The researchers, reporting in the journal Carcinogenesis, said female mice exposed to ultraviolet B (UVB) light took longer to develop skin tumors when they had access to a running wheel. They said exercise appeared to speed up the rate at which cancer cells die.

In the first part of the study,mice were exposed to ultraviolet B (UVB) three times a week for 16 weeks. Then for the next 14 weeks, in the absence of further UVB treatment, half the mice had access to running wheels while the other half did not.

All the mice in this study developed skin tumors, but exercising mice had fewer and smaller tumors, and they took an average of seven weeks to show signs of cancer, compared to an average of 3.5 weeks in the mice that took no exercise.

In the second part, mice were exposed to UVB light twice a week for 33 weeks. From the beginning of this study, half had access to a running wheel and half did not.

Again the exercising mice were slower to develop tumors, developed fewer tumors and those that they did develop were smaller. Analysis of samples found that exercise appeared to enhance programmed cell death (apoptosis) -- a process that removes sun-damaged cells.

"While UVB is triggering the development of tumors, exercise is counteracting the effect by stimulating the death of the developing cancer cells," said lead researcher Dr. Allan Conney. He added that the results also showed that animals with less fat developed fewer tumors.