Onions contain some mighty phytochemicals such as flavonoids – naturally occurring compounds that can trigger reactions in the human body that protect your health. Research on flavonoids has shown that they might help reduce the risk of cancer, Parkinson's disease, heart disease and stroke.
One flavonoid, quercetin, is particularly abundant in onions. It’s a strong antioxidant linked to cancer prevention, reducing the symptoms of bladder infections, promoting prostate health and lowering blood pressure. Quercetin also reduces allergic reactions by interfering with the production of histamines in your body, which are what makes you sneeze, cry and itch if you're having an allergic reaction.
Red onions have the most quercetin and white onions have the least; shallots and yellow onions are somewhere in the middle. In general, flavonoids in onions tend to be more concentrated in the outer layers of the flesh so make sure you don’t over-peel them.
Some good news is that quercetin is not degraded by heat. It seeps out of the onions and into the sauce or soup so you’re not losing out on this nutrient.
Other important phytochemicals in onions contain sulphur – responsible for their pungent smell – and have antimicrobial and anti-inflammatory properties. Onions are healthy both raw and cooked but raw onions have higher levels of these organic sulphur compounds which, apart from helping to fight infections, also act as mild blood thinners and thus help prevent the formation of artery plaques – a big risk for heart attacks or strokes. But beware of an extremely onion-rich diet (we’re talking several onions a day) as it could cause unhealthy blood thinning.
As with all whole plant foods, onions contain fibre but they are a particularly good source of one type of soluble fibre – oligofructose – which promotes good bacteria in your intestines.
While onions have many health benefits, they may also cause problems for some people. The carbohydrates in onions, especially when eaten raw, may cause gas and bloating and can worsen heartburn in people who suffer from chronic heartburn or gastric reflux. Eating large quantities of onions can also interfere with blood thinning drugs so you need to bear that in mind.
Reports that onions are toxic are largely based on animal studies so be aware of that when reading sensationalist articles, it’s often true that what applies to animals does not apply to people. For the same reason though, be careful to never feed onions (even in food leftovers) to dogs and cats as onions are toxic to them and cause severe or even life-threatening anaemia.
As for storing cut onions, as long as you put them in an airtight container that you place in the fridge, they are safe to eat for several days.