An onion sliced in half and into rings on a chopping board

Top 5 health benefits of onions

Registered nutritionist Jo Lewin advises on the the nutritional benefits and medicinal properties of the humble onion.

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.

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

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 ultimate makeover onion tart.

Nutritional profile of onions

An 80g serving of onion (raw) provides:

  • 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. Take a look at our printable infographic to discover what counts towards your five-a-day.

Red onions

Top 5 health benefits of onions

1. Rich in antioxidant compounds

Onions are loaded with plant chemicals including flavonoids, which have both an antioxidant 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.

Onions also have sulfur-containing compounds, which have been demonstrated to be protective against certain cancers. 

2. May support heart health

One of the flavonoids in onions, quercetin, has protective antioxidant and anti-inflammatory effects and it’s thought this, as well as other beneficial compounds present in onions, may contribute to the vegetable’s heart-friendly properties.

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

3. May support bone health

Including onions in the diet is associated with improved bone density. This may be because of their 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.

4. May support gut health

Onions are rich in fibre, especially the non-digestible type that is needed to maintain gut health. Although we can’t digest prebiotic fibre, the bacteria that live in our gut do and they use it as fuel to help increase their numbers and produce 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.

5. Antibacterial

Used in folk medicine for the relief of coughs, colds and catarrh, studies support that onions have valuable antibacterial properties against the likes of Escherichia coli, Pseudomonas aeruginosa and Staphyloccus aureus. What’s more, it’s older, stored onions that appear most potent. Once again, it seems quercetin is of value here, because it has the power to inhibit the growth of Helicobacter pylori and Methicillin-resistant Staphylococcus aureus (MRSA).

Onion being chopped

Are onions safe for everyone?

An allergy to onions is rare, but some people do have a sensitivity to them. As a result, those who are allergic may experience digestive issues, including heartburn and wind. Onions contain FODMAPs , a type of carbohydrate and fibre that some people find their digestive system cannot tolerate.

Leeks cooked in pan

How to select and store

Globe onions should be clean and hard and have dry, smooth skins. Avoid onions with a developed seed stem and those that show signs of decay. Onions should be stored at room temperature, away from bright light and in a well-ventilated area. Those that are more pungent in flavour, such as yellow onions, will keep longer than those with a sweeter taste, such as white onions, because the compounds that produce the sharp taste are natural preservatives. All onions should be stored away from potatoes, as they will absorb their moisture and ethylene gas, causing potatoes to spoil more easily. Do not store cooked onions in a metal bowl or storage container, as this will cause them to discolour. Freezing chopped onions will cause them to lose much of their flavour.

The compound allyl sulphate, which is produced when an onion’s ruptured cells are exposed to air, is responsible for producing tears. To reduce the production of this compound, chill the onions for 30 minutes or so before cutting to reduce the activity of the enzyme.

Recipe suggestions

Onions feature as the basis of many classic recipes worldwide. Here are just a few ideas:

Cheese & onion is a partnership not just for crisps:
Easy cheese & onion slice
Cheese & onion pork chops
Caramelised onion & feta pizzas
Cheese & onion soufflé tart

Onion transforms gravy:
Sausages with sticky onion gravy
Sausages with quick onion gravy

Raw onions for the brave and the bold:
Tuna & red onion salad
Tomato & onion salad

Onion soups:
Pea, mint & spring onion soup with Parmesan biscuits
Caramelised onion & barley soup with cheese croutons
Onion soup with herby garlic cheese bread

And let’s not forget the classic onion ring!
Crispy Cajun onion rings


This article was last reviewed on 31 August 2021 by Kerry Torrens.

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 BBC 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.

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