Researchers are saying that women whose exercise capacity is less than 85 percent of what is determined as normal in standard stress tests, are twice as likely as other women to die within eight years.
The researchers, using the results of 5,721 treadmill stress tests on women over 35 , determined what should be considered normal for them compared with the established fitness levels for men.
They women had no symptoms or history of heart problems.
The womens' exercise capacity was estimated in metabolic equivalents, or MET's, based on the treadmill's speed and grade.
One MET is the amount of energy or oxygen used to sit quietly for a minute, while moderate walking requires three to six MET's a minute, and running more than six.
The researchers found that a 50-year-old woman, for example, should be able to reach 8.2 MET's, while for a 50-year-old man, the predicted exercise capacity is 9.2.
The study which was led by Dr. Martha Gulati, a Chicago cardiologist, also tested the fitness equation to see how well it predicted survival during the eight years the volunteers were followed, and in another group of 4,471 women with heart-disease symptoms who were followed for five years.
In both groups, it was seen that women whose results were below 85 percent of their MET level had twice the risk of death compared with those above 85 percent.
The findings will be published in The New England Journal of Medicine.