Exercise capacity is a well-established predictor of cardiac mortality in women, but now a new study indicates how much exercise is enough.
In a report that compares the capacities of thousands of women with cardiovascular symptoms and without them, researchers have determined the degree of exercise required to assure cardiac health.
The paper appears in the Aug. 4 issue of The New England Journal of Medicine.
The 5,271 women without symptoms were recruited in the Chicago metropolitan area to participate in a study of heart disease in women.
The 4,471 women with symptoms had been referred from 1990 to 1995 for exercise stress tests for the evaluation of suspected coronary disease.
By following these women over more than a decade, the researchers were able to predict what effect the right amount of exercise would have. Age-predicted exercise capacity was established by averaging the results of the stress tests in asymptomatic women.
The study found that women who scored less than 85 percent of their age-predicted exercise capacity had double the risk of dying of any cause, and more than two and a half times the risk of dying of cardiac disease.
"Previous studies never told us the prognostic implications of not achieving your age-predicted exercise level," said Dr. Martha Gulati, the lead author. "This study is based on women we've followed since 1992. We know who's dead and who's alive, and we've found that if you achieve under 85 percent of your age-predicted exercise level, you're considerably more likely to die from cardiac disease."
Exercise capacity is measured in MET's (pronounced mets), or metabolic equivalents that indicate how much oxygen the body is consuming. (One MET is 3.5 millileters of oxygen per kilogram of body weight per minute.)
As the activity becomes more physically demanding, the number increases: just sitting still and breathing uses one MET; carrying golf clubs while walking around the course uses five. For the study, participants were tested on sophisticated hospital machines, but a hospital stress test is neither necessary nor recommended for people who are asymptomatic, even though it might provide useful information.
In any case, Dr. Gulati said, there is no need to get a stress test to determine the ideal level of exercise. "It's easy for people to see how hard they are working out," she said.
"On almost all modern gym machines," she added, "there is a MET's indicator, even though most people probably don't use the information it provides."
This is the first time exercise guidelines have been established for women taking their age into account, Dr. Gulati said, adding: "That's important because we've never known about women. Everything has been done with men, and the guidelines are very different."
The study has enabled researchers to create a chart, called a nomogram, that indicates the predicted exercise capacity in MET's for any age. Such nomograms are routinely used in clinical practice for men, but this is the first ever established for women.
"It's easy to use, and women should be using it to guide how hard they are working out," Dr. Gulati said. For a woman of 60, seven MET's is 100 percent of the predicted exercise capacity. But for a woman of 30, it is only 62 percent of capacity - not enough to lower her risk for cardiac illness.
The authors concede that their nomogram was created from data on asymptomatic, mostly white women, and that the comparison group was significantly more racially diverse, with a stronger representation of black women. A nomogram derived from a more racially diverse group might produce different recommendations.
The amount of time spent exercising is important, of course. But Dr. Gulati said, "If you achieve the maximum, even for a short duration, knowing that your heart can sustain it is very good news."