Many pale-skinned Britons suffer the annual indignity of spending the first few days of a sunshine break with stubbornly pasty limbs.
Now scientists have discovered why people do not tan immediately, even after a day of sunbathing at the beach.
The delayed tanning phenomenon occurs because the body prioritises repairing the DNA damage done to the skin by radiation. Only after the cells have repaired the genetic information sufficiently do they begin to increase melanin which darkens the skin, generating a physical protection to help withstand the next bout of Sun damage.
It suggests that tanners should take comfort in the fact the body is silently working beneath the skin to prevent cancer even if nothing is visible.
Nadav Elkoshi, a doctoral student from the Department of Human Molecular Genetics and Biochemistry at Tel Aviv University, who led the research said: “We have two mechanisms designed to protect the skin from exposure to dangerous UV radiation.
“The first mechanism repairs the DNA in the skin cells damaged by the radiation, while the second mechanism involves increased production of melanin, which darkens the skin in order to protect it from future exposure to radiation.
“It turns out that the mechanism that repairs our DNA takes precedence over all other systems in the cell, temporarily inhibiting the pigmentation mechanism.
“Only after the cells repair the genetic information to the best of their ability do they begin to produce the increased melanin.”
The skin cells that cause tanning are called melanocytes which reside deep in the outer layer of the skin and also account for the skin’s natural coloration. Light-skinned people have fewer melanocyte cells while dark-skinned people have more.
Although nobody can increase their number of melanocytes a limited increase in melanin production – tanning – can be achieved through increased exposure to UV light.
The experts had hypothesised that the body may prioritise damage repair, so to test their idea they activated the radiation DNA repair mechanism in animal and human skin tissue.
They found that after a short time, a tan developed even without exposure to sunlight, suggesting that one naturally follows on from the other.
Professor Carmit Levy, also of Tel Aviv University, added: “The DNA repair mechanism essentially tells all the other mechanisms in the cell, ‘Stop everything, and let me work in peace.’
“One system effectively paralyses the other, until the DNA correction reaches its peak, which occurs a few hours after the UV exposure.
“Only then does the pigment production mechanism get to work.”
They also discovered that a protein called ATM, plays a key role in the DNA repair, activating the regeneration mechanism while disabling tanning.
The team believes the breakthrough may offer new ways to activate the mechanism even without sunlight, to prevent skin cancer, and delay skin ageing.
Professor Levy added: “This scientific discovery has revealed a molecular mechanism that could serve as a foundation for further research that may lead to innovative treatments that will provide maximum protection of the skin against radiation damage; in the long run, it may even contribute to the prevention of skin cancer.”
The research was published in the Journal of Investigative Dermatology.