Unexpected health benefits

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 more than 7,000 years. In some ancient civilizations, onions were reputed to give strength and power. For example, the Egyptians, workers of the ancient pyramids used onions as food as well as to treat heart problems, headaches, and tumors. And in the Middle Ages, onions were so valuable that they were often used to pay off debts or even given as gifts. If these uses make us smile today, the fact remains that this respect for onions is well deserved, because this vegetable contains many molecules that are actively involved in the prevention of chronic diseases, especially certain types of cancer.

Onions: Valuable agents in the prevention and control of cancer

A large 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 that affect the digestive system such as stomach and colon cancer. The mechanisms responsible for this anticancer action are quite complex, but we have known for several years that they result from the formation of highly active molecules formed after onions are broken down. So when you chop an onion, a molecule called 1-propenyl-L-cysteine ​​sulfoxide comes into contact with alliinase, a highly active enzyme that converts this substance into sulfinic acids and thiosulphinate. These thiosulfins are largely responsible for the positive effects of onions on health, as they have the ability to accelerate the elimination of toxic, carcinogenic substances from our bodies, reducing the risk of these substances attacking our genetic material and causing mutations that can start cancer. These molecules also appear to have the property of stopping the growth of cancer cells and, in some cases, forcing them to commit suicide through the process of programmed cell death.

Onions: Makes you cry and we forgive them

In addition to causing the release of anti-cancer molecules, one of the known properties of onions is the production of a substance that irritates the eyes and can make you cry. This phenomenon is due to the presence of another enzyme (LF synthase) that converts 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 that leads to the production of tears. This formation of S-propanthial oxide is a maximum of 30 seconds after the onion is broken down and decreases thereafter. However, with some varieties of onions, this can be a bad time!

However, it is possible to reduce this irritation by adding a little water to the onion while cutting (the gas dissolves in the water) or by not cutting the base of the onion, since this area contains a greater amount of the enzyme responsible for the production of the irritating agent.

But if the tears stay there, you can console yourself with the idea that they reflect the production of anti-cancer molecules that help keep you healthy.


Chang et al. The growth-inhibitory effect of alk(en)yl thiosulfates derived from onions and garlic in human tumor and immortal cell lines. Cancer lite. 223: 47-55.

Imai et al. Onion enzyme that makes eyes water. temper nature; 419: 685.

* Presse Santé strives to impart medical knowledge in a language accessible to all. In no way can the information provided replace medical advice.

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