Onion (Allium cepa L.)

The onion is a biennial herbaceous species cultivated as an annual vegetable crop and is widely grown in temperate regions, where it represents one of the most important horticultural species. The plant forms a basal rosette of hollow, tubular leaves, typically green in colour, and develops an underground bulb composed of concentric, fleshy scales surrounded by papery outer tunics that protect the internal tissues. Bulb shape and size can vary considerably (spherical, flattened, or elongated), as can the colour of the outer tunic, which may be white, yellow, bronze or red, depending on the cultivar.

From an agronomic perspective, onion prefers well-drained soils of medium texture, with good fertility and adequate structure. Optimal growth is obtained under moderate temperature regimes, with excessive humidity and waterlogging being unfavourable, as they promote fungal diseases and reduce storability. The crop cycle depends on cultivar and planting date (direct seeding or transplanting), with harvest generally concentrated in summer to early autumn. Final quality is strongly influenced by water management, fertilization practices, plant density, and seasonal climatic conditions.

The onion bulb consists predominantly of water, with a fraction of carbohydrates mainly represented by glucose, fructose and sucrose. It also contains dietary fibre, potassium (with lower levels of calcium and magnesium), and a range of sulphur-containing compounds responsible for the characteristic pungent aroma and lachrymatory effect. In addition, the bulb provides flavonoids (such as quercetin), organic acids, and small amounts of water-soluble vitamins. The concentration of these components varies according to cultivar, growing conditions, agronomic management, and maturity at harvest.

From a technological and culinary standpoint, onion is an extremely versatile vegetable ingredient. It is consumed raw, cooked, caramelised, dehydrated, or processed into preserves, dried products, stocks and flavour bases. In many formulations it plays a key aromatic and flavour-building role, contributing to the sensory profile of dishes based on meat, fish, vegetables, sauces and soups. Commercial quality is assessed on the basis of bulb size, firmness of tissues, uniform colour, integrity of the outer tunics, storage potential, and absence of defects such as rots, early sprouting or abnormal desiccation. Different onion types and cultivars (round, flat, elongated; white, yellow, red) are suited to specific markets and uses, including fresh consumption, long-term storage, or industrial processing, with marked differences in pungency, aroma intensity, and sulphur compound content.

Botanical classification (APG IV system)

• Common name: Onion

• Botanical name: Allium cepa L.

• Kingdom: Plantae

• Clade: Angiosperms → Monocots

• Order: Asparagales

• Family: Amaryllidaceae

• Genus: Allium

• Species: Allium cepa L.

Indicative nutritional values per 100 g (raw, edible bulb)

Values may vary depending on cultivar, agronomic conditions and maturity stage.

Mini-glossary of acronyms

• SFA = Saturated fatty acids. When they largely replace unsaturated fats in the diet, their profile is considered less favourable for cardiovascular health.

• MUFA = Monounsaturated fatty acids. Generally beneficial when replacing saturated fats.

• PUFA = Polyunsaturated fatty acids. Include omega-6 and omega-3, essential in small amounts and involved in membrane structure and inflammatory balance.

Plant Characteristics

• Bulb: Underground storage organ composed of fleshy, concentric layers. It varies in color (white, yellow, red, purple) and size depending on the variety.
• Stem: Short and hidden within the bulb.
• Leaves: Hollow, tubular, and green, performing photosynthesis.
• Flowers: Small, white or greenish, arranged in spherical umbels on long flower stalks.
• Roots: Fibrous and shallow, staying close to the soil surface.
• Habitat: Cultivated worldwide, preferring temperate climates and well-drained soils.

Chemical Composition

Onions are rich in bioactive compounds and essential nutrients:

• Vitamins: High in vitamin C, B-complex vitamins (B6, folic acid), and small amounts of vitamin A.
• Minerals: Contain potassium, calcium, iron, magnesium, and zinc.
• Sulfur Compounds: Such as sulfides and thiosulfonates, responsible for flavor and health benefits.
• Flavonoids: Quercetin and other antioxidants with protective effects.
• Fiber: Supports digestion and gut health.
• Polyphenols: Contribute to anti-inflammatory and antioxidant properties.

