Healthy people who exercised reduced their risk of death by 35 percent, compared with healthy people who were inactive.
People who avoid exercising for fear of triggering a heart attack may actually court early death.
According to a University of Michigan study, In fact,hile an increasing number of physicians are recommending exercise for patients with chronic health problems, many physicians are still reluctant to prescribe exercise to high-risk patients, Richardson said.
"There's always the fear in the back of a physician's head that a patient might have a heart attack while exercising and that it would be his advice that led to it," she said. "That's sort of left over from our historical belief that physical activity was not good for people with risk factors."
The study looked at a representative sample of 9,611 Americans nationwide, starting in their 50s and early 60s, and following them for eight years.
It is the first study that looks at the effects of exercise on people of all risk levels, not just those who have had heart attacks or undergone surgery to repair clogged coronary arteries.
Among those with the highest risk of heart disease, those who did exercise cut their chance of dying over the eight-year period by 45 percent compared with high-risk individuals who remained sedentary, Richardson reported in the current issue of the journal "Medicine and Science in Sports and Exercise."
Healthy people who exercised reduced their risk of death by 35 percent, compared with healthy people who were inactive, she said.
"If you focus all the physical activity and public health message on healthy people to keep them healthy, you miss an opportunity to address the bulk of mortality that's attributable to being sedentary," Richardson said.
The Michigan study confirms the findings of smaller studies showing that people who have suffered heart attacks rebound faster and live longer when they undergo cardiac rehabilitation that involves exercise, said Dr. Gerald Fletcher, spokesman for the American Heart Association.
"This study is significant in that, safely done, people with high risk for heart disease can exercise," said Fletcher, a cardiologist at the Mayo Clinic in Jacksonville, Fla.
"They need to do it properly under the guidance of some type of health professional to make sure they're doing the right thing."
There is a general misconception about the risks of exercising for people with chronic diseases, Fletcher said. Many people only remember the rare cases of someone suffering a fatal heart attack while running a marathon or engaging in other exercise, he said.
"They rationalize ways not to exercise," he said.
The death-reducing benefit of exercise was seen among those who walked, gardened, golfed, bowled or went dancing a few times a week, Richardson said. Aerobic exercises, such as running, bicycling and swimming, provided further improvement, she said.
In the Michigan study, participants were placed in one of three cardiovascular risk groups based on five factors: smoking, diabetes, high blood pressure, a history of coronary artery disease including heart attack and angina, and a history of stroke.
Those with no risk factors were considered at low risk of early death. Those with one risk factor were at moderate risk, and those with two or more factors were considered high-risk.
After eight years, 810 of the participants had died. Of the people in the high-risk group who didn't exercise, 27.8 percent had died compared with 15.1 percent of the high-risk group who engaged in moderate to vigorous exercise. In the moderate-risk group, 8.9 percent who were sedentary had died compared with 4.3 percent of the moderate risk group who exercised.
In the low-risk group, 5.4 percent of the sedentary participants had died compared with 3.6 percent who exercised.
"This really shows -- especially for people who already have chronic diseases -- that they can really benefit from physical activity," said Dr. Miriam Nelson, director of Tufts University's John Hancock Center for Physical Activities and Nutrition.
"We certainly have to take precautions and make sure that people use good sound judgment, but it's not risky for them to exercise," she said. "There are very few people for which exercise is risky."