Red Onion Nutrition Facts per 100g: Calories, Carbs, Vitamins & Health Benefits
Introduction
Red onions are a staple in kitchens around the world — adding flavour, colour, and crunch to everything from salads to stir-fries. But how healthy are they? What are the actual red onion nutrition facts per 100 grams, and what do they mean for your body?
In this guide, we’ll break down the full nutritional profile of red onions, the vitamins and minerals they contain, and the key health benefits that make them more than just a tasty topping. Whether you’re counting calories, watching your carbs, or just want to eat smarter, this guide has everything you need to know.
Red Onion Nutrition Facts (Per 100g Raw)
| Nutrient | Amount |
|---|---|
| Calories | 40 kcal |
| Water content | ~89% |
| Carbohydrates | 9.3 g |
| └── Sugars | 4.2 g |
| └── Fibre | 1.7 g |
| Protein | 1.1 g |
| Fat | 0.1 g |
| └── Saturated fat | 0.02 g |
| Sodium | 4 mg |
| Potassium | 146 mg |
| Vitamin C | 8–10% RDI |
| Folate (B9) | 19 µg (5% RDI) |
| Vitamin B6 | 0.12 mg (6% RDI) |
| Calcium | 23 mg |
| Iron | 0.2 mg |
| Magnesium | 10 mg |
| Phosphorus | 29 mg |
| Manganese | 0.129 mg (7% RDI) |
💡 Values may vary slightly based on variety, growing conditions, and freshness.
Macronutrients Overview
🔥 Calories
- Just 40 kcal per 100g
- Ideal for low-calorie diets, meal prep, and healthy snacking
🍞 Carbohydrates
- 9.3g total, including 4.2g natural sugars
- Provides gentle energy release without blood sugar spikes (low GI)
🌾 Fibre
- 1.7g per 100g
- Supports digestion and gut health
- Aids in satiety and blood sugar regulation
🍗 Protein
- 1.1g — modest, but helpful when combined with protein-rich foods
🥑 Fat
- Virtually fat-free
- Great for heart-healthy or low-fat meal plans
Key Micronutrients
🍊 Vitamin C (Ascorbic Acid)
- Boosts immunity and collagen production
- Helps iron absorption
- 8–10% of your daily needs per 100g
🧠 Vitamin B6
- Supports brain function and metabolism
- Regulates hormones and mood
- 6% of your daily needs
🌱 Folate (B9)
- Crucial for pregnancy, cell repair, and DNA synthesis
- 5% of your RDI per 100g
🧂 Potassium
- Helps regulate blood pressure and hydration
- 146 mg per 100g (good support for electrolyte balance)
🧲 Manganese, Magnesium & Phosphorus
- Support bone health, nerve function, and energy metabolism
Red Onion Phytochemicals: Why They’re a Superfood
Red onions are rich in plant compounds that go beyond vitamins:
| Compound | Benefit |
|---|---|
| Quercetin | Antioxidant that reduces inflammation, protects heart and lungs |
| Anthocyanins | Give red onions their colour; protect cells and fight ageing |
| Sulphur Compounds | Support liver detox and immune response |
These compounds are especially concentrated in the outer layers and raw state — so don’t over-peel!
Health Benefits of Red Onions
1. Antioxidant Protection
- Fights oxidative stress and cellular damage
- Supports anti-ageing, brain health, and heart health
2. Heart Health
- Quercetin and potassium help lower blood pressure and cholesterol
- Sulphur compounds improve blood vessel elasticity
3. Blood Sugar Control
- Low GI, high fibre, and quercetin help stabilise glucose
- Especially helpful for those managing diabetes or PCOS
4. Cancer-Fighting Properties
- Sulphur compounds and anthocyanins help inhibit tumour growth
- Linked to reduced risk of colon, breast, and prostate cancers
5. Digestive Support
- Fibre and prebiotics feed healthy gut bacteria
- Helps prevent constipation and bloating
6. Bone Strength
- Onion intake linked to increased bone density, especially in women
Raw vs Cooked Red Onion Nutrition
| Nutrient/Compound | Raw Onions (100g) | Cooked Onions (100g) |
|---|---|---|
| Vitamin C | 8–10% RDI | ↓ Lower after cooking |
| Fibre | 1.7g | Slightly reduced |
| Quercetin | High | Moderate (heat sensitive) |
| Calories | 40 kcal | Similar |
| Taste | Sharp, crisp | Sweet, soft, mellow |
Tip: Lightly sauté or pickle if raw onions are too strong — you’ll retain some benefits while improving digestibility.
How to Add 100g of Red Onion to Your Diet
100g is roughly 1 medium red onion. Easy ways to include it:
- Add to salads, wraps, or grain bowls
- Use in omelettes, stir-fries, or tacos
- Pickle it with lime and vinegar for a gut-healthy condiment
- Blend into dressings, dips, or sauces
- Roast with other veg for a nutrient-dense side dish
Red Onion vs White Onion: Nutritional Comparison
| Nutrient | Red Onion (100g) | White Onion (100g) |
|---|---|---|
| Calories | 40 kcal | 42 kcal |
| Sugar | 4.2g | 4.6g |
| Fibre | 1.7g | 1.2g |
| Vitamin C | 8–10% RDI | 7% RDI |
| Quercetin | Higher | Moderate |
| Anthocyanins | Present | Absent |
| Flavour | Mild-sweet | Sharper-spicy |
Red onions offer more antioxidants and fibre, making them a smarter choice for health-focused meals.
Are Red Onions Keto-Friendly?
Yes — red onions can fit into a low-carb or keto diet when used in moderation.
- 100g = ~9g total carbs
- Net carbs = ~7.6g (after subtracting fibre)
- Use in small portions (25–50g) to stay within carb limits
Add to high-fat meals like avocado bowls, cheese platters, or roasted meats.
Are Red Onions Good for Weight Loss?
Absolutely:
- Low in calories and fat
- High in flavour, helping reduce the need for heavy sauces
- Contains fibre for fullness and appetite control
- May improve fat metabolism through sulphur compounds and B vitamins
Great for volume eating, intermittent fasting, or low-calorie meal planning.
Conclusion
Red onions are more than a garnish — they’re a nutrient-rich, low-calorie superfood with powerful health benefits. Just 100 grams delivers a solid mix of:
✅ Antioxidants
✅ Fibre
✅ Vitamins C and B6
✅ Prebiotics and detox support
✅ Anti-inflammatory and anti-cancer compounds
Whether raw, cooked, or pickled, adding red onions to your meals is an easy way to boost flavour and health at the same time.
Top 10 FAQs About Red Onion Nutrition per 100g
1. How many calories are in 100g of red onion?
About 40 kcal — making them very low in energy.
2. Is red onion healthier than white onion?
Yes — red onions have more antioxidants and fibre.
3. How many carbs in 100g of red onion?
About 9.3g total carbs, including 4.2g sugars.
4. How much fibre in red onion?
Roughly 1.7g per 100g.
5. Is red onion keto-friendly?
Yes, in moderation — use 25–50g per serving for keto.
6. Do red onions have sugar?
Yes, around 4.2g of natural sugars per 100g.
7. Is red onion good for diabetics?
Yes — the low GI, fibre, and quercetin help regulate blood sugar.
8. What vitamins are in red onion?
Mainly vitamin C, B6, folate, plus antioxidants.
9. Does cooking destroy nutrients in red onion?
Some, yes — especially vitamin C and quercetin. Light cooking retains more.
10. How much is 100g of red onion?
About one medium red onion, or ~1 cup sliced.