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By Nikola · Precision Nutrition L1 · Updated 4 November 2026

Onions (Allium cepa) are the most common base vegetable in European cooking: almost every soup, stew and sauce starts with one. A raw onion is mostly water, 89 g per 100 g, and gives only 40 kcal, 1.7 g of fibre and 7.4 mg of vitamin C per 100 g. A medium onion of about 110 g has roughly 44 kcal. Onions are not a nutrient powerhouse on their own; their value is that they make vegetables, beans and grains taste good with very little energy. For people with irritable bowel syndrome they can be a trigger, because they contain FODMAPs.

Onion nutrition per 100 g and per portion

Nährstoff	Raw onion, 100 g	Portion: 1 medium onion (110 g)
Energie	40 kcal	44 kcal
Wasser	89.1 g	98 g
Eiweiß	1.1 g	1.2 g
Fat	0.1 g	0.1 g
Carbohydrate	9.3 g	10.3 g
of which sugars	4.2 g	4.7 g
of which fibre	1.7 g	1.9 g
Vitamin C	7.4 mg	8.1 mg
Vitamin B6	0.12 mg	0.13 mg
Folat	19 µg	21 µg
Kalium	146 mg	161 mg
Kalzium	23 mg	25 mg
Manganese	0.13 mg	0.14 mg
Natrium	4 mg	4 mg

Source: USDA FoodData Central, “Onions, raw”, FDC ID 170000, checked 4 November 2026. The record gives a medium onion (about 6.4 cm across) as 110 g, a large one as 150 g and a small one as 70 g; the 110 g column is calculated. Values rounded.

A medium onion gives about 10% of the EU reference intake for vitamin C (80 mg) and close to 8% of the 25 g of fibre EFSA considers adequate for adults. Modest numbers, but onions are eaten often and in generous amounts, so they add up over a week.

Glycemic index of onions

Onions are not listed in the international GI tables (Atkinson 2008). A medium onion has about 10 g of carbohydrate, of which almost 2 g is fibre, and it is usually shared across several portions of a dish, so its effect on blood sugar is small.

Health benefits: what the evidence says

Onions are often presented online as a cure for many things. We only list what we could check in reliable sources.

• A vegetable that counts. WHO advises adults to eat at least 400 g of fruit and vegetables a day. Onions are an easy way to add to that total, especially in cooked dishes where a whole onion disappears into the sauce.
• Few calories, a lot of flavour. At 40 kcal per 100 g, onions add volume and taste without much energy. Slowly cooked onions give sweetness to a dish, which can reduce the need for added sugar in sauces and dressings.
• Fibre. 1.7 g per 100 g. Onions alone will not cover your fibre needs, but together with beans, whole grains and other vegetables they help you reach the 25 g a day EFSA considers adequate for normal bowel function.
• Less salt in cooking. WHO recommends less than 5 g of salt a day. Onions, Knoblauch, herbs and spices are the classic way to keep food tasty with less salt.

Onions also contain plant compounds such as flavonoids that are studied for possible health effects. We have not included specific claims here because the human evidence for onions as a food is limited.

Ayurveda: onions in the traditional system

In Usha and Vasant Lad’s food guidelines, the form of the onion matters. For Vata, raw onion is on the list to avoid and cooked onion is favourable in moderation. For Pitta, raw onion is to be avoided and cooked onion is favourable. For Kapha, onions are favourable. In short, the tradition prefers cooked onions for most people. This is a traditional view, not a clinical finding.

How to buy, store and use onions

• Choosing: pick firm onions with dry, papery skins and no soft spots, sprouts or dark patches.
• Storing: keep whole onions in a cool, dry, dark and airy place. Once cut, wrap the rest and keep it in the fridge.
• Yellow onions are the all-purpose cooking onion. Red onions are milder and are the usual choice for salads. Spring onions are eaten with their green tops.
• Softening the bite of raw onion: slice it thinly and leave it for 10 minutes in cold water or in a little lemon juice or vinegar with a pinch of salt.
• The base of a dish: soften chopped onion slowly in Olivenöl before adding Tomaten, lentils, weiße Bohnen oder Pilze. Add Knoblauch a minute later so it does not burn.
• Caramelised onions: cook sliced onions over low heat for 30-40 minutes, stirring now and then, until deep golden. They keep in the fridge for a few days and lift sandwiches, grain bowls and omelettes.
• Salads: red onion with Tomate, Gurke, olives Und feta; or with chickpeas, Petersilie and lemon.

Who should be careful

• Irritable bowel syndrome. Onions and garlic are among the foods containing FODMAPs, carbohydrates that are hard to digest and can cause bloating, pain and changes in bowel habits in people with IBS. A low-FODMAP diet, best done with a dietitian, reduces or avoids these foods (NIDDK).
• Sensitive digestion. If onions bother you, keep a simple food diary for a couple of weeks and discuss it with your doctor or a dietitian before cutting out whole groups of foods.
• Reactions after eating onions. If you get itching, swelling, a rash or breathing problems after eating onions, see a doctor.

Related entries and articles

• Knoblauch – the other kitchen allium
• Lauch – a milder cousin for soups
• Tomaten – onion and tomato, the base of countless sauces
• Linsen – lentil soup starts with onions
• Mediterranean diet
• Natural remedies for better digestion
• Plant-based meals for weight loss

FAQ

How many calories are in an onion?

A medium raw onion of about 110 g has around 44 kcal, a large one (150 g) about 60 kcal, according to USDA data. Frying adds the energy of the oil, which is usually much more than the onion itself.

Are raw onions healthier than cooked ones?

Both are good choices. Cooking softens the taste and brings out sweetness, while raw onions keep their crunch and sharpness. Eat them in the form you enjoy and tolerate.

Why do onions cause bloating?

Onions contain FODMAPs, carbohydrates that are hard to digest and are fermented in the gut. Most people handle them without problems, but in people with irritable bowel syndrome they can cause bloating and discomfort. A dietitian can help you find out how much you tolerate.

Sources

1. USDA FoodData Central: Onions, raw (FDC ID 170000). fdc.nal.usda.gov
2. Regulation (EU) No 1169/2011, Annex XIII – nutrient reference values. EUR-Lex
3. EFSA NDA Panel (2010). Scientific Opinion on Dietary Reference Values for carbohydrates and dietary fibre. EFSA Journal 8(3):1462. efsa.europa.eu
4. World Health Organization: Healthy diet, fact sheet. who.int
5. Atkinson FS, Foster-Powell K, Brand-Miller JC. International tables of glycemic index and glycemic load values: 2008. Diabetes Care 2008;31(12):2281-2283. PMC
6. NIDDK: Eating, diet and nutrition for irritable bowel syndrome. niddk.nih.gov
7. Lad U, Lad V. Food Guidelines for Basic Constitutional Types, from Ayurvedic Cooking for Self-Healing, 2nd ed. The Ayurvedic Institute. ayurveda.com

This page is for information only and is not medical or dietary advice.