosulfur compounds. Alterations in oxidant/antioxidant markers including lipid peroxidation (LPO), glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), and MDA have been observed by studies investigating the effects of A. cepa and its constituents.

Anti-inflammatory Pathways

The anti-inflammatory effect of onion has been attributed to its potential in preventing the formation of leukotrienes and thromboxanes via inhibition of COX and LOX pathways. Onion and its derivatives including saponins, aglycones, quercetin, cepaenes, flavonoids, organosulfurs, and phenolic compounds have shown various pharmacological properties and therapeutic effects. In vitro studies suggest that high concentrations of quercetin (>1 µM) have anti-inflammatory effects.

Cardiovascular Mechanisms

Quercetin has prominent medicinal value in cardiovascular disease, such as antioxidation, antiplatelet aggregation, reducing myocardial fibrosis, improving ventricular remodeling and cardiac function, protecting vascular endothelium, anti-arrhythmia, anti-heart failure, preventing ischemia-reperfusion injury, and regulating blood pressure. A number of proposed mechanisms may be responsible for observed blood pressure decreases, such as antioxidant effects, inhibition of angiotensin-converting enzyme activity, and improved endothelium-dependent and -independent function.

Antidiabetic Mechanisms

Research indicates that Allium cepa exhibits significant potential in managing diabetes through various mechanisms, including antioxidant activity, modulation of glucose metabolism, and improvement of insulin sensitivity. Ethanolic extracts derived from dried onion peel demonstrate inhibition of alpha-glucosidase, leading to reduced blood glucose levels, and this inhibition prompts an increase in insulin production. Allium cepa may also act as a hypoglycemic agent through extra-pancreatic effects, acting directly on tissues such as liver and muscles.

Antimicrobial Activity

Onion extracts and their derived bioactive compounds, such as thiosulfinate compounds, phenolic compounds, polysaccharides, and essential oils, have been reported to possess potent antibacterial properties, antifungal activities, and antiviral effects. Recent studies have identified two primary antimicrobial agents in onions: quercetin and allicin.

5. Scientific Evidence by Area of Use

5.1 Cardiovascular Health and Blood Lipids

Lipid Profile (Meta-analysis Level Evidence): Ten trials with 446 participants in total were included in a meta-analysis. The pooled findings suggested that onion supplementation significantly improved HDL cholesterol (MD: +2.29 mg/dl; 95% CI: 0.87, 3.72; I² = 0%) and lowered LDL cholesterol (MD: −6.64 mg/dl; 95% CI: −10.91, −2.36; I² = 32%), while onion supplementation did not significantly lower triglycerides (MD: −6.55 mg/dl; 95% CI: −15.64, 2.53; I² = 45%). Analysis of nine trials showed a significant reduction in total cholesterol (MD: −5.39 mg/dl; 95% CI: −10.68, −0.09; I² = 49%).

Broader Metabolic Meta-analysis: A systematic review and meta-analysis (through October 2022) found that onion supplementation improved body fat percentage, blood lipid profile (except triglycerides), systolic blood pressure, and blood levels of adiponectin and AST. However, consumption of onion supplementation had no significant effect on anthropometric measurements (except body fat percentage), glycemic indices, blood triglyceride levels, blood leptin levels, or diastolic blood pressure.

Endothelial Function: In a study where healthy men (44±10 years, n=23) received 4.3 g of onion extract (containing 51 mg of quercetin) once a day for 30 days, flow-mediated vasodilation (FMD) responses were measured before and after. Maltose loading significantly decreased FMD both before and after chronic onion extract intake.

Onion peel extract containing quercetin at 100 mg/day for 12 weeks improved endothelial function in a small group of healthy overweight and obese subjects.

Cohort studies have associated dietary intake of flavonoids in general and quercetin in particular with a decreased risk for CVD. Although the physiological mechanisms accounting for this benefit remain incompletely defined, animal studies and human intervention studies have identified relevant effects — for example, supplementation of quercetin may reduce platelet aggregation and plasma concentrations of atherogenic oxidised LDL (oxLDL).

Evidence strength: Moderate. A 2021 meta-analysis of RCTs supports lipid improvements; however, individual trial populations are small and heterogeneous, and the evidence is stronger for LDL/HDL than for triglycerides.

