Onions contain important sulphur-containing compounds, which are responsible for many of their health benefits, as well as being responsible for their pungent odour. They are members of the Allium family, together with garlic and leeks but, unlike garlic, their health benefits are often overlooked.
Onions come from the Middle East and Asia and have been cultivated for more than 5000 years. They have been revered over the centuries, for both their use in food and as a medicine. The Egyptians found them so important that they placed them in the tombs of kings such as Tutankhamen for the afterlife, as well as using them as currency to pay the workers who built the pyramids. They were also used as a medicine in India as early as the 6th century.
When the onion bulb is cut or crushed, two sulphur-containing compounds in the onion, allium and allyl disulphide, convert to allicin, which works in a similar way to statin drugs to reduce cholestrol, but without the side effects. These sulphur-containing compounds have also been shown to have cancer-fighting properties, as well as being able to help with blood sugar regulation. They have been found to decrease blood vessel stiffness, by releasing nitric oxide, which may reduce blood pressure, inhibit blood clot formation and help reduce the risk of heart disease and stroke.
Onions are a good source of Vitamin C and B6, iron, folate, potassium and manganese. They are also a good source of polyphenols, including the flavonoid quercetin, which is a well known antioxidant that has been shown to have anti-cancer, anti-inflammatory and anti-diabetic benefits. A study even showed that the antioxidant protection from the quercetin in onions may protect us better than the quercetin in supplement form. Red onions are usually higher in total flavonoids than white onions, with yellow onions falling somewhere in between.
Several servings of onion each week are sufficient to statistically lower your risk of some types of cancer, including colorectal, laryngeal, prostate and ovarian cancer. For decreased risk of oral and esophageal cancer, however, you need to consume one onion serving per day (approximately 1/2 cup). It is not known what the exact mechanism of cancer inhibition is, but it is believed that compounds in onions inhibit the growth of tumours and prevent free radical formation.
Daily consumption of onions has been shown to increase bone density and especially be important for women who are experiencing loss of bone density around menopausal age. Post menopause, daily onion consumption may also lower a woman’s risk of hip fracture, but it is important to note that the benefit was only found on daily consumption.
The high sulphur content of onions may be useful for our connective tissue too, including our joints, as many of our connective tissue components need sulphur for their formation.
The folate, found in onions, may help with depression by preventing an excess of homocysteine in the body. Excess homocysteine interferes with the production of the feel-good hormones serotonin, dopamine, and norepinephrine, which regulate not only mood, but also sleep and appetite too.
How to get the most out of your onions
If you don’t want to loose the benefits of the flavonoids in your onions, then be careful not to peel too much off your onion, since the flavonoids seem to be concentrated higher in the outer layers of the flesh.
When onions are simmered to make a soup, their quercetin does not get destroyed, but rather gets transferred into the broth. If you keep to a low heat in the soup preparation you can preserve onion’s health benefits that are associated with this flavonoid.
The sharp fragrance and flavour emitted by onions is due to the sulphur compound allyl propyl disulphide; it’s allyl sulphide that brings you to tears when peeling one, serving the good purpose of washing the thin epithelial layer of the eyes. Holding peeled onions under cold water for several seconds before slicing minimizes this effect.
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