Members of the lily family, onions are a highly aromatic vegetable used in cuisines around the world. You may have heard rumours that onions become toxic after they're cut, or that onions are too starchy for those on low-carbohydrate eating plans. However, these rumours are quickly dispelled after reviewing the research on onions.

Onion Nutrition Facts

The following nutrition information is provided by the USDA for one medium raw onion (110g) measuring approximately 2 1/2 inches in diameter.

Calories: 44

Fat: 0.1g

Sodium: 4.4mg

Carbohydrates: 10g

Fibre: 1.9g

Sugars: 4.7g

Protein: 1.2g

Carbs

One medium onion contains 10 grams of carbohydrates. Of these, 4.7 grams are sugar and 1.9 grams are fibre. Onions have a low glycemic index between 10–15.

Fats

On their own, onions contain minimal fat. Onions are often prepared with added fat, however, either sautéed in oil or butter, covered in salad dressing or breaded and deep-fried.

Protein

Onions are not high-protein food with just a little over 1 gram of protein per serving.

Vitamins and Minerals

Onions provide vitamin C, folate, and manganese. Onions also contain some calcium and potassium.

Health Benefits

Onions contain numerous health-boosting phytochemicals to help ward off disease. Along with sulphur-based compounds, onions are rich in quercetin, a powerful antioxidant.

Fights Heart Disease

Onions contain powerful compounds, called allyl sulphides, that help prevent heart disease. The vitamin C in onions also acts as a powerful antioxidant that is beneficial for cardiovascular health. Finally, onions possess an impressive flavonoid, called quercetin, that reduces blood pressure and inflammation. All of these factors, combined with their natural fibre and potassium content, make onions a clear choice for anyone hoping to improve heart health.

Prevents Cancer

The allyl sulphides in onions are also protective against cancer. Specifically, diallyl trisulfide is responsible for regulating several pathways associated with cancer development. These include arresting the cell cycle, preventing the migration of cancer cells, and inducing apoptosis (programmed cell death).

Reduces Obesity Risk

Studies demonstrate multiple mechanisms by which onions inhibit excess fat storage. Onion extracts block the generation of new fat cells and increase energy expenditure through thermogenesis. Quercetin is thought to be effective at transforming white fat cells into brown fat cells, which are more metabolically active.

Supports Male Fertility

Onions show promise in the treatment of male infertility. The quercetin in onions is beneficial for both sperm motility and viability. 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 lead to the development of infertility medications.

Improves Memory

Onions contain inulin, a type of soluble, prebiotic fibre that feeds healthy bacteria in the gut. It is believed that a gut-brain axis exists, connecting messages from the digestive system with brain activity. A double-blind, placebo-controlled study tested the impact of inulin on mood and cognitive performance. The ingestion of inulin was associated with improvements in the participants' short-term recall and recognition. Although more studies are needed to determine cause and effect, the inulin in onions may be beneficial for cognitive functioning.

Allergies

Allergy symptoms, like asthma, skin rash, and runny nose, are more often seen as the result of handling onions rather than ingesting them. Nonetheless, anaphylaxis due to an IgE-mediated 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 you have an onion allergy, speak to your doctor for a full evaluation.

Adverse Effects

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 propanethial S-oxide, a sulphur compound that wafts upward toward your eyes, making them burn and tear. To reduce irritation, try wearing goggles or chilling the onion before slicing.

Onions contain sulphurous compounds that can be absorbed into the bloodstream when ingested and can escape from our pores when sweating. Therefore, some people may "smell like onions" after eating them. Onions, particularly raw onions, can sometimes make your breath smell. To get rid of onion breath, try eating some parsley.

Onions also contain fructans. Anyone following a low FODMAP diet for irritable bowel syndrome should be aware that onions may cause abdominal pain, bloating, and gas.

When onions are eaten as a whole food ingredient (rather than taken in a supplement form) they are generally considered safe. Taking high doses of onion supplements, however, could potentially interact with some medications, so let your doctor know about any supplements you take.

Varieties

Onion bulbs come in three colours: red, yellow, or white. Depending on how much daylight is required for onion bulb formation, onions are classified by farmers as short-day (under 12 hours), medium-day, or long-day (14 hours or more). Onion varieties are tested based on their sweetness and pungency. Onion bulbs of the same colour may vary in taste based on their chemical composition. Disease-resistance also varies among the different types of onions.

You can buy fresh onions or purchase dehydrated onions or onion powder to add flavour 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 that were 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. Avoid pre-cut onions that appear dried out or slimy. Always use pre-cut onions by the expiration date listed on the package.

Storage and Food Safety

Use fresh onions within 2–4 weeks or purchasing. Store 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 but should be discarded if you notice any signs of mould.

To preserve onions in the freezer, chop into 1/4 to 1/2 inch sections. Onions that are peeled, washed, and sliced don't need to be blanched prior to freezing. Freeze them flat on a baking sheet and then place them into airtight containers. This lets you remove pieces of frozen onion as needed, rather than the pieces sticking together. Onions will keep in the freezer for up to a year.

How to Prepare

Use onions to flavour sauces, stews, chilli, or soup. Top sandwiches, wraps, or burgers with a slice of onion. Dice raw, grilled, or sautéed onions and put them in your salads, side dishes, or eggs.