A fascinating study on mice from the University of Copenhagen in Denmark might explain the reason for the decreased risk factor. Exercise reduced the risk of cancer by 60% in animals with five different kinds of tumours. Exercising animals receiving implants of melanoma skin cancer cells were less likely to develop full-blown cancer than sedentary animals. Exercise increase adrenaline, natural killer cells and interleukin-6, which combined to become potent cancer killers. The researchers speculated that adrenaline acts as a signal to interleukin-6, which turn on the natural killer cells that fight the cancer. It is not known whether these results apply to humans.