Onion
The most used vegetable in Indian cooking — and one of the most medicinally important. Raw onion preserves quercetin; cooked onion preserves sulphur compounds.
TLDR — What You Need to Know
- Quercetin, the primary flavonoid in onions, is most concentrated in the outermost layers — do not discard the first 1-2 layers of dry skin
- Raw onion has significantly more quercetin than cooked — heat degrades flavonoids; cook gently and briefly to preserve more
- Sulphur compounds (allyl sulphides) are responsible for the pungency, tears, and the anti-inflammatory, antimicrobial, and cardioprotective effects
- Onion is a rich source of fructooligosaccharides (FOS) — prebiotic fibres that selectively feed Lactobacillus and Bifidobacterium in the colon
- Red onions have more antioxidants (quercetin + anthocyanins) than white or yellow onions — choose red for raw applications
- High FODMAP food — a significant trigger for IBS patients; those with GERD or fructose intolerance should moderate intake
What Is an Onion?
The onion (Allium cepa) is one of the oldest cultivated vegetables in human history — archaeological evidence suggests cultivation in ancient Egypt, Persia, and India dating back over 5,000 years. It belongs to the Allium genus along with garlic, leek, chive, and shallot — a family defined by high sulphur content and strong antimicrobial properties.
India is the world’s second-largest producer and exporter of onions. Karnataka, Maharashtra, and Madhya Pradesh are the leading growing states. The onion is the foundational flavour base of Indian cooking — no curry begins without it.
Three main colour types are commercially available in India:
- Red onion — most common in India; pungent, high in quercetin and anthocyanins; best eaten raw or briefly cooked
- White onion — milder, lower in antioxidants; preferred in some regional dishes
- Yellow/Brown onion — the standard in international cuisines; moderate flavour and antioxidant content
Shallots (small, clustered onions) are particularly popular in South Indian cooking and have a slightly higher quercetin concentration than regular onions.
Nutritional Profile
Onion — Nutrition Facts per 100g Raw
Per 100g raw
| Nutrient | Amount | % Daily Value |
|---|---|---|
| Energy | 40 kcal | — |
| Protein | 1.1 g | — |
| Total Fat | 0.1 g | — |
| Carbohydrates | 9.3 g | — |
| Dietary Fiber | 1.7 g | — |
| Vitamin C | 7 mg | 8% |
| Quercetin | 22 mg | — |
| Folate | 19 µg | 5% |
| Potassium | 146 mg | — |
| FOS (Prebiotic Fibre) | 2–6 g | — |
| Calcium | 23 mg | — |
Health Benefits
1. Quercetin — anti-inflammatory and cardiovascular protection
Quercetin is a flavonoid antioxidant with broad anti-inflammatory activity. It inhibits the production of pro-inflammatory cytokines (including IL-6, TNF-alpha, and COX-2 pathway mediators) through multiple mechanisms. A meta-analysis in the British Journal of Nutrition (2016) found that quercetin supplementation significantly reduced systolic blood pressure and LDL cholesterol. Dietary quercetin from onions is well absorbed — with bioavailability of around 52% from raw onions, compared to only 24% from apple (another quercetin source). The outer two or three dry layers of onion contain the highest quercetin concentration; when you peel onions, retain as much as possible of the innermost papery layer.
2. Sulphur compounds and antimicrobial activity
When onion cells are cut or crushed, an enzyme called alliinase converts alliin (a sulphur amino acid) into a cascade of reactive sulphur compounds — allyl sulphides, thiosulfinates, and syn-propanethial-S-oxide. These compounds have demonstrated antimicrobial activity against Staphylococcus aureus, E. coli, and Helicobacter pylori (a key bacterium in stomach ulcer formation). The anti-H. pylori activity of onion extract is well documented in vitro and has led to dietary recommendations for raw onion consumption in populations with high H. pylori burden.
3. Prebiotic FOS and gut health
Onions are one of the richest dietary sources of fructooligosaccharides (FOS) — short-chain fructose polymers that the human body cannot digest but gut bacteria can. FOS selectively feeds Lactobacillus acidophilus and Bifidobacterium species while being poorly metabolised by pathogenic bacteria. Regular FOS consumption is associated with improved stool regularity, higher beneficial bacteria counts, and improved mineral absorption (particularly calcium and magnesium). The FOS content survives cooking, though some is lost in cooking water.
4. Heart health — multiple pathways
Onion consumption is associated with cardiovascular benefit through several mechanisms: quercetin reduces LDL oxidation; sulphur compounds inhibit platelet aggregation (reducing clotting tendency); the chromium content improves insulin signalling; and the prebiotic effect indirectly reduces systemic inflammation. A large Dutch cohort study found an inverse association between onion consumption and mortality from cardiovascular disease.
5. Bone health
An unexpected benefit: onion contains the peptide GPCS (gamma-L-glutamyl-trans-S-1-propenyl-L-cysteine sulphoxide), which has been shown in animal studies to inhibit osteoclast activity — the cells that break down bone. Human observational studies have shown higher onion consumption associated with greater bone mineral density in middle-aged and older women.
Why Onions Make You Cry
The tear-inducing compound is syn-propanethial-S-oxide, a volatile sulphur gas released when the enzyme alliinase contacts alliin during cutting. The gas diffuses through the air, dissolves in the water on the eye’s surface, and forms sulphuric acid — which irritates the tear glands. Chilling the onion before cutting (30 minutes in the fridge) slows the enzymatic reaction. Cutting near running water or a fan disperses the gas. Wearing goggles is 100% effective and underutilised.
