New research challenges the notion that you can be fat and fit, finding that being active can lower but not eliminate heart risks faced by heavy women. "It doesn't take away the risk entirely. Weight still matters," said Dr. Martha Gulati, a heart specialist at Northwestern Memorial Hospital.

Previous research has gone back and forth on whether exercise or weight has a greater influence on heart disease risks.

The new study involving nearly 39,000 women helps sort out the combined effects of physical activity and body mass on women's chances of developing heart disease, said Gulati, who wasn't involved in the research.

The study by Harvard-affiliated researchers appears in Monday's Archives of Internal Medicine.

Participants were women aged 54 on average who filled out a questionnaire at the study's start detailing their height, weight and amount of weekly physical activity in the past year, including walking, jogging, bicycling and swimming. They were then tracked for about 11 years. Overall 948 women developed heart disease.

Women were considered active if they followed government-recommended guidelines and got at least 30 minutes of moderate activity most days of the week, including brisk walking or jogging. Women who got less exercise than that were considered inactive.

Weight was evaluated by body mass index: A BMI between 25 and 29 is considered overweight, while obese is 30 and higher.

Compared with normal-weight active women, the risk for developing heart disease was 54 percent higher in overweight active women and 87 percent higher in obese active women. By contrast, it was 88 percent higher in overweight inactive women; and 2 1/2 times greater in obese inactive women.

About two in five U.S. women at age 50 will eventually develop heart attacks or other cardiovascular problems. Excess weight can raise those odds in many ways, including by increasing blood pressure and risks for diabetes, and by worsening cholesterol. Exercise counteracts all three.

"It is reassuring to see that physical activity really does make an impact," said lead author Dr. Amy Weinstein of Boston's Beth Israel Deaconess Medical Center. However, she added, "If you're overweight or obese, you can't really get back to that lower risk entirely with just physical activity alone."

University of South Carolina obesity expert Steven Blair, a leading proponent of the "fit and fat" theory, said the study is limited by relying on women's self-reporting their activity levels. That method is not as reliable as a more objective fitness evaluation including exercise treadmill tests, Blair said. These tests include heart-rate measures to see how the heart responds to and tolerates exercise.

In Blair's research, overweight people deemed 'fit' by treadmill tests did not face increased risks of dying from heart disease.

Dr. Laura Concannon, who specializes in treating overweight patients at Chicago's Advocate Illinois Masonic Medical Center, said the study's message that exercise can help reduce health risks isn't new, but it's important.

"Anything that can motivate the public is useful because heart disease is becoming a bigger and bigger problem as levels of obesity increase," Concannon said.