"There is currently a controversy over the amount of exercise you need," says study leader John M. Jakicic, director of the University of Pittsburgh Physical Activity and Weight Management Research Center. "Some people say this number, some people say that number. This is the first big clinical trial to address the issue."

So here are the numbers, as reported in the Sept. 10 issue of the Journal of the American Medical Association (news - web sites): "If you can get people to exercise 30 minutes a day and reduce calorie intake by 20 percent, you get a reasonable reduction in weight," Jakicic says. "If you increase that to 60 minutes a day, you dramatically increase weight loss."

And while the effects of weight reduction on the risk of conditions such as heart disease and diabetes are obvious, another report in the journal adds a less well-known benefit: a lessened risk of breast cancer (news - web sites).

For those women who are not exercise-prone, the good news from the Pittsburgh study is that a brisk 30-minute walk every day fills the bill, Jakicic says: "While 60 minutes gives better results, 30 minutes is pretty good."

But here's the catch: Exercise alone doesn't do it. "If you do exercise without a change in eating habits, you won't see the benefit," Jakicic says.

But there is something of a bright tinge to that finding, he adds. It's the amount, not the kind, of food you eat that matters: "It's really about the calories. Anything you do to reduce calories is good."

Those numbers come from a two-year study of 201 non-exercising, chubby women, average age 37, average body mass index of 32.7, which translates to a body weight of 192 pounds for someone 5-foot-4.

All the women were told to reduce their food intake, to between 1,200 and 1,500 calories a day, with fatty foods only 20 percent to 30 percent of the diet. Then they had one of four exercise regimens prescribed, from vigorous intensity/high duration to moderate intensity/moderate intensity, which translates to a brisk 30-minute walk every day. After a year, the average weight loss for those who worked hardest was 19.6 pounds, and for those who did the minimum was 13.9 pounds.

Jakicic looks at that last number as encouraging. "It is realistic to get overweight, sedentary women to do 30 minutes of exercise a day," he says. On the eating front, the recipe is simple: "Put what you normally put on your plate, and then reduce that by 20 percent."

The benefit of exercise in reduced risk of breast cancer is outlined in a study by Dr. Anne McTiernan and colleagues at the Fred Hutchinson Cancer Research Center in Seattle. It is a big study, looking at exercise patterns of more than 74,000 women.

Again, a good benchmark was a daily brisk walk, adding up to between 75 and 150 minutes a week. Women who walked that much lowered their breast cancer risk by 18 percent, compared to women who did no exercise. More vigorous exertion, up to 10 hours of walking a week, reduced the risk slightly more. Starting exercise early in life gave the most benefit, but women of all ages benefited -- and the greatest benefits were found in the thinnest women.

This is not startling news, says Dr. Rachel Ballard-Barbash, associate director of the National Cancer Institute (news - web sites) applied research program, because a review published last year found more than two dozen studies showing "convincing evidence that physical activity protects against breast cancer."

But one reason why this new study is important is that it provides evidence that exercise at any age is good, she says: "Many women between the ages of 50 and 79 saw benefits."

The mechanisms of protection appear clear, says Dr. I-Min Lee, an associate professor of medicine at Harvard Medical School (news - web sites) who wrote an accompanying editorial.

"Weight reduction favorably alters estrogen levels in the body," Lee says. "It also affects insulin and growth factor levels."

Eating and exercising are inextricably intertwined, she adds: "If you are willing to cut your food intake a bit more, you may not have to exercise more."

More information

An overview of obesity, its prevalence, problems and prevention, is available from the Centers for Disease Control and Prevention. You can learn to eat well from the American Dietetic Association.