Skin cancer is on the rise
Exercise might protect against skin cancer, a US study suggests.
The researchers found female mice exposed to a form of ultraviolet light took longer to develop skin tumours if they had access to a running wheel.
The Rutgers University team found exercise appeared to speed up the rate at which cancer cells die.
However, experts warned the study, published in the journal Carcinogenesis, was not an excuse to go out in the sun unprotected.
In the first part of the study, labelled the high risk model, 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 in their cages, while the other half did not.
In the second part mice were exposed to UVB light twice a week for 33 weeks, and, from the beginning, half had access to a running wheel and half did not.
All the mice in the high risk part of the study developed skin tumours.
But exercising mice took an average of seven weeks to show signs of cancer, compared to an average of just 3.5 weeks in the mice which took no exercise.
The tumours in the exercising mice were also less numerous and smaller.
Non-malignant tumour size per mouse was decreased by 54% and malignant tumour size per mouse by 73%.
The second part of the study produced similar results. Again the exercising mice were slower to develop tumours, developed fewer tumours and those that they did develop were smaller.
This time non-malignant tumour size per mouse was decreased by 75% and malignant tumour size per mouse by 69%.
Analysis of samples found that exercise appeared to enhance programmed cell death (apoptosis) - a process that removes sun-damaged cells - both in the skin, and in tumours.
Lead researcher Dr Allan Conney said: "While UVB is triggering the development of tumours, exercise is counteracting the effect by stimulating the death of the developing cancer cells."
Dr Conney said the results also showed that animals with less fat developed fewer tumours.
He said this might be a significant factor - particularly as obesity rates were rising throughout the Western world.
Henry Scowcroft, of the charity Cancer Research UK, said other research had found that taking regular exercise, coupled with a healthy diet, reduced the risk of several types of cancer.
He said: " Despite the fact that exercise lessened the severity of skin cancer in mice in this study, this doesn't mean people should strip off and start jogging in strong sunlight without proper protection - UV rays from the sun are a major cause of skin cancer and are best avoided when the sun is most intense, especially by the fair skinned."