The anti-cancer effects of exercise are due to increases in a protein that blocks cell growth and induces cell death, according to Australian researchers.

The 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 identified new cases of colorectal cancer in the Melbourne Collaborative Cohort Study, a prospective study of 41,528 adults recruited between 1990 and 1994.

The investigators looked at baseline body mass index and level of physical activity reported and compared baseline levels of IGF-1or IGFBP-3 with those measurements.

Analyses centered on 443 colon cancer patients followed for more than 5 years.

Among subjects who were physically active, an increase in IGFBP-3 was associated with a 48 percent reduction in colon cancer-specific deaths. No association was apparent for IGF-1.

For the physically inactive, there was no association between IGF-1 or IGFBP-3 and colon cancer survival.

Haydon told Reuters Health that that “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.

“We did not look at the amount of physical activity needed to reduce colorectal cancer incidence, as we only looked at those from our cohort who had CRC,” Haydon pointed out.

“Other studies that have looked at this have shown a dose-effect, meaning the more exercise the lower the risk, however our study did not try to address this issue. We were examining the effect of physical activity on one’s prognosis following a diagnosis of bowel cancer and the possible mechanisms behind this effect.”

SOURCE: Gut, May 2006.