CHICAGO -- Overweight, sedentary people should take it slow to avoid heart attacks when they finally get off the couch and start exercising.
A study found that people who are extremely inactive are more than 30 times more likely to have a heart attack during exertion than at any other time. Those who are just plain inactive are 21 times more likely.
"If you don't get much exercise and you're middle-aged with a lot of risk factors, you ought to be pretty careful about picking up that snow shovel," said Dr. Paul Thompson, one of the authors of the study in Wednesday's Journal of the American Medical Association.
The warning shouldn't be construed as an excuse to avoid exercise, the researchers said.
Out-of-shape people should begin exercise with caution and check with their doctors first, said Thompson and Dr. James Fletcher, a cardiologist with the Mayo Clinic in Jacksonville, Fla.
"Long-term, carefully done exercise is very good and prevents heart attacks," said Fletcher, who wasn't involved in the study. "The problem is a small number of people who become enthusiastic and decide to jump in there."
Fletcher said if the body is not accustomed to vigorous exercise and suddenly gets it, the circulation system becomes stressed, with blood pressure rising and the heartbeat increasing, which can lead to a heart attack.
The study of 640 patients at Hartford Hospital in Connecticut compared 64 people whose heart attacks were related to exertion with 576 whose heart attacks were not.
Of those whose heart attacks were related to exertion, 84 percent were inactive. Eighty-six percent were men and 62 percent had high levels of fat in their blood. Fifty-nine percent were smokers.