Onion Nutrition Facts and Health Benefits

Members of the lily family, onions are a highly aromatic vegetable used in cuisines around the world. They offer nutrients and flavor without a lot of calories or fat. Onions are also cholesterol- and gluten-free, so they can fit into most healthy eating plan.

You may have heard that onions become toxic after they're cut or that they promote hair growth. However, research does not validate these common onion myths.

Onion Nutrition Facts

One medium-sized onion (110g) provides 44 calories, 1.2g of protein, 10.3g of carbohydrates, and 0.1g of fat. Onions are a good source of vitamin C, vitamin B6, potassium, folate, and manganese. The following nutrition information is provided by the USDA for a raw onion measuring approximately 2.5 inches in diameter.

Carbs

One medium onion contains just over 10 grams of carbohydrates. Of these, 4.7 grams are sugar and 1.9 grams are fiber. Onions have a low glycemic index rating, between 10 and 15. This means that they have minimal impact on blood sugar levels.

Fats

On their own, onions contain minimal fat. That said, they are often prepared with added fat: sautéed in olive oil or butter, covered in salad dressing, or breaded and deep-fried. All of these can increase the amount of fat in your dish.

Protein

Onions aren't high in protein at just over 1 gram per serving. If you're looking to increase your protein intake, use onions to add flavor and nutrients to higher protein food sources, such as eggs or lean meat.

Vitamins and Minerals

Onions provide a variety of nutrients, notably vitamin C, vitamin B6, folate, potassium, and manganese. You'll also get a little calcium, iron, magnesium, phosphorus, zinc, copper, selenium, choline, and other vitamins and minerals when consuming onion.

Calories

A medium-sized onion that is 2.5 inches in diameter (110 grams) supplies around 44 calories. A thin slice of onion (9 grams) provides approximately 3.6 calories while a thick slice (about a quarter-inch thick, or 38 grams) is just over 15 calories.

Health Benefits

Onions contain numerous phytochemicals that can help ward off disease and boost health.

Fights Heart Disease

Onions contain powerful compounds called allyl sulfides that help prevent heart disease. The vitamin C in onion also acts as an antioxidant that is beneficial for cardiovascular health, while its flavonoid, quercetin, reduces blood pressure and inflammation.

May Prevent Cancer

The allyl sulfides in onions also offer cancer protection. They are responsible for regulating several pathways associated with cancer development. This includes arresting the cell cycle, preventing the migration of cancer cells, and inducing apoptosis (programmed cell death).

Reduces Obesity Risk

While most lab studies investigating onion's effects on weight have involved animals, one 2021 review compiled studies that looked at its effect on humans. It noted that many studies have connected onion's active compounds with reductions in body weight.

Supports Male Fertility

Onion peel extract has been shown to enhance the activity of the Hv1 channel, which plays a crucial role in sperm function. Studies suggest that the effects of onion peel are so powerful, they could even lead to the development of infertility medications.

Improves Memory

Onions contain inulin, a soluble prebiotic fiber that feeds healthy bacteria in the gut. One study tested the impact of inulin on mood and brain performance. It found that taking inulin was associated with improvements in short-term recall and recognition. Although more studies are needed, this suggests that the inulin in onions may be beneficial for cognitive function.

Allergies

Allergy symptoms—like asthma, skin rash, and runny nose—are more often a result of handling onions rather than ingesting them. Nonetheless, anaphylaxis due to an IgE-mediate response (as with a typical food allergy) has been observed following the intake of cooked onions.

It is possible to experience allergy symptoms from touching onions or eating them, either immediately after exposure or several hours later. If you believe that you may have an onion allergy, speak to your doctor for an evaluation.

Adverse Effects

Onions can irritate the eyes during the cutting process. When you cut an onion, you break its cells. Enzymes that were initially separated mix with sulfenic acids to produce propanethial S-oxide, a sulfur compound that wafts up toward the eyes, making them burn and tear. 

Eating onions, especially raw, can make your breath smell. Onions also contain sulfurous compounds that can be absorbed into the bloodstream when ingested and escape from the pores in sweat. Therefore, some people may smell like onions after eating them.

Onions contain fructans as well. This means that anyone following a low FODMAP diet for irritable bowel syndrome needs to be aware that onions may cause abdominal pain, bloating, and gas. 

Onion is generally considered safe. Though, if cooked, it may increase the effects of warfarin. Also, since onion can improve blood sugar control, it may lead to a change in the dosage of drugs taken for this effect. Talk to your doctor if you are taking prescription medications.

Varieties

Onion bulbs come in three colors: red, yellow, and white. Onion bulbs of the same color may vary in taste based on their chemical composition. Disease resistance also varies among different types of onions.

You can buy fresh onions, dehydrated onions, or onion powder to add flavor when cooking. In addition to typical bulb onions, green onions or scallions are smaller and more mild-tasting. These leafy varieties are actually just immature onion bulbs picked before reaching maturity.

When It's Best

Onions can be found in supermarkets at any time of the year. Choose onions that are free of bruises and cuts. Onions should be firm, not squishy.

If you buy pre-cut onions, avoid ones that appear dried out or slimy. Always use by the expiration date listed on the package.

Storage and Food Safety

Use fresh onions within 2 to 4 weeks of purchasing and store them in a dry, well-ventilated area at room temperature. Wash onions under running water before cutting into them.

Once you slice an onion, it can be stored in the refrigerator for up to 7 days. However, it should be discarded if you notice any signs of mold.

To preserve onion in the freezer, chop it into quarter or half-inch sections, or a small dice. Onions that are peeled, washed, and sliced don't need to be blanched prior to freezing.

Freeze onion flat on a baking sheet and then place it in airtight containers. This lets you remove pieces of frozen onion as needed. Onion can also be pureed into a paste before freezing. Onion will keep in the freezer for up to a year.

How to Prepare

Dice onions up and use them to flavor sauces, stews, chili, or soup. Diced onion also adds flavor and crunch to veggie-filled, nutrient-dense salads. Slice the onion instead and it's ready to top your sandwich, wrap, or burger.

You can also grill or sauté onions and use them in side dishes, add them to eggs (whether scrambled or an omelet), or serve them on top of a steak. Dehydrated onion helps season all of these dishes as well.