5.2 Blood Pressure

Key RCT (Onion Skin Extract / Quercetin): Subjects (n=70) were randomised to receive 162 mg/day quercetin from onion skin extract powder or placebo in a double-blinded, placebo-controlled crossover trial with 6-week treatment periods separated by a 6-week washout period. In the total group, quercetin did not significantly affect 24-hour ambulatory blood pressure (ABP) parameters and office BP. However, in the subgroup of hypertensives, quercetin decreased 24-hour systolic BP by −3.6 mmHg.

Epidemiological studies have reported that intake of quercetin, found in onions, is inversely related to cardiovascular disease. More recent work using hypertensive animals and humans (>140 mmHg systolic and >90 mmHg diastolic) indicates a decrease in blood pressure after quercetin supplementation. The majority of mechanisms have been identified using animal models treated with quercetin, and relatively few have been corroborated in human studies.

In humans, the blood pressure-lowering effects of chronic quercetin supplementation are difficult to predict. There have been a few small human studies exploring the effects of quercetin supplementation on heart health, but the results have been inconsistent, and more well-designed clinical trials are needed.

Evidence strength: Modest and preliminary. A significant effect in hypertensive subgroups has been observed in an RCT, but the overall evidence is limited by small sample sizes and inconsistency across trials.

5.3 Glycemic Control and Diabetes

Preliminary Clinical Evidence: In assessment of hypoglycaemic activity of Allium cepa in type 1 and type 2 diabetic patients, ingestion of crude Allium cepa (100 g) caused a considerable reduction in fasting blood glucose levels.

Some studies show decreases in blood glucose levels in healthy volunteers and non-insulin-dependent diabetic subjects with aqueous onion extract and raw dietary onion. However, in overweight-to-obese patients with hypertension or pre-hypertension, glucose metabolism was not affected by supplemental onion skin extract (quercetin 162 mg/day over 6 weeks), and fasting blood sugar was also unaffected in a trial evaluating effects of raw red onion consumption in overweight or obese women with polycystic ovary syndrome.

The chemical constituents of onions, such as quercetin, sulfur compounds, and flavonoids, have been shown in some studies to lower blood glucose levels and enhance insulin secretion. However, in vitro and clinical studies show heterogeneous and sometimes inconclusive results. Further research is needed to establish optimal dosages, mechanisms of action, and long-term efficacy.

Evidence strength: Weak to preliminary for human clinical evidence. While mechanistic (in vitro and animal) data are more substantial, clinical trials are small, short-term, and of variable quality. The effect in already-medicated diabetic patients requires careful monitoring.

5.4 Antioxidant and Anti-inflammatory Effects

A double-blind, placebo-controlled randomized clinical trial showed that after 8 weeks of oral treatment with 500 mg quercetin or placebo in 88 myocardial infarction patients, the serum total antioxidant capacity of the quercetin group was significantly improved, while the inflammatory factor TNF-α was also significantly reduced. However, there were no significant changes in IL-6, C-reactive protein (CRP), and blood pressure in both groups.

A large number of in vitro and in vivo studies have shown that quercetin has various functions such as anti-inflammatory, antioxidant, antihypertensive, hypoglycemic, neurovascular protection, anticancer, anti-aging, and immune enhancement. However, many of these mechanisms remain primarily established in cell or animal models.

Evidence strength: Preclinical evidence is robust; clinical evidence for anti-inflammatory outcomes is limited and inconsistent.

5.5 Cancer — Epidemiological and Preclinical Evidence

Many epidemiological studies have confirmed that dietary consumption of onions is associated with a reduced risk of developing many forms of cancer and cardiovascular and neurodegenerative diseases. A meta-analysis by Zhou et al. (2011) showed that higher consumption of Allium vegetables, including onions, is associated with a significantly reduced risk of gastric cancer.

In adrenocortical carcinoma, quercetin-rich extracts have been shown to induce cell cycle arrest and apoptosis more effectively than the standard drug mitotane in cell studies. In the presence of onion extracts, a significant decrease in DNA damage was observed, indicating a protective role on DNA strand breaks.

Evidence strength: Epidemiological association exists for certain cancers (particularly gastric cancer); direct human clinical intervention evidence is lacking. Cellular and animal data are promising but have not been translated to clinical trials.

5.6 Antimicrobial Activity

Onion extracts and their derived bioactive compounds, including thiosulfinate compounds, phenolic compounds, polysaccharides, and essential oils, have been reported to possess potent antibacterial properties, antifungal activities, and antiviral effects.

