Onions: Nutrition Facts

Calories in Onions and Their Health Benefits

Varieties of onions
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Members of the lily family, onions are a strongly flavored, highly aromatic, non starchy vegetable that are used in almost every culture as a vegetable or for flavoring. According to the National Onion Association, onions are the third most consumed fresh vegetable in the United States and are available all year long.

Onions range in size from less than one-inch in diameter to over 4.5 inches, and the bulbs can be yellow, red, or white in color.

There are many varieties of onions that can be used in cooking. Even though it might not be your first choice, some may also be eaten raw.

Varieties range from the popular Spanish, Vidalia, yellow, white, red onion, cippolini, and pearl, and there are varieties such as scallions, shallots, and leeks. Their flavor ranges too—and can be mild, sweet, or spicy.

Onions are very low in calories and carbohydrate, which is a bonus because a little bit of onion goes a long way.

Onion Nutrition Facts
Serving Size 1 slice raw, medium (1/8" thick) (14 g)
Per Serving% Daily Value*
Calories 6 
Calories from Fat 0 
Total Fat 0g0%
Saturated Fat 0g0%
Polyunsaturated Fat 0g 
Monounsaturated Fat 0g 
Cholesterol 0mg0%
Sodium 1mg0%
Potassium 20.44mg1%
Carbohydrates 1.3g0%
Dietary Fiber 0.3g1%
Sugars 0.6g 
Protein 0.2g 
Vitamin A 0% · Vitamin C 2%
Calcium 0% · Iron 0%

*Based on a 2,000 calorie diet

One medium slice of raw onion contains only 6 calories and 1.3 grams of carbohydrate, making it a very low calorie and almost carbohydrate free food choice.

As with any food, the portion will affect the calorie and carbohydrate content. For example, if you are consuming a larger portion of onions, let's say 1/2 cup raw, you'll still consume very little calories, about 21 calories and 5 grams of carbohydrate. This makes onions a great ingredient to use in cooking, adding flavor and volume to meals.

Health Benefits of Onions

Onions contain vitamin C and are a very good source of chromium. Vitamin C is important in cell repair, including wound healing. It is also important in boosting immunity and has anti-aging properties. Chromium is important and plays a role in insulin action. Insulin is the hormone that helps to regulate blood sugar by enabling the body to use glucose for energy and storage.

Onions are also high in quercetin, a flavonoid that has antioxidant properties. Animal research and research using cell cultures have found that quercetin may help to protect against certain cancers by killing cancer cells. Notably, these types of studies can suggest possible helpful effects, but they do not provide proof that such effects can be achieved in humans. Preliminary studies also suggest that quercetin may help reduce the risk of heart disease.

Some studies have found that the consumption of onions may help to reduce the incidence of certain diseases, such as gastric ulcers by scavenging free radicals and preventing growth of the ulcer-forming microorganism, Heliobacter pylori. However, the myth of putting an onion in a room to prevent the cold or flu is not true.

Common Questions About Onions

Why does cutting onions make me cry?

Onions can irritate the eyes due to sulfuric acid, which is produced and released during the cutting process. When you cut an onion you break cells, releasing their contents. Enzymes that were kept separate mix with the sulfenic acids to produce propanethiol S-oxide, a sulfur compound that wafts upward toward your eyes, making them burn and tear. To reduce irritation, try wearing goggles or chilling the onion before slicing.

Will eating onions make me smell?

Onions contain sulfurous compounds that can be absorbed into the blood stream when ingested and can escape from our pores when sweating. Therefore, some people may "smell like onions" after eating them.

Onions can sometimes make your breath smell too, especially raw onions. To get rid of onion breath, try eating some parsley.

How many calories do onion rings have?

The nutrition profile of onion rings will vary based on the size of the onion ring and the quantity served. To give a baseline, a small serving of onion rings from a fast food restaurant is about: 320 calories, 16 grams fat, 41 grams carbohydrate, 840 mg of sodium, and 3 grams of protein.

Picking and Storing Onions

Choose onions that do not have any cuts, bruises, or blemishes. When purchasing whole peeled onions, select ones with an outside layer that does not appear to be dehydrated. If you are purchasing fresh, pre-cut onions, make sure to use them before the expiration date.

Onions can be stored for quite a while if kept in proper conditions. Store dry bulb onions in a cool, dry, dark place with plenty of air movement. Do not store them in plastic, as lack of ventilation will reduce their storage life.

Refrigerate whole onions only when you are trying to extend the shelf life of sweet or mild onion varieties. Place them in a low humidity setting because they must be kept dry.

If you cut your onions and don't plan to use them all, store them in a sealed container for up to seven days. Pre-cut onions should be kept refrigerated and used before the expiration date.

Foods such as scallions and leeks should be stored in the refrigerator.

Healthy Ways to Prepare Onions

There are so many ways to use onions.Use onions to flavor sauce, stews, chili, or soup. Top whole grain sandwiches, wraps, or burgers with a slice of onion or dice up some raw, grilled, or sauteed onions and put them in your salads, side dishes, or eggs.

Aim to avoid deep frying your onions or consuming onion rings, blooming onions, etc. as these types of food are high in calories, sodium, and unhealthy fat.

Recipes With Onions 

Cook up some onions as an addition to your meals or eat them raw. Try them for breakfast, lunch, snack or dinner.

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Article Sources
  • Galeone C, Pelucchi C, Levi F, Negri E, Franceschi S, Talamini R, Giacosa A, La Vecchia C. Onion and garlic use and human cancer. Am J Clin Nutr. 2006 Nov;84(5):1027-32.
  • American Cancer Society. Quercetin. 
  • National Onion Association. How to Select, Cut, Prepare, and Store Onions.