While vegetables are widely acclaimed for their nourishing and wholesome characteristics, some have better reputations than others. It is often said that deeply tinged produce, such as broccoli and spinach, have added nutritional value compared to more pallid vegetables varieties, such as celery and cucumber, and a recent study bears testament to this fact. However, all this emphasis on eating our greens has tended to overshadow the nutritional attributes offered by less colourful fare: there is evidence to suggest that the onion packs an eye-watering nutritional punch.

Onions are rich in a class of compounds known as the flavonoids. Also found in foodstuffs such as tea, apples and red wine, flavonoids are believed to have the capacity to quell disease-precipitating processes in the body. One flavonoid that has caught the attention of researchers over the last decade is quercetin, with several studies suggesting that this nutrient has the ability to protect us against a variety of conditions, and onions are especially rich in this.

Studies show that onions have the capacity to 'thin' the blood and may help to reduce blood pressure, too. Onion eating has also been found to be associated with lower levels of blood fats known as triglycerides, which are believed to play some part in the clogging-up of arteries around the body. The physiological and biochemical boons onions offer appear to translate into considerable benefits for the heart. Increased flavonoid intake, including those coming from onions, has been associated with a significantly reduced risk of heart disease. In one study, high flavonoid intake was associated with a halving in the risk of succumbing to this condition.

There is also some evidence that compounds in onions have beneficial effects on the hormone insulin in the body. Insulin is principally responsible for moderating the level of sugar in the bloodstream.

A reduction in the amount or effectiveness of insulin predisposes the body to diabetes - a condition characterised by higher-than-normal levels of sugar in the bloodstream. Onions appear to prolong the effect of insulin, and more than one study shows that they help to keep blood-sugar levels in check. These facts mean that onions are a particularly healthy choice for diabetics.

In addition to their rich stash of flavonoids, onions have also been found to harbour significant amounts of sulphur-containing compounds. These biochemical entities seem to help the liver deactivate potentially toxic substances, an effect which might help to keep the body free from cancer. This idea is borne out by studies linking onion-eating with a reduced risk of cancer of the stomach and prostate. Other evidence points to a particularly strong relationship between onions and a reduced risk of lung cancer. Onions are also believed to help reduce the airway spasm that is characteristic of asthma, and are believed to help keep symptoms of this condition at bay.