How to Cultivate Allium cepa

Onions are relatively easy to cultivate:

1. Climate: Grow best in temperate climates with temperatures between 13°C and 24°C.
2. Soil Requirements: Prefer well-drained, loose, organic-rich soil.
3. Planting: Can be grown from seeds, sets (small bulbs), or transplants, depending on the variety and season.
4. Irrigation: Require moderate watering, avoiding excess moisture that can cause root rot.
5. Weed Control: Keeping the field weed-free is essential for good growth.
6. Harvesting: Ready when leaves turn yellow and begin to bend over naturally.
7. Storage: After harvest, onions are dried in a well-ventilated area before storage.

Uses and Benefits

• Culinary Uses:

  • A staple ingredient in various cuisines, adding flavor to numerous dishes.
  • Can be eaten raw, cooked, fried, baked, or pickled.
  • Used in soups, stews, sauces, salads, and meat or vegetable dishes.

• Health Benefits:

  • Antioxidant Properties: High quercetin levels help combat free radicals.
  • Anti-inflammatory: Sulfur compounds contribute to reducing inflammation.
  • Cardiovascular Health: Helps regulate blood pressure and cholesterol levels.
  • Antibacterial & Antiviral: Traditionally used to support immune function.
  • Blood Sugar Regulation: Studies suggest onions may help maintain stable glucose levels.
  • Digestive Aid: Stimulates gastric juices and supports gut health due to prebiotic fiber.

• Agricultural and Environmental Uses:

  • Natural Pest Repellent: Its strong odor deters certain insects in horticulture.
  • Organic Fertilizer: Onion scraps can be composted to enrich soil nutrients.

• Cosmetic Uses:

  • Onion extract is found in skincare products for scar reduction and skin brightening.
  • Some traditional remedies use onion juice to promote hair growth.

Applications

• Food Industry:

  • Dried and powdered for seasonings and flavor enhancers.
  • Used in pickles and preserves.

• Traditional Medicine:

  • Used in home remedies for colds, coughs, and respiratory infections.
  • Found in some supplements for cardiovascular and digestive health.

• Agriculture:

  • Planted in crop rotation systems to improve soil health.
  • Being studied for potential applications in organic pest control.

Environmental and Safety Considerations

• Environmental Benefits:

  • Onion cultivation contributes to agricultural biodiversity.
  • Some local varieties are drought-resistant, making them suitable for sustainable farming.

• Safety:

  • Excessive onion consumption may cause gastric irritation in sensitive individuals.
  • Onion juice can irritate the eyes and mucous membranes during preparation.

Research and Future Potential

Onions are being extensively studied for their bioactive compounds and potential health benefits. Recent research explores their role in preventing chronic diseases such as diabetes, cardiovascular conditions, and certain cancers. The food industry is developing new onion-based products to harness their nutritional and functional properties. In agriculture, researchers are working on climate-resilient onion varieties and improving post-harvest storage techniques to reduce food waste.

Studies

Onions have antibacterial properties and can help reduce the risk of certain chronic diseases. Quercetin, found in onions, has been linked to preventing age-related diseases such as diabetes, cancer, and neurodegenerative diseases.

Animal studies have shown the effectiveness of onion as an anticoagulant.

Human studies have found the usefulness of onion combined with other vegetables to prevent cardiovascular problems.

As an anti-inflammatory, a component of the onion, Onionin A found in the bulb, has been shown to perform an action to counteract inflammatory blood activity (1).

There would also appear to be an action to improve the balance of blood sugar.

It is recommended, along with garlic, shallots and chives to prevent prostate cancer (2).

The best known red onions in Italy are the red head of Tropea and Certaldo.

Red onion studies

References________________________________________________________________________

(1) Fujiwara Y, Horlad H, Shiraishi D, Tsuboki J, Kudo R, Ikeda T, Nohara T, Takeya M, Komohara Y. O

(1) Fujiwara Y, Horlad H, Shiraishi D, Tsuboki J, Kudo R, Ikeda T, Nohara T, Takeya M, Komohara Y. Onionin A, a sulfur-containing compound isolated from onions, impairs tumor development and lung metastasis by inhibiting the protumoral and immunosuppressive functions of myeloid cells.  Mol Nutr Food Res. 2016 Nov;60(11):2467-2480. doi: 10.1002/mnfr.201500995.