Side Effects and Who Should Avoid
IBS and FODMAP sensitivity: Onions are high in fructooligosaccharides and fructose — both classified as FODMAPs (Fermentable Oligo-, Di-, Mono-saccharides and Polyols). These are poorly absorbed in the small intestine of susceptible individuals and are fermented rapidly in the colon, producing gas, bloating, and abdominal cramping. Onion is one of the highest-FODMAP foods and is often the primary trigger in FODMAP elimination diets for IBS. Even small amounts of raw or cooked onion trigger symptoms in highly sensitive individuals.
GERD and acid reflux: Onions relax the lower oesophageal sphincter, allowing stomach acid to reflux. Those with chronic GERD typically find that even small amounts of onion worsen heartburn.
Bad breath: The same sulphur compounds responsible for health benefits are also metabolised to volatile sulphur compounds by mouth bacteria, causing prolonged bad breath after onion consumption. Chewing fresh parsley, mint leaves, or cardamom seeds metabolises some of the sulphur compounds.
Blood-thinning medications: Onion inhibits platelet aggregation. Those on anticoagulant medications (warfarin, aspirin therapy) should maintain consistent onion intake and discuss with their doctor if they significantly increase consumption.
Organic vs Conventional
Onions do not appear on the EWG Dirty Dozen — they rank among the lowest-pesticide produce. The strong sulphur chemistry that makes onions pungent also repels most insects naturally, reducing the need for insecticide application. However, fungicides are commonly applied post-harvest to prevent mould during storage. Organic onions are still preferable where available, but onions are a lower priority than thin-skinned produce when making organic purchasing decisions.
How to Select and Store
Selecting: Choose onions that are firm and dry with no soft spots, no sprouting, and no mould. The outer papery skin should be crisp. Heavy onions for their size indicate high water content and freshness.
Storing: Keep in a cool (10–15°C), dry, dark, well-ventilated location — a mesh bag or open bowl in a pantry. Onions can last 1–2 months under these conditions. Do not store in the refrigerator (humidity promotes mould) and never store with potatoes (each produces ethylene and moisture that accelerates spoilage of the other). Once cut, store in an airtight container in the fridge and use within 2 days.
Red vs White vs Shallot vs Spring Onion per 100g
| Parameter | Red Onion | White Onion | Shallot | Spring Onion |
|---|---|---|---|---|
| Energy | 40 kcal | 42 kcal | 72 kcal | 32 kcal |
| Quercetin | 22mg | ~4mg | ~30mg | ~14mg |
| Anthocyanins | Present | Absent | Absent | Trace |
| Pungency | High | Mild | Medium-high | Mild |
| Best use | Raw, salads, pickles | Cooking, gravies | South Indian tempering | Garnish, raw |
| FOS prebiotic | High | High | High | Medium |
Shallots have the highest quercetin density; red onions have the broadest antioxidant profile including anthocyanins. White onions are milder and best for cooked applications.
A cooling yogurt-onion condiment from North India. The raw onion preserves maximum quercetin; yogurt provides probiotics. Ready in 5 minutes.
Key Ingredients
1 medium red onion, finely chopped · 200g plain curd (yogurt) · 1/2 tsp roasted cumin powder · 1/4 tsp black pepper · Salt to taste · 1 tbsp fresh coriander, chopped · Optional: 1/4 tsp chaat masala
Home Test: Fresh vs Chemical-Treated Onion Storage Test
Steps
- 1 Select 2-3 onions from the batch
- 2 Examine the outer papery skin closely — natural onions have dry, slightly flaky skin with a matte finish
- 3 Cut one onion in half and smell the cut surface immediately
- 4 Check the outer 1-2 layers for unusual shine or slippery feel, which can indicate chemical coating
- 5 Soak one onion in water for 5 minutes and observe whether the water changes colour — synthetic dyes used on some red onions will bleed into the water
Pure / Pass
Dry, matte papery skin with natural variability in colour. Strong pungent smell when cut. No colour bleeding into water. No shine or coating on outer layers. Onion is natural and untreated.
Adulterated / Fail
Unusually uniform deep red colour. Pink or red tinge in soaking water indicates synthetic dye has been applied to enhance colour of lower-quality onions. Discard dyed onions.
Available at Organic Mandya
Organic Onions
No synthetic pesticides. No post-harvest fungicide coating. Grown in natural soil — full sulphur profile intact.
Frequently Asked Questions
Q Is it safe to eat raw onion every day?
Is it safe to eat raw onion every day?
Yes, for most healthy adults. 50-100g of raw onion daily provides meaningful quercetin, prebiotic FOS, and sulphur compounds. Regular raw onion consumption has been associated with better heart health markers and reduced H. pylori burden. The only concerns are for those with IBS (high FODMAP), chronic GERD, or those on blood-thinning medications who should maintain consistent intake.
Q Why do onions make you cry, and how can you stop it?
Why do onions make you cry, and how can you stop it?
The culprit is syn-propanethial-S-oxide, a volatile gas released when the enzyme alliinase contacts the onion cell contents during cutting. The gas irritates tear gland receptors. Effective remedies: chill the onion in the refrigerator for 30 minutes before cutting (slows enzymatic reaction), cut near a running exhaust fan (disperses gas), or wear goggles (blocks the gas entirely).
Q Can people with diabetes eat onions?
Can people with diabetes eat onions?
Yes — onions are actually beneficial for diabetics. The chromium in onion improves insulin sensitivity, quercetin has anti-diabetic properties, and the FOS prebiotic fibre slows carbohydrate absorption. Raw onion consumption has been shown in small studies to reduce fasting blood glucose. The caloric content is low (40 kcal/100g) and the glycaemic index is approximately 10 — negligible.
Last updated: March 2026
Disclaimer: This article is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your doctor or a qualified healthcare provider before making dietary changes, especially if you have a medical condition.