A sunless tanning cream that mimics the protective effects of dark skin may help ward off skin cancer, tests on mice suggest.

Unlike lotions that colour the skin, the cream changes skin pigmentation the way a suntan does, but without exposure to the sun's harmful ultraviolet rays.

People who tan easily or have naturally dark skin are less likely to develop skin cancer. As fair-skinned people moved to sunnier climes, melanoma and other skin cancer rates have increased, the researchers said. 

Melanoma is now the fastest-increasing form of cancer worldwide, according to the American Cancer Society.

Many fair-skinned redheaded people have a defect in the hormonal pathway that leads to the production of the skin pigment melanin. 

In Thursday's issue of the journal Nature, Dr. David Fisher, director of the melanoma program at Dana-Farber Cancer Institute in Boston, Mass., and his colleagues showed the pigmentation pathway can be restored in genetically engineered fair-skinned mice lacking hair.

"The study involved using a small molecule to essentially mimic the process that occurs when skin cells are struck by ultraviolet light from the sun," said Fisher.

A molecule in the cream recreates the way a tan forms in people who darken naturally. The sun's ultraviolet rays initially cause damage to DNA in the skin, but then a tan forms to curb the effect.

The experiments suggested UV radiation acts on different cells in the skin than previously thought during the protective pigment-making process.

The cream's active ingredient, which is derived from the root of the forskohlii plant found in India, has not yet been tested on humans.

Although the mice showed no side-effects, the animals have thinner skins than people.