Innate Fitness May Affect Cancer Risk

It's been known for several years now that being physically fit reduces a woman's risk of breast cancer. But how? Is there something about actively engaging in an exercise regimen that reduces risk, or is it just a function of being naturally fit (called "innate" fitness)? Or both?

To try to address this question, researchers took advantage of groups of rats that had been bred to be either genetically "fit" or genetically "not fit". The two populations were created over many generations by exercising rats on a treadmill; those that could run long distances were bred to other rats that also ran long distances; those that were poor exercisers were bred to other poor exercisers. Over time, the groups diverged in terms of innate fitness.

In recent experiments, then, female offspring of the "fit" and "not fit" groups were exposed to a known carcinogen. None of the rats in either group engaged in exercise after birth, so any differences between the two groups must be attributable to innate fitness, rather than exercise per se. The results were strikingly different - the incidence of breast cancer was reduced by more than 70% in the "fit" rats, compared to the "not fit" rats.

It's not known yet whether an active exercise regimen would reduce the risk of breast cancer in the "not fit" rats. Undoubtedly that will be tested next. But for now, the new information is that there's something about the metabolism of innately fit rats (and perhaps humans) that protects against breast cancer, even if they're not routinely exercising.

The researchers looked only at breast cancer as their measured endpoint in these experiments, but perhaps fitness affects the risks other cancers as well.