Onions: unsuspected virtues for health

Native to Central Asia (between the Caspian Sea and China), the onion (Allium cepa) is a member of the garlic family that has been an integral part of the human diet for over 7000 years. In some ancient civilizations, the onion was reputed to give strength and power. For example, the Egyptian workers of the ancient pyramids used onions as food as well as for the treatment of heart problems, headaches and tumors. And in the Middle Ages, onions were so highly valued that they were often used to pay off debts or even given as gifts. If these uses can make us smile today, the fact remains that this respect for the onion is completely deserved, because this vegetable contains several molecules which actively participate in the prevention of chronic diseases, in particular certain types of cancer.

Onions: valuable agents in the prevention and fight against cancer

A significant number of studies have shown that people who regularly eat vegetables from the garlic family, such as onions, have a lower risk of being affected by certain types of cancers, especially those of the digestive system such as stomach and colon cancers. The mechanisms responsible for this anti-cancer action are very complex, but we have known for some years that they are due to the formation of very active molecules formed following the breaking of the onion. So when you cut the onion, a molecule called 1-propenyl-L-cysteine ​​sulfoxide comes into contact with alliinase, a very active enzyme that converts this substance into sulphenic acids and thiosulphinates. These thiosulphinates are largely responsible for the positive effects of onions on health, because they have the ability to accelerate the elimination of carcinogenic toxic substances from our body, which reduces the risk of these substances attacking our genetic material and cause mutations that can start cancer. It seems that these molecules also have the property of stopping the growth of cancer cells and even, in some cases, of forcing them to commit suicide through the process of apoptosis.

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Onions: they make you cry but we forgive them

In addition to causing the release of anticancer molecules, one of the well-known peculiarities of the onion is to generate a substance that irritates the eye and can make you cry. This phenomenon is due to the presence of another enzyme (LF synthase) which transforms 1-propenyl-sulphenic acid into a volatile and very irritating gas called propanthial-S-oxide. This gas diffuses through the air and reaches the eye, causing irritation which induces the secretion of tears. This formation of propanethial-S-oxide is maximal 30 seconds after the breaking of the onion and decreases thereafter. However, with some onion varieties, this can be a bad time!

However, it is possible to reduce this irritation by adding a little water to the onion during the cut (the gas dissolves in the water) or by not cutting the base of the onion, because this region contains the greater quantity of the enzyme responsible for the production of the irritating factor.

But if the tears remain there, you can console yourself with the thought that they reflect the production of anticancer molecules that help maintain your health.


Chang et al. Growth inhibitory effect of alk(en)yl thiosulfates derived from onion and garlic in human immortalized and tumor cell lines. Cancer Lett.; 223: 47-55.

Imai et al. An onion enzyme that makes the eyes water. Nature; 419:685.

* Presse Santé strives to transmit medical knowledge in a language accessible to all. In NO CASE can the information given replace medical advice. []

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