Onions can vary in size, shape, color, and flavor. The most common types are red, yellow, and white onions. Flavors can be sweet and juicy to sharp, spicy, and pungent, often depending on the season in which they are grown and consumed.
It is estimated that 105 billion pounds of onions are harvested each year, worldwide. According to the Food and Agriculture Organization of the United Nations (FAO), China is the biggest producer.
The possible health benefits of consuming onions include lowering the risk of several types of cancer, improving mood, and maintaining the health of skin and hair.
Consuming fruits and vegetables of all kinds has long been associated with a reduced risk of many lifestyle-related health conditions.
Many studies have suggested that increasing consumption of plant foods like onions decreases the risk of overall mortality, diabetes, and heart disease.
Plant foods also promote a healthful complexion, hair, increased energy, and overall lower weight.
Allium vegetables have been studied extensively in relation to cancer, especially stomach and colorectal cancers. Their beneficial and preventive effects are likely due in part to their rich organosulfur compounds.
The exact mechanism by which these compounds inhibit cancer is unknown, but hypotheses include the inhibition of tumor growth and mutagenesis, and prevention of free radical formation. Onions are also a source of the strong antioxidant vitamin C that helps to combat the formation of free radicals known to cause cancer.
High fiber intake from all fruits and vegetables are associated with a lowered risk of colorectal cancer. One study showed that consuming greater than 7 servings of onions per week was associated with a decrease in colorectal cancer.
In a paper, published by the Journal of the National Cancer Institute, researchers used a population-based, case-controlled study to investigate the relationship between allium vegetable intake and prostate cancer. They found that men with the highest intake of allium vegetables had the lowest risk of prostate cancer.
Esophageal and stomach cancer
Frequent intake of allium vegetables has been shown to reduce the risk of esophageal and stomach cancer. Several survey-based human studies have demonstrated the potential protective effects of consuming alliums, as well as reports of tumor inhibition of allium compounds in animal experiments.
Sleep and mood
Folate, found in onions, might help reduce depression. Homocysteine prevents blood and other nutrients from reaching the brain, and folate prevents this chemical from building up. Excess homocysteine also interferes with the production of the feel-good hormones serotonin, dopamine, and norepinephrine, which regulate not only mood, but also sleep and appetite.
Skin and hair
Onions are high in vitamin C, which is needed for the building and maintenance of collagen, which provides structure to skin and hair.
Onions are a nutrient-dense food, meaning that while they are low in calories, they are high in beneficial nutrients like vitamins, minerals, and antioxidants.
One cup of chopped onion contains approximately 64 calories, 15 grams of carbohydrate, 0 grams of fat, 0 grams of cholesterol, 3 grams of fiber, 7 grams of sugar, 2 grams of protein, and 10 percent or more of the daily value for vitamin C, vitamin B-6, and manganese.
Onions also contain small amounts of calcium, iron, folate, magnesium, phosphorus, and potassium, and the antioxidants quercetin and sulfur.