Researchers at the Beth Israel Deaconess Medical Center, Boston, have found that exercise reduces the risk of heart disease in overweight or obese women.
However, the researchers also found that though the risk is reduced, it is not completely eliminated by higher levels of physical activity.
Both obesity and physical inactivity are modifiable risk factors for coronary heart disease, according to background information in the article.
"Obesity is recognized as a major public health issue owing to its dramatically rising prevalence and deleterious impact on many chronic diseases, including coronary heart disease," the authors wrote.
"In addition, the majority of Americans are inactive and not meeting the Surgeon General's goal for adequate physical activity," they added.
Amy R. Weinstein, M.D., M.P.H., of Beth Israel Deaconess Medical Center, Boston, and colleagues studied the interaction between these two risk factors in 38,987 women who were participants in the recently completed Women's Health Study.
At the beginning of the study in 1992, the women reported their height and weight (used to calculate their body mass index or BMI), the average time per week spent performing physical activities, other health habits and medical history. They were then followed for an average of 10.9 years and completed regular follow-up questionnaires about heart events and risk factors.
At the beginning of the study, 34 percent of women were considered physically active based on the Surgeon General's guidelines, 31 percent were overweight and 18 percent were obese. During the follow-up period, 948 women developed coronary heart disease.
Both BMI and physical activity were individually associated with the risk of heart disease. Risk was lowest for women of normal weight who were active, slightly higher for women of normal weight who were inactive, higher still for women who were overweight or obese and active, and highest for women who were overweight or obese and inactive.
Fat cells or adipocytes release chemicals that may have adverse effects on the heart by accelerating the hardening of the arteries and increasing inflammation, clotting and dysfunction of the blood vessels, the authors note. Physical activity, on the other hand, improves blood vessel function and reduces the risk for blood clots.
"We postulate that the beneficial effect of physical activity may directly reduce and combat the ill effect of the prothrombotic factors released by adipocytes," the authors wrote.
However, physical activity did not eliminate the negative effects of being overweight.
"Even high quantities of physical activity are unlikely to fully reverse the risk of coronary heart disease in overweight and obese women without concurrent weight loss," the authors wrote.
"Regardless of body weight, these data highlight the importance of counseling all women to participate in increasing amounts of regular physical activity and maintaining a healthy weight to reduce the risk of coronary heart disease," they concluded.
The study is published in the April 28 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.