(2) Colli JL, Amling CL. Chemoprevention of prostate cancer: what can be recommended to patients?  Curr Urol Rep. 2009 May;10(3):165-71. Review.

Abstract. Prostate cancer is third to lung and colon cancer as the cause of cancer-related deaths in American men. It is estimated that there will have been more than 28,000 deaths and 186,000 new cases in 2008 that will impose a significant burden on national health care costs. Chemoprevention aims to reduce both incidence and mortality through the use of agents to prevent, reverse, or delay the carcinogenic process. This study provides clinicians with information on some chemoprevention agents that have been considered to reduce prostate cancer risks, including 5-alpha-reductase inhibitors; statins (a class of compounds used to reduce cholesterol); NSAIDs; selenium; vitamins E and D; lycopene; allium vegetables (garlic, scallions, onions, chives, and leeks); soy/isoflavones; and green tea polyphenols. The evidence to support prostate cancer risk reduction benefits for each chemoprevention agent based on a review of the literature is provided.

Tsuboki J, Fujiwara Y, Horlad H, Shiraishi D, Nohara T, Tayama S, Motohara T, Saito Y, Ikeda T, Takaishi K, Tashiro H, Yonemoto Y, Katabuchi H, Takeya M, Komohara Y. Onionin A inhibits ovarian cancer progression by suppressing cancer cell proliferation and the protumour function of macrophages. Sci Rep. 2016 Jul 12;6:29588. doi: 10.1038/srep29588.

Abstract. It is well known that tumour-associated macrophages (TAMs) play an important role in tumour development by modulating the tumour microenvironment, and targeting of protumour activation or the M2 polarization of TAMs is expected to be an effective therapy for cancer patients. We previously demonstrated that onionin A (ONA), a natural low molecular weight compound isolated from onions, has an inhibitory effect on M2 macrophage polarization. In the present study, we investigated whether ONA had a therapeutic anti-ovarian cancer effect using in vitro and in vivo studies. We found that ONA reduced the extent of ovarian cancer cell proliferation induced by co-culture with human macrophages. In addition, we also found that ONA directly suppressed cancer cell proliferation. A combinatorial effect with ONA and anti-cancer drugs was also observed. The activation of signal transducer and activator of transcription 3 (STAT3), which is involved in cell proliferation and chemo-resistance, was significantly abrogated by ONA in ovarian cancer cells. Furthermore, the administration of ONA suppressed cancer progression and prolonged the survival time in a murine ovarian cancer model under single and combined treatment conditions. Thus, ONA is considered useful for the additional treatment of patients with ovarian cancer owing to its suppression of the protumour activation of TAMs and direct cytotoxicity against cancer cells.

Li Q, Wang Y, Mai Y, Li H, Wang Z, Xu J, He X. Health Benefits of the Flavonoids from Onion: Constituents and Their Pronounced Antioxidant and Anti-neuroinflammatory Capacities. J Agric Food Chem. 2020 Jan 22;68(3):799-807. doi: 10.1021/acs.jafc.9b07418. 

Abstract. Onion is the most widely cultivated vegetable around the world. In this study, the isolation, concentration, quantification, and bioactivity evaluation of the phenolics in onion peels were investigated. Thirty-four phenolics, including 17 flavonoids and 17 nonflavonoid phenolics, were purified and identified. Among them, there were 2 new unusual epoxyflavanones and a new phenolic constituent, as well as 13 unreported constituents from the genus Allium. The total flavonoids were concentrated, and finally obtained 90.25% of flavonoid content extract. Fifteen main flavonoids were quantified using an ultra-performance liquid chromatography-photodiode array (UPLC-PDA), and quercetin (36.94%) and quercetin 4'-O-β-d-glucopyranoside (15.81%) were the richest contents of flavonoids. The antioxidant and anti-neuroinflammatory capacities were evaluated, and the bioactive results indicated that the flavonoids in onion exhibited pronounced activities. The study suggested that the flavonoids in onion peels could be used in functional food.

