When the thirties come (and sometimes even before), they appear to cover the entire head. They are white / gray hair. What is the reason for this inevitable and irreversible phenomenon that affects all human beings?

A question of cycles?

The growth of hair (human beings) is governed by cycles, which usually last from four to seven years for the woman and two to four years for the man. After about ten cycles, the melanocytes - the cells responsible for the pigmentation of the skin or the hair - would lose effectiveness, which would explain the bleaching of the hair. This theory was verified by Harvard scientists in 2005, whose conclusion was clear: "Failed stem cells would no longer produce the proper number of melanocytes to maintain color." However, that would not be the only explanation.

White hair: Hydrogen peroxide on the head?

In 2009, British researchers carried out a thorough study on hair bleaching and deduced that it was the presence of hydrogen peroxide in the scalp that bleached the hair. It is the hair follicles that produce this lightening substance, but during the first 30 years of life of the human being, this same substance is destroyed by an enzyme called catalase which itself would lose its effectiveness with age.

In reality, it is the combination of these two phenomena that would be responsible for white hair.

A genetic predisposition?

Another data discovered recently complements all its characteristics: it is the genetic predisposition. Indeed, researchers at University College London have just discovered that all the aspects already known of the canities are amplified by the genes.

"We found an allele (variant of a gene that often determines the appearance of different hereditary characters, editor's note) that predisposes to grizzled hair," says Kaustubh Adhikari of University College London and lead author of the study . "We have already identified genes responsible for baldness and hair color, but this is the first time a gene linked to hair aging has been defined," he adds.

"The allele of aging is essentially observed in Europeans," says the researcher. The average age of the appearance of the white hair is around 35 years for the Caucasians, a little before the quarantine for the Asians and only towards 45 years for the Africans. "

Are you wearing white hair?

If white hair is the result of a disorder of enzyme and pigment, where does the expression "get white hair" comes from when someone is worried? Here too scientists provide a response: that of oxidative stress *, a phenomenon due to anxiety that accelerates the process of degradation of melanocytes. A phenomenon that explains for example that the age of people who see their first white hair appear is totally variable.

* Type of aggression of cell constituents due to oxygenated reactive species. Sources of Oxidizing Pressures? Alcohol, tobacco, food imbalance, pollution, drugs or overwork, lack of physical activity.