Question: We have been eating a lot of onions lately, possibly as a result of the casserole, curry and similar slow-cooker recipes in our dinners. What are the health benefits, or otherwise, of onions? And are pre-cut onions just as good, or do they lose nutritional value, like pre-crushed garlic?
Answer: Onions are armed with some serious chemical weapons, as anyone who has cut a fresh onion through a blur of tears will know. Onions have a stockpile of sulphur-containing molecules that, when cut, turn into reactive stinging airborne missiles that make people’s eyes water. The good news is, those chemical weapons also have numerous health benefits once you get past the challenge of cutting an onion.
Onions contain vitamin C, folate, niacin, potassium and other micronutrients in modest amounts. But given New Zealanders eat a lot of onions, they do make a helpful contribution. What is of genuine interest in onions is the range of other health-promoting chemical compounds, known as phytochemicals.
For starters, onions are a rich source of antioxidants called flavonoids. Antioxidants deactivate harmful free radicals and other oxidants, thus preventing them from causing damage to cell components that may result in cancer or cardiovascular disease.
Onions contain a sub-group of flavonoids called flavonols. These pigmented compounds are responsible for the common brown onion’s yellow flesh and brown skin. Anthocyanins, another sub-group of flavonoids, give the reddish-purple colour to red onions.
Flavonoid antioxidants are the subject of much research interest. One flavonol in onions, quercetin, reduced the growth of certain cancer cells in laboratory-based studies. Quercetin has also been linked to improved healing of stomach ulcers in animals.
Onions also contain sulphur-containing compounds that give them their distinctive flavour. When onions are cut or chewed, the sulphur-containing compounds are converted by enzymes into sulphides, reducing the risk of blood clotting and, potentially, heart disease. Sulphur-containing compounds may also be responsible for the antibacterial and antifungal properties of onions.
On the downside, the sulphur-containing compounds in onions are toxic to cats and dogs, damaging their red blood cells, so make sure your pets never eat onions or garlic.
Another group of compounds found in onions are saponins. These compounds improve blood cholesterol levels by encouraging bile acids to bind on to dietary fibre in the gut, forcing the bile acids to then be excreted from the body. The body must then use the cholesterol in the blood to make more bile acids, thereby lowering blood cholesterol levels.
Onions also contain soluble fibre in the form of fructo-oligosaccharides or fructans. These compounds pass through the gut undigested and reach the bowel, where our intestinal microflora ferments them and short-chain fatty acids are produced. These encourage the growth of good bacteria and reduce the proliferation of harmful bacteria. Dietary intake of fructans helps prevent constipation and is associated with improved cholesterol levels and a reduced risk of certain cancers.
Unfortunately, the fructans in onions are a prime offender in causing painful gastrointestinal symptoms in people who suffer from irritable bowel syndrome (IBS). An estimated one in six New Zealanders suffers from IBS. If you are an IBS sufferer who loves the taste of onions, your best bet is to use spring onions and eat only the green part; this will provide the onion taste without the fructan load.
Researchers used fresh-cut samples to identify most of the health benefits ascribed to onions, rather than pre-cut and stored onions. Studies looking at the effects of cutting and storage on the phytochemical content of onions have produced varying results. One study found compounds such as quercetin increased during storage of cut onions. In contrast, several other studies found the phytochemical content in cut onions progressively reduced over time. To maximise the health benefits of your onions, cut them immediately before use, slice rather than dice (if possible), and eat them raw or moderately cooked.
This article was originally published July 17 2021
