Staying active has long been thought to lower the risk of getting cancer, but a new report says it may also be an important prescription for recovery.

The study, released yesterday, found that women who exercised after being diagnosed with breast cancer reduced their chance of dying from the disease by one-quarter to one-half, depending on how active they were.

"We know that physical activity has been shown to improve the quality of life for women with breast cancer," said Dr. Michelle Holmes of Brigham and Women's Hospital, in Boston. "We conclude it may also help them live longer as well as better."

People who walk and get other kinds of exercise are less likely to develop many common health problems, including heart disease, high blood pressure, osteoporosis and diabetes. Several studies also suggest exercise can prevent breast and colon cancer, and it may also help stop endometrial, kidney and esophageal cancer.

Generally, doctors recommend at least 45 minutes of moderate to vigorous exercise five times a week.

Holmes' results were based on the Nurses Health Study, which has followed the health of almost 122,000 female nurses since 1976.

During up to 16 years of follow-up, it turned out that those who got lots of exercise were most likely to survive their disease, although even a little bit clearly helped.

Most of the women walked for exercise. Those who put in one to three hours a week at a leisurely 3 mph lowered their risk of dying from breast cancer by one-quarter, compared with the most sedentary women. Those who walked between three and eight hours a week cut their risk in half.

Evidence is building that really heavy people may be able to greatly improve their odds of dodging weight-related illnesses while remaining very heavy.

The secret: lose just a few pounds. Weight reduction, it appears, is powerful medicine for the large, no matter how seemingly insignificant the dose.

Many obesity experts agree that getting down to a normal size may not be necessary to avoid much of the bad effects of being big. Dropping just 10 or 15 pounds – too little to even miss on many people – can have a surprising and substantial effect on the body processes that obesity disrupts,

If true – and the idea still has some skeptics – this means that at least a partial antidote to the apocalyptic predictions about the obesity epidemic may be within reach, even if people remain vastly overweight by every measure.

"The bad news is people are getting more and more obese," says Dr. Christie Ballantyne, a cardiologist at Methodist Hospital in Houston. "The good news is losing a modest amount of weight can have really profound health benefits."

Doctors say the best reason for getting control of weight is to be healthier.

"The goal should be to become healthy, not become a fashion model," says Dr. Judith Fradkin, diabetes endocrinology head at the National Institute of Diabetes and Digestive and Kidney Diseases. "If you move in the right direction even a little bit, that can make a big difference in health."

Just a loss of 15 pounds for someone 90 pounds too heavy can make a difference, she says.

There's little doubt among experts that obesity is a potentially deadly condition, blamed for about 300,000 U.S. deaths yearly.