Exploring Nutrition: What chemicals does onion contain?

October 6, 2025 •

2 min read

More than 25 different flavonoids have been characterized in onions, making them one of the richest dietary sources of these potent antioxidants. Understanding what chemicals does onion contain reveals a complex nutritional profile that contributes to its numerous health benefits, going far beyond a simple flavoring agent.

Quick Summary

Onions are rich in organosulfur compounds, flavonoids like quercetin, and various vitamins and minerals. These bioactive phytochemicals are responsible for the vegetable's potent antioxidant, anti-inflammatory, and antimicrobial properties.

Key Points

Organosulfur Compounds: Formed when the onion is cut, these are responsible for its pungent flavor and offer potent antioxidant, anti-inflammatory, and cardiovascular benefits.
Quercetin: The most abundant flavonoid in onions, this powerful antioxidant and anti-inflammatory compound is most concentrated in red and yellow varieties.
Anthocyanins: These flavonoids give red and pink onions their color and are primarily found in the outer, dry skin, contributing to antioxidant activity.
Fructooligosaccharides (FOS): As a prebiotic, this type of carbohydrate feeds beneficial gut bacteria, promoting digestive health.
Cooking Effects: Boiling onions can cause water-soluble quercetin to leach out, while dry-heat cooking methods like frying or roasting better preserve the flavonoid content.
High Vitamin and Mineral Content: Onions are nutrient-dense, providing significant amounts of Vitamin C, Vitamin B6, folate, potassium, and manganese.

The Pungent Power of Organosulfur Compounds

Organosulfur compounds (OSCs) are key to the onion's pungent aroma and flavor. Formed when onions are cut through enzymatic reactions involving S-alk(en)yl-L-cysteine sulfoxides (ACSOs) and alliinase, they result in compounds like diallyl sulfide (DAS), diallyl disulfide (DADS), and diallyl trisulfide (DATS). These OSCs offer antioxidant, anti-inflammatory, antimicrobial, and potential cardiovascular benefits by helping to regulate cholesterol and blood pressure and preventing platelet aggregation. The specific OSCs present vary based on the onion's type, growth, and preparation.

Flavonoids: The Antioxidant Arsenal

Onions are a significant source of antioxidant flavonoids, primarily quercetin in forms like quercetin-4'-O-beta-glucoside and quercetin-3,4'-O-beta-diglucoside. Red and yellow onions typically have more quercetin than white varieties. Quercetin helps reduce oxidative stress and has potential anti-inflammatory and cardiovascular benefits. Red onions also contain anthocyanins, flavonoids contributing to their color, concentrated mostly in the outer layers.

Essential Vitamins and Minerals

Onions also provide essential nutrients, including Vitamin C (for immunity and collagen), Vitamin B6 (for brain function and metabolism), Folate (for cell growth), Potassium (for fluid balance and nerve function), and Manganese (for brain and nervous system health).

Other Notable Phytochemicals

Other beneficial compounds in onions include fructooligosaccharides (FOS), a type of prebiotic that supports gut health by feeding beneficial bacteria, and saponins, which may benefit heart health and immune function.

How Cooking Affects Onion Chemicals

Cooking methods impact onion compounds differently. Boiling can reduce quercetin levels as it leaches into water, while frying and sautéing maintain or even increase phenolic compound content. Microwaving better preserves flavonoids and Vitamin C than boiling, and dry-heat methods are generally better at retaining flavonoids than wet cooking.

Comparing Chemical Profiles by Onion Color

The color of an onion indicates its chemical profile, especially antioxidants:


Feature	Red Onions	Yellow Onions	White Onions
Flavonoids/Quercetin	Highest, including quercetin and glycosides.	High, mainly quercetin.	Lowest.
Anthocyanins	Present, providing red/purple color.	Absent.	Absent.
Organosulfur Compounds	Significant levels.	Significant levels, potentially higher than red or white depending on type.	Significant levels.

Conclusion: A Chemical Compound Powerhouse

Onions are rich in organosulfur compounds and flavonoids like quercetin and anthocyanins, along with prebiotics, vitamins, and minerals. These compounds give onions their distinct flavor and provide significant health benefits, including antioxidant and anti-inflammatory effects. The nutritional value varies by color, with red and yellow onions generally having more antioxidants. Adding onions to your diet is a simple way to benefit from these natural compounds.

Visit the National Institutes of Health to learn more about the beneficial compounds in onions.

Frequently Asked Questions

When you cut an onion, enzymes convert sulfur compounds into thiopropanal S-oxide, a lachrymatory factor that irritates your eyes and causes tears.

Red onions typically contain higher levels of flavonoids, including quercetin and anthocyanins, giving them a greater antioxidant capacity compared to white onions.

Cooking methods impact nutrient retention. Boiling can reduce water-soluble compounds like quercetin, while dry-heat methods may better preserve flavonoids and can sometimes increase the bioavailability of phenolic compounds.

Flavonoids, particularly quercetin, are the primary antioxidants in onions. Organosulfur compounds also contribute to the onion's antioxidant properties.

Onion's organosulfur compounds have antioxidant, anti-inflammatory, and antimicrobial effects. They may also support cardiovascular health by influencing cholesterol, blood pressure, and platelet function.

Onion skins contain significantly higher concentrations of flavonoids, especially quercetin, than the edible bulb. Red onion skins are also rich in anthocyanins.

Flavonoids in onions act as antioxidants, neutralizing free radicals and reducing oxidative stress. They also possess anti-inflammatory and protective properties beneficial for cardiovascular health and potentially reducing cancer risk.