The study involved four groups of hairless mice. For two weeks, one group exercised on a running wheel, another drank caffeinated water, a third exercised and consumed caffeine, and the fourth did neither.

All of the animals were exposed to ultraviolet-B radiation, the component of sunlight primarily responsible for sunburns.

The researchers found that the exercise group and the caffeine group each experienced a modest increase in apoptosis of UVB-damaged skin cells.

However, the combination of the two enhanced apoptosis of these cells. In fact, the effect was "more than additive," meaning exercise and caffeine somehow worked synergistically to boost each other's effects.

Whether the results might translate to humans is not yet known. But some past studies suggest the mouse findings "may have human relevance," Dr. Allan H. Conney, the study's senior author, told Reuters Health.

Population studies have linked coffee drinking to lower risks of non-melanoma skin cancer, as well as lower risks of liver cancer and certain other tumors, explained Conney, who directs Rutgers' Cullman Laboratory for Cancer Research.

Similarly, regular exercise has been tied to lower risks of certain cancers.

Among the next steps is to see whether the exercise-caffeine combo lowers the incidence of actual skin tumors in animals. In previous work, Conney and his colleagues found that caffeine or exercise alone can do this.

It will also be important to uncover the mechanisms at work, according to Conney. One of the "intriguing" findings in the current study, he said, was that exercise and caffeine decreased the animals' body fat.

There's evidence that fat tissue produces substances that impede damaged body cells from apoptosis, and it's possible that decreased fat stores help explain the benefits that caffeine and exercise had on apoptosis, Conney noted.

However, he added, this appears to be only "part of the story," and other mechanisms must play a role.

So should you add a coffee to your morning run? According to Conney, regular exercise is certainly a good habit to develop, but it's too soon for people to take up coffee drinking for cancer prevention. Caffeine can have negative effects, such as increased blood pressure.

However, healthy people who already drink a modest amount of coffee need not feel they should give it up.

"I think coffee, in moderation, can be a good thing," Conney said.

SOURCE: Proceedings of the National Academy of Sciences, July 31, 2007.

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