What are onions?

Onions are botanically part of the lily (Liliaceae) family, along with garlic, shallots and leeks. Although not valued as highly as garlic, onions contain potent compounds which are known to benefit health.


Onions form the basis of a wide variety of dishes, whether eaten raw, sautéed, baked, steamed or boiled.

The health benefits of onions include:

  • Rich in antioxidant compounds
  • Contain anti-cancer compounds
  • Are antibacterial
  • May support heart health
  • May support bone health
  • May support gut health
  • May help regulate blood sugar
  • May support the respiratory system
  • May protect the digestive system
  • May help with weight management

Discover our full range of health benefit guides or check out some of our best onion recipes, from our tangy red onion chutney to our lighter onion tart.

Bowl of sliced red onions on wood

Nutritional profile of onions

An 80g serving of onion (raw) provides:

More like this
  • 28kcal / 120kj
  • 0.8g protein
  • 6.4g carbohydrate
  • 5.0g sugars
  • 1.8g fibre
  • 2.0mg vitamin C

One medium onion or 80g counts as one portion of your five-a-day.

Are onions good for you?

1. Rich in antioxidant compounds

Onions are loaded with plant compounds, including flavonoids, which have both a protective and anti-inflammatory effect. When consumed regularly and in sufficient quantity, these compounds may help protect against chronic conditions such as cancer and diabetes. In fact, onions contain over 25 different flavonoids and are one of the richest sources in our diets.

2. Contain anti-cancer compounds

Onions also contain sulfur-containing compounds, which have been shown to be protective against certain cancers. Some studies suggest that people who eat a diet rich in allium vegetables, like onions, have a lower risk of certain cancers including colorectal cancer. Although the evidence looks promising, clinical trials are needed to fully understand how onions may contribute towards this protection.

3. They're antibacterial

They’ve been used in folk medicine for the relief of coughs, colds and catarrh for decades and now studies support the fact that onions have valuable antibacterial properties. These properties are effective against the likes of E.coli, Pseudomonas aeruginosa and Staphylococcus aureus (MRSA).

What’s more, it’s the older stored onions that appear most potent. It’s a protective flavonoid called quercetin we have to thank here, because it has the power to inhibit the growth of Helicobacter pylori and Methicillin-resistant Staphylococcus aureus.

4. May support heart health

Quercetin also has a protective anti-inflammatory effect and it’s thought that this, along with other beneficial compounds in onions, may contribute to the vegetable’s heart-friendly properties. Red onions have twice as much quercetin as white onions and a staggering 14 times the amount of that in garlic.

Studies suggest eating onions may help reduce your risk of heart disease by lowering blood pressure, managing cholesterol levels and reducing inflammation.

Slicing a brown onion

5. May support bone health

Including onions in the diet is associated with improved bone density. This may be because of their protective antioxidant properties, which reduces oxidative stress and appears to reduce bone loss.

A study looking at the effect on peri- and post-menopausal women reported that frequent onion consumption decreased the risk of hip fracture. A further study on middle-aged women showed onion juice consumption reduced bone loss and improved bone density.

6. May support gut health

Onions are rich in fibre, especially the non-digestible type that helps to maintain gut health. Although we can’t digest prebiotic fibre, the bacteria that live in our gut do and they use it as a fuel source to help increase their numbers and produce beneficial by-products called short-chain fatty acids (SCFAs). Research shows that these SCFAs are important for maintaining the health and integrity of the gut and supporting our immunity and digestion.

Onions do, however, contain inulin and fructo-oligosaccharides – these are classed as FODMAPs – types of carbohydrates that may be problematic for some people.

7. May help regulate blood sugar

Including onions in the diet may help regulate blood sugar levels; there is still much for us to learn in this area, but a small study of type 1 and 2 diabetics found eating 100g raw red onion significantly lowered fasting blood sugar after four hours.

This effect may once again be down to the plant compound quercetin, which appears to interact with cells in the small intestine, pancreas, liver and skeletal muscle.

8. May support the respiratory system

Potentially helpful for conditions like asthma and bronchitis, several studies suggest compounds in onions may relax the smooth muscle of the airways, improve allergic asthma and reduce inflammation in the lungs.

9. May protect the digestive system

Including onion in the diet may have a protective effect on the digestive system. To date, much of this research has been conducted on animal models but the findings suggest onion may mitigate gastric ulcers and alleviate colitis.

However, more research is needed to assess whether these effects may be replicated in humans.

10. May help with weight management

Several animal studies and clinical trials have reported that onion may be effective in the prevention and management of obesity. One study assessed the effect of a 9g serving of onion powder per day for 12 weeks and reported that onion powder (and potentially its quercetin content) may reduce the risk of obesity and improve liver function. Similarly, an extract from steamed onion appeared to reduce total body fat and most notably abdominal visceral fat, in a group of overweight people.

Are onions safe for everyone?

An allergy to onions is rare, but some people do have a sensitivity to them. As a result, these people may experience digestive issues, including heartburn and wind.

As onions contain FODMAPs, a type of carbohydrate and fibre that some people find their digestive system cannot tolerate, this is particularly relevant for those with a diagnosis of irritable bowel syndrome (IBS).

Overall, are onions good for you?

Onions add flavour to a dish, are nutrient packed and contain potent plant compounds that may promote digestive health, immune function and heart health. For the majority of people onions make a useful addition to a balanced diet, however, those diagnosed with IBS may need to exercise caution.

Healthy onion recipes

Aubergine dhal with tomato & onion raita
Crunchy baked tomato & onion gratin
Socca pancakes with hummus & lemony onions
Healthier French onion soup

Enjoyed this? Now read…

Top health benefits of garlic
Top health benefits of kale
Top health benefits of broccoli
Top 20 healthiest vegetables

This page was reviewed on 10 October 2023 by Kerry Torrens, Registered Nutritionist

Kerry Torrens is a qualified Nutritionist (MBANT) with a post graduate diploma in Personalised Nutrition & Nutritional Therapy. She is a member of the British Association for Nutrition and Lifestyle Medicine (BANT) and a member of the Guild of Food Writers. Over the last 15 years she has been a contributing author to a number of nutritional and cookery publications including Good Food.

Jo Lewin is a registered nutritionist (RNutr) with the Association for Nutrition with a specialism in public health. Follow her on Twitter @nutri_jo.


All health content on goodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.

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