"The anti-cancer effects of exercise are due to increases in a protein that blocks cell growth and induces cell death, according to Australian researchersThe protein, called insulin-like binding protein-3 (IGFBP-3), inhibits another protein called insulin-like growth factor-1 (IGF-1), thereby blocking IGF-1's proliferative effect on cell growth, the study hints. Dr. Andrew M. M. Haydon and colleagues at Manash Medical School in Melbourne"Haydon also added:
"physical activity can increase IGFBP-3 levels, which, in turn, reduces the amount of free IGF-1." IGF-1 has been shown to stimulate cell growth, inhibit cell death, and promote angiogenesis -- the formation of new blood vessels, which tumors need to grow."
see also: The Surprising Effect of Exercise