WASHINGTON (AP) -- New research indicates adding a cup or two of coffee to the exercise routine increases protection from skin cancer.

The combination of exercise and caffeine increased destruction of precancerous cells that had been damaged by the sun's ultraviolet-B radiation, according to a team of researchers at Rutgers University.

Mice bodies provide a protective effect from both caffeine and voluntary exercise, and when both are provided -- not necessarily at the same time -- protection is even more than the sum of the two, said Dr. Allan H. Conney of the laboratory for cancer research at Rutgers.

"We think it likely that this will extrapolate to humans, but that has to be tested," Conney said in a telephone interview.

Nonetheless, he added, people should continue to use sunscreen.

Exposing the mice to ultraviolet-B light causes some skin cells to become precancerous.

Cells with damaged DNA are programmed to self-destruct, a process called apoptosis, but not all do that, and damaged cells can become cancerous.

The researchers report in Tuesday's issue of Proceedings of the National Academy of Sciences that they studied hairless mice in four groups. Some were fed water containing caffeine, some had wheels on which they could run, some had both and a control group had neither.

"The most dramatic and obvious difference between the groups came from the caffeine-drinking runners, a difference that can likely be attributed to some kind of synergy," Conney said.

Compared with the control animals, those drinking caffeine had a 95 percent increase in apoptosis in damaged cells. The exercisers showed a 120 percent increase, and the mice that were both drinking and running showed a nearly 400 percent increase.

Just what is causing that to happen is not yet clear, though the researchers have several theories.

"We need to dig deeper into how the combination of caffeine and exercise is exerting its influence at the cellular and molecular levels, identifying the underlying mechanisms," Conney said.

"With an understanding of these mechanisms we can then take this to the next level, going beyond mice in the lab to human trials," he said. "With the stronger levels of UVB radiation evident today and an upward trend in the incidence of skin cancer among Americans, there is a premium on finding novel ways to protect our bodies from sun damage."

Conney said the researchers were originally interested in the effects of green tea in preventing skin cancer and were doing tests on regular and decaffeinated teas.

They found the regular tea had an effect, but not the decaffeinated brew.

And, he said, researchers also observed that mice drinking caffeine were more active than those that did not get it, so they decided to study the effects of exercise too.

They put running wheels into some of the cages. The mice "love to go on it," he said, and will jump on the wheels and run for several minutes, then get off for a while, and then get on and run some more.

And they found that both caffeine and exercise helped eliminate damaged skin cells, but the combination worked better than either alone.

"What we would like to see next is a clinical trial in people," Conney said.

Dr. Michael H. Gold, a dermatologist and a spokesman for the Skin Cancer Foundation, said he believes "the concept of systemic caffeine should be addressed further."

"I think the concept potentially has a lot of merit," he said in a telephone interview. But mice and humans are different and studies need to be done to be sure this also applies to people.

In the meantime, he said: "If you go outside, you have to wear a sunscreen ... it has to be caffeine and exercise with your sunscreen."

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