Onion skin extracts have been tested in vitro against multiple pathogens including Listeria monocytogenes, Escherichia coli O157:H7, Bacillus cereus, Enterococcus faecalis, Pseudomonas aeruginosa, Staphylococcus aureus, Salmonella typhimurium, and Yersinia enterocolitica.

Evidence strength: Exclusively in vitro. No human clinical trials have examined onion as a standalone antimicrobial intervention.

5.7 Respiratory and Asthma — Preclinical

In an allergic asthma model, onion bulb extract (OBE), administered prior to allergen challenge, significantly inhibited the allergen-induced increase in airway cellularity and histopathological changes, and also pro-inflammatory Th2 cytokines such as IL-4 and IL-5, while enhancing levels of the anti-inflammatory cytokine IL-10. There is good preclinical evidence that onion/OBE has anti-inflammatory action, particularly when administered immediately before an inflammatory-inducing agent.

Evidence strength: Animal/preclinical only. Human clinical translation has not been demonstrated.

6. Body Systems and Health Areas Associated with Onion

A. cepa and its pharmacologically active constituents display broad-spectrum activities including antioxidant, anti-inflammatory, lipid-modifying, anti-obesity, antihypertensive, and antidiabetic effects.

• Cardiovascular system: LDL/HDL cholesterol modulation, blood pressure effects, endothelial function, platelet aggregation inhibition, and oxidized LDL reduction.
• Metabolic/Endocrine system: Blood glucose modulation, insulin sensitivity, body fat percentage effects.
• Gastrointestinal system: Prebiotic fructooligosaccharides supporting gut microbiota; higher intakes of onion may worsen existing heartburn, though it does not appear to cause heartburn in people who do not already have it.
• Immune/Inflammatory system: COX/LOX inhibition, cytokine modulation, antioxidant enzyme support.
• Respiratory system: Traditional use in respiratory ailments; preclinical anti-asthmatic data only.
• Antimicrobial: In vitro activity against a broad range of bacteria and fungi.
• Oncological (preventive/epidemiological): Associated with reduced gastric cancer risk in epidemiological studies; preclinical antiproliferative data in cancer cell lines.
• Hepatic: Onion and its main components have shown benefits including anti-gastric ulcer properties, and hepatoprotective effects have been demonstrated in animal models.

7. Dosage Forms and Reported Doses in Studies

No standardized therapeutic dosage for onion as a supplement has been established by regulatory agencies. The following are doses as reported in cited sources:

• Quercetin from onion skin extract, 162 mg/day: Used in a double-blinded, placebo-controlled crossover trial (n=70, 6-week treatment periods) examining blood pressure in overweight-to-obese patients with pre-hypertension and stage I hypertension.
• Quercetin from onion skin extract, 100 mg/day for 12 weeks: Used in a small clinical study improving endothelial function in healthy overweight and obese subjects.
• Onion extract, 4.3 g containing 51 mg quercetin, once daily for 30 days: Used in a study of healthy men (n=23) examining postprandial endothelial function (flow-mediated vasodilation).
• Quercetin, 500 mg/day for 8 weeks: Used in a double-blind, placebo-controlled, randomized clinical trial in 88 myocardial infarction patients, demonstrating improved total antioxidant capacity and reduced TNF-α.
• Crude Allium cepa, 100 g (raw fresh onion): Used in a preliminary clinical study of hypoglycemic effects in type 1 and type 2 diabetic patients.
• Quercetin from onion peel extract, 150 mg/day (supra-nutritional dose): Reported in patients with high cardiovascular risk phenotype in a prior trial, with significant reductions in systolic blood pressure observed.
• Quercetin, 730 mg/day for 4 weeks: Used in a study of human subjects examining blood pressure response, described as a pharmacological dose.

Methods for the qualitative assessment of the flavonoids have been detailed, and quercetin glycosides have been shown to be heat stable and transferable to cooking water.

8. Safety Considerations and Interactions

General Safety Status

Onion has "generally recognized as safe" (GRAS) status when used as food. No formal contraindications have been identified at ordinary food consumption levels. With large intake, the stomach may be affected, and frequent contact with onion rarely may cause allergic reactions.

Gastrointestinal Effects

Onions are a high-FODMAP food, meaning they can worsen IBS symptoms in some people. Higher intakes of onion may worsen existing heartburn, though it does not seem to cause heartburn in people who do not already have it.

