Why do tans appear only after we've left the beach?

Beachgoers are familiar with the experience of spending hours in the sun, going home, and noticing only hours later that their skin has changed colour. A research team led by Tel Aviv University has now uncovered the science behind the mystery of why the body’s tanning process does not occur immediately after sun exposure, with their results published in the Journal of Investigative Dermatology.

“We have two mechanisms designed to protect the skin from exposure to dangerous UV radiation,” explained study co-leader Nadav Elkoshi. “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.”

To test their hypothesis, the researchers activated the DNA repair mechanism in both animal models and human skin tissues. In both, a tan developed even without any exposure to UV radiation, substantiating their findings.

“The genetic information must be protected from mutations, so this repair mechanism takes precedence inside the cell during exposure to ultraviolet radiation from the sun,” said study co-leader Professor Carmit Levy. “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.

“In our previous research, we showed that a protein called MITF, which is activated during exposure, is responsible for regulating these two mechanisms. In the current study we show that another protein, called ATM, which plays a key role in DNA repair, activates one mechanism while disabling the other. This process likely harnesses the pigmentation mechanism’s components to maximise the chances of the cell surviving without mutations following radiation exposure.

“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,” Levy concluded. “In the long run, it may even contribute to the prevention of skin cancer.”

Image credit: iStock.com/Ralf Nau