Onions: Health Benefits and Nutrition

Medically Reviewed by Zilpah Sheikh, MD on January 18, 2024
Written by Kim Painter, Emily Delzell, Kristin Mitchell, WebMD Editorial Contributor
8 min read

Onions are vegetables that belong to the genus allium and are closely related to garlic, shallots, and leeks. The average person consumes roughly 20 pounds of this pungent and versatile food per year, eating onions raw, cooked, pickled, or powdered.

They're rich in chemicals that can help protect your heart, lower your risk of some cancers, and make it easier for your body to make insulin. Onions are also one of the greatest vegetable sources of quercetin, a plant compound with many health benefits.

 

You can choose from a variety of onions, including yellow, red, white, and green onions (also called scallions) and shallots.

Red onions

All types of onions are good for you, but this dark reddish-purple variety, sometimes called purple onion, has especially high levels of quercetin and other helpful plant compounds known as flavonoids. Flavonoids give many fruits, vegetables, and flowers their colors.

Red onions have a mild, sweet taste and add color and flavor to many dishes. Use them raw as a crunchy addition to salads, sandwiches, and salsa.

Yellow onions

Try cooking these strongly flavored yellow-brown onions to tame their sharp taste. Sautéing, roasting, grilling, or caramelizing them brings out their sweetness.

White onions

White onions are milder than yellow onions. Dice them to use raw in guacamole and as a garnish for tacos, or slice them to use as a pizza topping.

Shallots

These small, sweet onions have a subtle flavor with just a hint of garlic. Dice them finely to use in vinaigrettes, salads, and other dishes in which you'd like a mild onion flavor.

Scallions/green onions

These long, slender onions are often thinly sliced and used raw. Try them as a garnish for soups, baked potatoes, stir-fry, and other dishes to add a dash of color and a mild onion taste.

Scientists have linked onions to many possible health benefits, with most of them coming from the antioxidants in onions. Antioxidants help prevent cell damage in your body. Research shows that one particular antioxidant, called quercetin, protects health in several ways, such as fighting inflammation and boosting the immune system.

Onions are also a good source of vitamins, minerals, and fiber.

Some possible health benefits of onions include:

Lower risk of cancer

Many kinds of onions contain chemicals that can help fight cancer.

One study found that people who ate the most onions were the least likely to have cancer of the colon, throat, and ovaries. Another showed that men who ate the most vegetables of the allium family were the least likely to have prostate cancer.

Some researchers believe that quercetin and other antioxidants in onions are responsible for their cancer-fighting properties. A diet full of quercetin has been associated with a lower risk of developing lung cancer.

Antibacterial action

Onions may kill a wide range of bacteria, according to some lab research. In one experiment, onion and garlic extracts slowed the growth of several microbes. More research is needed to show how onion affects bacteria in the body.

Digestive health

Onions have fructooligosaccharides, substances that act as prebiotics (food for your gut's healthy bacteria), which can help with digestion. They pass through the small intestine and feed the healthy bacteria in the large intestine. 

Diseases ranging from diabetes to colon cancer and depression have been tied to not having enough healthy gut bacteria.

Bone health

Onions may play a role in preventing osteoporosis, a condition that weakens your bones. One study in people near or past menopause found that those who ate onions daily had greater bone density, resulting in stronger bones.

Lower risk of heart disease and stroke

Onions contain organic sulfur compounds, which give them their sharp, strong taste and smell. These compounds can help reduce the level of cholesterol in your body and may help break down blood clots, lowering your risk for heart disease and stroke.

You should eat onions raw rather than cooked to get the most sulfur compounds from them.

Diabetes control

Both quercetin and organic sulfur compounds found in onions are known to boost insulin production, making them a helpful vegetable choice if you have diabetes.

Lower risk of Alzheimer's disease

One study found that a long-term diet high in flavonoids decreases the risk of Alzheimer's disease.

What about onions for hair?

Some people use onion juice as a home remedy for hair loss, dandruff, itchy scalp, and other hair-related problems. But research on any possible benefits is scarce. One small study found a possible benefit in people who lost hair due to an immune disorder called alopecia areata. In that study, people who washed their hair with onion juice grew more hair than those who washed it with water. 

Some people swear by drinking onion water to fight cold and flu symptoms. The idea may be that if onions can fight off inflammation and kill bacteria and viruses (at least in test tubes), onion water is a good way to get a potent dose. But there's no research to back up that idea.

Onion water--made by chopping up peeled, clean onions and letting them sit in water for a while--probably won't hurt you. In fact, it may have the benefits of other waters infused with fruits, vegetables, or herbs: if you drink more liquid than you would otherwise, it can help keep you hydrated. But keep in mind that infused water will contain less fiber, vitamins, and other nutrients than the food it's made from. Also, you need to refrigerate it if you don't drink it all at once.

