Cancer Rx: Move?

· Reduces levels of circulating estrogen and testosterone, two hormones linked to breast, endometrial and prostate cancers. Even with medication to suppress estrogen production, some estrogen is stored in fat cells. Exercise may help by converting fat to muscle.

· Prevents weight gain and promotes weight loss, important because obesity is associated with lower rates of survival for many forms of cancer. For women with breast cancer, obesity at the time of diagnosis and weight gain afterwards are associated with worse outcomes. The heavier and less active a person is, the more likely her cancer will return.

Most of the scientific work so far has focused on women with breast cancer. But studies have shown exercise also has positive effects for survivors of colorectal and prostate cancers. Among men older than 65, three hours of vigorous activity a week was associated with a decline in death from prostate cancer.

Exercise is now considered so beneficial that cancer experts are even encouraging patients to begin or resume exercise while treatment is underway.

Workouts might need to be scaled back in intensity and pace, but "evidence strongly suggests that exercise is not only safe and feasible during cancer treatment, but that it can also improve physical functioning and some aspects of quality of life," according to the American Cancer Society.

Lockey Maissoneuve, a 41-year-old personal trainer, went through two mastectomies and chemotherapy two years ago. She is now training for a triathlon.

"If you're in treatment, the first week or two you try to do anything, you need to take a nap," she said. "If there's a day you want to exercise, do it."

Wearing a wig was uncomfortably hot in the gym, so she switched to a kerchief. With her immunity reduced by chemo, she wiped down the equipment before she used it. She is now certified to work with cancer patients.

"The trainer is almost like your bodyguard," said Julie Percy of Parisi Sports Club in Midland Park, N.J., who also specializes in work with cancer patients. "We maneuver you to the right equipment, give you a sense of security."

When scar tissue forms after surgery, it limits flexibility. Percy helps women who have had mastectomies and underarm incisions restore their range of motion.

Trainers have to be particularly attentive when someone has had surgery to remove lymph nodes.

If the tiny valves in the vessels that transport lymph around the body fail, that can lead to lymphedema, a dreaded side effect of cancer surgery. The arm, for breast patients, or the leg, for prostate patients, becomes permanently swollen.

"We watch the amount of weights they use," Percy said.

She starts light and increases gradually. Women who have lymphedema, or a heightened risk of it, wear a compression sleeve.