Exercise has proven benefits
Prostate cancer patients have been told to carry on exercising despite research in mice which appears to suggest it speeds the growth of tumours.
The US study found tumours expanded twice as fast in mice given exercise wheels compared to those without.
The researchers said improved blood flow to the tumour was a possible cause, but encouraged patients to remain active.
Sedentary lifestyles raised the risk of other serious diseases, they said.
The results of the mouse experiment, presented at a US cancer conference, could actually help reveal ways to improve prostate cancer treatments, said researchers from Duke University Medical Center.
The research team implanted 50 human prostate tumours into mice and then placed half of them in cages where exercise was impossible. The remainder were allowed to run for an average of half a mile a day.
Dr Lee Jones, one of the researchers, said: "Our study showed that exercise led to a significantly greater tumour growth than a more sedentary lifestyle.
"Among the mice that had the opportunity to voluntarily exercise, tumours grew approximately twice as fast as they did among the mice that did not have the opportunity to exercise."
However, he said that exercise could be a way to improve treatment by delivering drugs more effectively to prostate tumours, which can generally have a poor blood supply.
Dr Stephen Freedland, another Duke researcher, said the circumstances were very different to human prostate cancer, and said that the results should be interpreted with caution.
"These mice were not receiving treatment and we were allowing aggressive tumours to grow unchecked for the sake of the experiment. Patients would not find themselves in the same situation."
He said that the benefits of exercise in improving cardiovascular health, and reducing the risk of diabetes, obesity, and other chronic conditions meant that it was still advisable for prostate patients, and older men in general, to take regular exercise.
A spokesman for the Prostate Cancer Charity agreed that men should not avoid exercise for fear of the disease.
A spokesman for the charity said: "It is important to remember that a balanced healthy diet together with regular exercise will benefit your overall health and reduce your risk of all cancers as well as other common conditions such as heart disease and diabetes."