Drinking coffee and exercising may prevent skin cancer by killing off cells damaged by the sun's ultraviolet-B (UVB) radiation, said a study of hairless laboratory mice published Monday.

The coffee-exercise combination produced a "dramatic" fourfold difference in apoptosis -- the programmed death of pre-cancerous cells -- between laboratory mice that did and did not follow the regime, said the researchers of New Jersey's Rutgers University.

Researchers compared UVB radiation effects on groups of hairless mice that drank caffeinated water (the human equivalent of one or two cups of coffee a day); that exercised on a running wheel; that had caffeine and ran; and a control group that had no caffeine or exercise at all.

Compared to the control group, mice that only drank coffee showed a 95 percent increase in UVB-induced apoptosis, those that only exercised showed a 120 percent increase, while those who drank and exercised showed an almost 400 percent increase.

"The differences between the groups in the formation of UVB-induced apoptotic cells -- those cells derailed from the track leading to skin cancer -- were quite dramatic," said Allan Conney, one of the study's authors.

The promising results, however, were likely due to "some kind of synergy ... still somewhat of a mystery" which, until better understood, precludes taking the research to "the next level ... human trials," he added.

Sunlight-induced skin cancer is the most prevalent cancer in the United States, with more than one million new cases each year, according to the National Cancer Institute.