Breast-cancer risk linked to level of fitness

Anew study with female rats found that those that were the most fit were much less likely than other animals to develop cancer after exposure to a known carcinogen, even if they did not exercise.

The findings offer tantalizing new clues into the relationship between fitness, exercise and malignancies.

A large percentage of our aerobic fitness, perhaps as much as half, according to some studies, is innate. This genetically determined fitness level varies widely from family to family and person to person. Exercise can augment it, but a person’s baseline, genetic fitness is his or hers from birth.

For the new study, which was published in July in Carcinogenesis, researchers at Colorado State University, Memorial Sloan Kettering Cancer Center in New York City and the University of Michigan opted to focus on breast cancer.

Over multiple generations, rats were tested on treadmills. Those that ran the farthest before tiring were subsequently mated with one another, while those that pooped out early likewise were paired up, until, ultimately, the pups displayed a large difference in inborn fitness. These young animals did not exercise, so their fitness depended almost exclusively on genetics.

Before the pups reached puberty, they were exposed to a chemical known to be a potent breast cancer trigger. The researchers then checked them frequently for palpable tumors throughout adulthood. They also looked, after the animals’ deaths, for signs of malignancies that had been too small to feel and microscopically examined breast cells for various markers of cell health.

The differences between the animals with high and low fitness turned out to be striking. The rats with low natural fitness were about four times as likely to develop breast cancer as the rats with high fitness were, and showed more tumors once the disease began. They also tended to contract the disease earlier and continue to develop tumors later in life compared with fit rats.