Allergic Reactions

Onion seeds have been reported as occupational allergens, but frequent contact with onion rarely causes allergic reactions. Certain sulfur compounds, such as propanethial-S-oxide, escape from onion in vapor form and hydrolyze to sulfuric acid when cut, causing eye irritation and lacrimation.

Onions belong to the Liliaceae family of flowering plants; research suggests that allergenic cross-reactivity may occur among different Liliaceae plants as well as grass pollens. People with allergies to mugwort and celery might also be allergic to onion.

Drug Interactions

• Antidiabetic medications: There is a rated moderate interaction between onion and antidiabetes drugs. Onion may decrease blood sugar, and diabetes medications are also used to lower blood sugar. Concurrent use in medicated diabetic patients warrants monitoring.
• Lithium: Onion might have an effect like a diuretic. Taking onion might decrease how well the body gets rid of lithium, potentially increasing how much lithium is in the body and resulting in serious side effects.
• CYP2E1 substrates: Some medications are changed and broken down by the liver. Onion may slow down how quickly the liver breaks down some medications, potentially increasing the effects and side effects of CYP2E1 substrates. Medications that might be affected include acetaminophen, chlorzoxazone, ethanol, theophylline, and certain anesthetics such as enflurane, halothane, isoflurane, and methoxyflurane.
• Aspirin and onion allergy: Some people are allergic to onions. Aspirin might increase sensitivity to onions in allergic individuals, though this has only been reported in one person.

Use in Supplemental vs. Dietary Amounts

Pharmacokinetic studies suggest that supplemental quercetin (166 mg) would be comparable to about 10 mg of quercetin aglycone equivalent from red onion consumed as food, and safety has not been established for amounts greater than those found in foods.

References

• Chemical Composition and Biological Activity of Allium cepa L. and Allium × cornutum Methanolic Extracts — PMC
• Onion (Allium cepa) and its Main Constituents as Antidotes or Protective Agents against Natural or Chemical Toxicities — PMC
• The effects of Allium cepa L. (onion) and its active constituents on metabolic syndrome: A review — PMC
• Onion (Allium cepa L.) bioactives: Chemistry, pharmacotherapeutic functions, and industrial applications — Food Frontiers (Wiley)
• An ethnopharmacological, phytochemical, and pharmacological overview of onion (Allium cepa L.) — Journal of Ethnopharmacology (ScienceDirect)
• Onion (Allium cepa L.) Organosulfur Compounds: From Traditional Use to Modern Pharmacological Insights — PMC
• Research progress of quercetin in cardiovascular disease — PMC
• Effect of onion on blood lipid profile: A meta-analysis of randomized controlled trials — PMC / Food Science & Nutrition
• Effects of a quercetin-rich onion skin extract on 24 h ambulatory blood pressure and endothelial function in overweight-to-obese patients with (pre-)hypertension — PMC / British Journal of Nutrition
• Chronic intake of onion extract containing quercetin improved postprandial endothelial dysfunction in healthy men — PubMed
• Therapeutic potential of quercetin to decrease blood pressure: review of efficacy and mechanisms — PubMed
• Preliminary Study of the Clinical Hypoglycemic Effects of Allium cepa (Red Onion) in Type 1 and Type 2 Diabetic Patients — PMC
• Quantification of the Anti-diabetic Effect of Allium cepa — PMC
• Onion supplementation and health metabolic parameters: A systematic review and meta-analysis of randomized controlled trials — PubMed / Clinical Nutrition ESPEN
• A review of anti-inflammatory, antioxidant, and immunomodulatory effects of Allium cepa and its main constituents — PMC
• Recent Advances in Bioactive Compounds, Health Functions, and Safety Concerns of Onion (Allium cepa L.) — PMC
• Treatment with onion bulb extract both prevents and reverses allergic inflammation in a murine model of asthma — PMC
• Onion nutritional and nutraceutical composition and therapeutic potential of its phytochemicals assessed through preclinical and clinical studies — ScienceDirect
• Onion Uses, Benefits & Dosage — Drugs.com (Natural Product monograph)
• Onion: Health Benefits, Side Effects, Uses, Dose & Precautions — RxList
• History of Onion — New Mexico State University
• Enhanced Yield of Bioactivities from Onion (Allium cepa L.) Skin and Their Antioxidant and Anti-α-Amylase Activities — PMC