Trimming down cancer: fat could hinder body's fight against disease.

Fatty tissue secretes substances that make it harder for the body to battle cancer, a study in mice suggests.

Previous studies showed that obese people have excess risk of getting cancers such as those of the breast and colon. However, obesity changes many aspects of a person's overall health, so scientists aren't sure what facet of obesity is responsible for the increased cancer risk.

Several years ago, Allan Conney of Rutgers University in Piscataway, N.J., and his colleagues noticed that when lab mice were exposed to ultraviolet (UV) light and then given caffeine or encouraged to exercise regularly on a running wheel, they were less likely to get skin cancer than were UV-exposed mice that didn't receive these interventions. Since both caffeine and exercise decreased the animals' body fat, the researchers wondered whether fat itself might be the deciding factor in cancer susceptibility.

In a new experiment, Conney's team separated mice into two groups. Only one group of animals was placed in cages with exercise wheels.

After 2 weeks, all the animals were similar weights, but those in the running group had significantly more muscle and less body fat than the non-exercisers did.

After researchers exposed the animals' skin to UV light, lab tests showed that the light-damaged cells in the runners were twice as likely to die as were cells in the nonexercisers. This cell death stopped the majority of damaged cells from developing into tumors.

Working with some mice that had formed tumors, Conney's team found a similar effect: Tumor cells in exercisers were more likely to die spontaneously than were tumor cells in sedentary mice.

To make sure that these effects weren't purely due to physical activity, the researchers surgically removed a layer of fat from the bellies of some non-exercising mice, and then exposed them and other non-exercisers to UV. Twice as many UV-damaged cells and tumor cells died in the surgically lean animals as in the animals that had retained the fat. The team reports its findings online Oct. 23 and in an upcoming Proceedings of the National Academy of Sciences.

Conney and his colleagues suggest that body fat might be leaching some substance that keeps damaged and cancerous cells alive. "Fat secretes a lot of different substances--it's not an inert tissue," says Conney.

Jens Bulow, who studies obesity at Bispebjerg Hospital in Copenhagen, finds the researchers' speculation plausible. He notes that if further studies can track down the cancer-supporting substance, researchers might be able to develop drugs to block its action.

In the meantime, Bulow advises cancer patients not to try to lose weight. "It's weight loss induced by cancer that often kills these patients," he says.