Getting regular exercise may lower the risk of skin and bowel cancers, two new studies show.
The studies are published in the May 13 online edition of the journal Carcinogenesis. Both teams of researchers studied mice, not people. It’s not yet known if the results apply to humans.
One study focused on skin tumors. The other concentrated on precancerous intestinal polyps.
Both experiments showed that tumors and polyps were rarer, slower to appear, and smaller in size in mice that voluntarily exercised on running wheels, compared with those without access to running wheels.
Here’s a closer look at the findings and what clues they may offer about exercise and cancer.
Skin Cancer Study
The skin cancer study was conducted by scientists including Laura Michna of the Joint Graduate Program in Toxicology at Rutgers, the State University of New Jersey, and the University of Medicine and Dentistry of New Jersey-Robert Wood Johnson Medical School.
Few other studies have tracked skin cancer and exercise, note Michna and colleagues.
Michna’s team studied hairless female mice exposed to various doses of ultraviolet B (UVB) and ultraviolet A (UVA) light. The light came from lamps that give off ultraviolet light.
Ten mice shared each cage. For 14 weeks, the researchers kept one group of mice in cages with access to running wheels. They kept the other mice in cages without access to running wheels.
Both groups of mice ultimately developed skin tumors from UV light exposure. On average, mice with no access to running wheels developed tumors in 3.5 weeks, compared with seven weeks for the mice with access to running wheels. The total number of tumors was 32% less in the exercise group of mice.
Throughout the study, the mice with no access to running wheels had tumors that were larger in volume than those with running-wheel access.
The study didn’t pinpoint how far each mouse ran on the running wheels. Up to four mice could run on the wheel together, the researchers note.
Bowel Cancer Study
The researchers who worked on the bowel cancer study included Lisa Colbert, PhD, MPH, of the University of Wisconsin’s Comprehensive Cancer Center.
Observational studies in large groups of people have linked lower colorectal cancer risk to high levels of physical activity. But it’s not clear how that process works, or if exercisers have other factors working in their favor against cancer.
Colbert and colleagues studied 47 male mice that had a genetic tendency to develop intestinal polyps that can turn into cancer. For 10 weeks, half of the mice had access to running wheels in their cages. The other mice had no access to running wheels.
The mice were studied for 17 weeks. During that time:
Six mice died before the end of the study due to complications from polyps; all of them had no access to running wheels. The mice with access to running wheels had fewer, smaller polyps than those without access to running wheels.
The mice with access to running wheels ran about 2.4 miles (3.8 kilometers) per day, on average. They weren’t forced to run.
“In summary, 10 weeks of voluntary wheel running exercise increased survival and decreased polyp number by 25 percent in [the] mice compared with mice without wheel access,” the researchers write.
Why the Difference?
The studies don’t show exactly why cancers were rarer, slower to appear, and smaller in mice that were allowed to exercise.
In Michna’s study on skin cancer, the mice with running-wheel access lost body fat and gained muscle, compared with the mice with no access to running wheels.
“How decreased body fat or increased exercise may inhibit carcinogenesis is poorly understood,” Michna and colleagues write. “Carcinogenesis” means the development of cancer.
Michna’s team suggests that possibly exercise helps trigger the death of skin cancer cells.
In Colbert’s study on bowel cancer, the mice with running-wheel access actually ended up with more fat than the nonexercising mice. That finding might be because the nonexercising mice were already wasting away because of their disease, the researchers note.
Colbert and colleagues point out that they cut the calories of the exercising mice in their study so that those mice burned more calories than they consumed. It’s not clear if that calorie gap influenced the results or if exercise was what mattered most.
By Miranda Hitti, reviewed by Louise Chang, MD
SOURCES: Michna, L. Carcinogenesis, May 13, 2006; online edition. Colbert, L. Carcinogenesis, May 13, 2006; online edition. News release, Oxford University Press.