Nutrients per serving

A one-cup serving of chopped, raw onion has:

  • Calories: 64
  • Carbohydrates: 15 grams
  • Protein: 2 grams
  • Fat: 0 grams
  • Fiber: 3 grams
  • Sugar: 7 grams

Onions are also a good source of:

 

 

Eating onions, especially raw onions, can have a few side effects, including:

Gas and bloating. The same substances in onions that feed the healthy bacteria in your gut can be hard for some people to tolerate. Onions are among foods that sometimes cause trouble for people with irritable bowel syndrome. If you think onions bother you, avoid or limit them, especially when they're raw. 

Changes in body odor. When your body breaks down the sulfur compounds found in onions, they can react to sweat on your skin, creating what is generally considered to be an unpleasant body odor. And, of course, they can also cause bad breath.

Food poisoning. Health authorities in the U.S. have traced several outbreaks of salmonella and other bacteria to whole and  diced onions. Outbreaks like these have been rare in onions, because their papery outer skin can protect against contamination, and the drying process that prepares onions for the market further decreases the chances of the growth of bacteria. To reduce risks further, keep cut onions in the refrigerator.

 

Whole bulb onions are best kept in a cool, dry place, not in the refrigerator. The exception is onions that are already peeled. Those should be refrigerated. If you have too many onions to use in the near future, you can preserve them by freezing, pickling, canning, or dehydrating them. Note that if onions have been frozen, they should only be used for cooking.

When you're ready to use them in your favorite recipes, try to prepare onions close to the time you will serve them. Their flavor lessens, and their smell grows stronger the longer they sit after cutting. Any cut onions you don't use need to go into the refrigerator.

You can sauté, braise, boil, steam, bake, grill, caramelize, roast, or fry onions. They offer a flavor boost to any dish and can be eaten at breakfast, lunch, and dinner. Here are some ways to add more onions to your diet:

  • Mix diced onion, jalapeño, tomato, and cilantro with lime juice for a spicy guacamole dip.
  • Combine rice with caramelized onions and broth for a rice dish with a little sweetness.
  • Bread fresh-cut onions, then fry them to make delicious onion rings.
  • Mix grated onions with rice vinegar, soy sauce, sugar, and vegetable oil for a refreshing salad dressing.
  • Add cooked onions to omelets, frittatas, and quiches.
  • Add onions to stir-fry dishes.
  • When making chili, add onions to kick up the flavor.
  • Try adding caramelized onions to savory baked goods such as cornbread, focaccia, and cheddar biscuits.

Many recipes call for red onions. Here are a few ideas to try: 

  • Make your own quick pickled red onions with vinegar, water, sugar, and salt. Add them to salads, sandwiches, or other dishes.

  • Add red onion to make a classic tomato and cucumber salad.

  • Throw red onion and green pepper on the grill with skirt steak for beef fajitas.

  • Add sliced red onion to your beef or veggie burger.

  • Toss chunks of red onion and other vegetables with a little olive oil and oven roast.

  • Add red onions to the classic combo of green beans and new potatoes.

  • Sub red onions for the green ones in tabbouleh.

Green onions are best stored in the refrigerator. Before you cook with green onions, be sure to rinse them and trim off the roots and the top ends of the greens. All the rest of the white and green parts are good to eat -- raw, if you like -- and can be used in:

  • Soups and stews
  • Dips, sauces, and relishes
  • Stir-fries
  • Omelets and frittatas 

Many recipes using onion call for chopped or sliced onion. Knowing the best way to cut an onion can improve your cooking--and keep you from getting nasty cuts.

Here's the best step-by-step advice:

  • Make sure you have a sharp knife and a clean cutting board.
  • Cut off the stem end of the onion and peel off the skin.
  • Cut the whole peeled onion in half, from the root to the stem end. 
  • Place the onion halves on the cutting board, cut side down.
  • For dicing or mincing, make evenly spaced lengthwise cuts, from the stem to root end, keeping the root intact. Carefully make one or two horizontal cuts through the onion, parallel to the cutting board. Then make cuts across your first cuts to make even pieces.
  • For slices or wedges, start with the same peeled onion halves and then make even cuts along the ridges.
  • For rings, start with a whole peeled onion on it's side and slice crosswise.

For any onion chopping task, keep the onion steady with the clawed fingers of your non-chopping hand to keep them clear of the knife.

Onion chopping work-arounds

If chopping an onion is difficult for you, look into devices that make the task easier, such as finger guards, weighted knives, or assistive devices made especially for chopping onions or other vegetables. Using a food processor is another option. 

Reducing onion tears

Wondering why cutting onions makes  you cry? When you pierce an onion, it releases sulfur compounds that can irritate your eyes and cause tearing.

 To prevent tearing up while you're cutting onions, try:

  • Chilling them for 30 minutes beforehand
  • Leaving the root end attached while you cut (It has the most sulfur compounds)
  • Wearing onion goggles, available from online retailers
  • Opening a window or using your cooking vent to clear the air
  • Using spring onions and sweet onions, such as Vidalias. They are less likely to cause irritation.