Pale skin burns more easily, and is more vulnerable to cancer
Scientists have given mice a tan without exposing them to the sun.
They have developed a cream, which has not yet been tested on humans, that switches on the tanning machinery in skin cells.
The breakthrough also raises the prospect of a new way to protect fair-skinned people from skin cancer caused by exposure to sunlight.
The study, by Dana-Farber Cancer Institute and Children's Hospital Boston, is published in Nature.
The cream contains a small molecule that essentially mimics the process that occurs when skin cells are struck by ultraviolet light from the sun.
It is thought people with fair skin and red hair cannot tan properly because of a defect in a pouch-like receptor called MC1R on the surface of the pigment-producing melanocyte skin cells.
This leads to reduced production of a chemical called cAMP, which stimulates the melanocytes to produce pigment.
As a result, fair-skinned people tend to burn, rather than tan in the sun, increasing the risk of DNA damage to skin cells which can lead to cancer.
The World Health Organization has estimated that as many as 60,000 people a year worldwide die from skin cancer as a result of too much exposure to the sun.
The researchers genetically engineered fair-skinned, red-haired mice who did not tan when exposed to low levels of UV radiation, but did burn when the dose was cranked up.
They then treated the skin of the animals with a compound known to increase cAMP levels.
The compound, forskolin, is derived from the root of the forskohli plant found in India.
The mice turned dark, proving that melanocytes in redheads can make pigment if appropriately stimulated.
Further experiments showed that this sunless tanning process was virtually indistinguishable from that in dark-haired mice that tan naturally.
The researchers also found that the tans acquired through forskolin treatment protected the skin against cancer caused by exposure to UV light.
Lead researcher Dr David Fisher said it was not yet clear whether forskolin would penetrate deeply enough in human skin to activate melanocytes.
However, he said: "These studies suggest that a drug-induced 'rescue' of the tanning mechanism may correspondingly rescue at least some aspect of skin cancer protection.
"Such sunless tanning may also dissuade sun-seeking behaviours, which undoubtedly contribute significantly to high skin cancer incidence."
Hazel Nunn, cancer information officer at the charity Cancer Research UK, said the study gave new insights into why fair-skinned people rarely tan.
She said: "We welcome this advance in our understanding of the tanning process but it is important to point out that a tan-inducing cream alone is unlikely to be enough to protect those with fair skin from sunburn or to prevent skin cancer.
"A UV-induced tan only provides protection equivalent to that of sunscreen with SPF4."