Yoo Y, Kim S, Lee W, Kim J, Son B, Lee KJ, Shin H. The prebiotic potential of dietary onion extracts: shaping gut microbial structures and promoting beneficial metabolites. mSystems. 2025 Jan 21;10(1):e0118924. doi: 10.1128/msystems.01189-24. 

Abstract. Onions are well-known vegetables that offer various health benefits. This study explores the impact of onion extracts on gut microbiome using an in vitro fecal incubation model and metabolome analysis. Fecal samples were collected from 19 healthy donors and incubated in the presence or absence of onion extracts for 24 h. To reduce inter-individual variability in the gut microbiome, we employed enterotyping based on baseline fecal microbiota: 14 subjects with a Bacteroides-dominant type (enterotype B) and 5 subjects with Prevotella-dominant type (enterotype P). Alpha diversity was significantly reduced in the onion-treated group compared to the non-treated control group in both Bacteroides- and Prevotella-dominant types. However, significant structural differences in bacterial communities were observed based on weighted UniFrac distance. Notably, short-chain fatty acid (SCFA)-producing bacteria, such as Bifidobacterium_388775, Feacalibacterium, and Fusicatenibacter, were overrepresented in response to onion extracts in enterotype B. Furthermore, genes related to butyrate production were significantly overrepresented in the onion-treated group within enterotype B. Consistent with the enriched taxa and the predicted metabolic pathways, SCFAs and their related metabolites were significantly enriched in the onion-treated group. Additionally, tryptophan metabolism-derived metabolites, including indolelactate (ILA) and indolepropionate (IPA), were elevated by 4- and 32-fold, respectively, in the onion-treated group compared to the control group. In vitro growth assays showed an increase in lactobacilli strains in the presence of onion extracts. These results provide evidence that onion extracts could serve as promising prebiotics by altering gut microbial structure and promoting the production of beneficiary metabolites, including SCFAs and indole derivatives, and enhancing the growth of probiotics.IMPORTANCEThis study is significant as it provides compelling evidence that onion extracts have the potential to serve as effective prebiotics. Utilizing an in vitro fecal incubation model and enterotyping to reduce inter-individual variability, the research demonstrates how onion extracts can alter gut microbial structure and promote the production of beneficial metabolites, including SCFAs and indole derivatives like ILA and IPA. Additionally, onion extract treatment enhances the growth of beneficial probiotics. The findings underscore the potential of onion extracts to improve gut health by enriching specific beneficial bacteria and metabolic pathways, thereby supporting the development of functional foods aimed at improving gut microbiota composition and metabolic health.

Dorrigiv M, Zareiyan A, Hosseinzadeh H. Onion (Allium cepa) and its Main Constituents as Antidotes or Protective Agents against Natural or Chemical Toxicities: A Comprehensive Review. Iran J Pharm Res. 2021 Winter;20(1):3-26. doi: 10.22037/ijpr.2020.112773.13940. 

Abstract. Onion (Allium cepa) is a member of the family Amaryllidaceae and one of the most widely cultivated species of the genus Allium. Onion has plentiful chemical compounds such as allicin, quercetin, fisetin, other sulphurous compounds: diallyl disulphide and diallyl trisulphide. Onion and its main components in specific doses have shown a lot of benefits including free-radical scavenging and antioxidant properties, anticholesterolemic, anti-heavy metals toxicity, antihyperuricemia, antimicrobial, anti-gastric ulcer, and anticancer. This study summarizes numerous in-vitro and animal studies on the protective effects of onion against natural and chemical toxicities. Onion and its main components can ameliorate the toxicity of chemical agents in kidney, liver, brain, blood, heart, reproductive system, embryo, pancreas through reducing lipid peroxidation, antioxidant effect, radical-scavenging, anti-inflammatory, chelating agent, cytoprotective activities, increasing protein synthesis in damaged tissues, suppressing apoptosis, as well as modulation of PKC-