Brisk walking three hours a week can cut the risk of heart disease in women by as much as 40 percent, equivalent to the benefits of regular aerobics, jogging or other vigorous physical activity, according to a new study by Harvard University researchers.

The study, the largest of its kind, is the first to show the effectiveness of walking in reducing heart disease risk in women. Only a few studies have examined the effects of walking on the heart, and nearly all were conducted on men. The most recent--a Honolulu Heart Program study of nearly 2,700 elderly men--reported in July that men who walked less than a quarter-mile a day had twice the heart disease risk of those who walked at least 1.5 miles per day.

In the new study, women who walked at 3 mph or faster at least three hours per week achieved the same reduction in heart disease risk as those who engaged in vigorous exercise such as aerobics, jogging or bicycling.

"We've long known that walking was a great exercise for most Americans," said former U.S. surgeon general C. Everett Koop. "But this study shows the strong health benefit that it can have in protecting women against heart disease."

Heart disease afflicts nearly 59 million Americans, according to the federal Centers for Disease Control and Prevention. More than 950,000 people die of heart disease each year, making it the leading cause of death in the United States, according to the National Center for Health Statistics.

Regular, vigorous physical activity helps cut the risk of heart disease. Federal guidelines call for 30 minutes of moderate-intensity physical activity on most, if not all, days of the week. Estimates are, however, that 60 percent of Americans do not engage in regular exercise--one reason that rates of obesity and overweight have soared.

Walking is an easy, inexpensive, readily accessible activity that requires nothing more than a good pair of shoes. But experts have long questioned whether walking alone could provide a strenuous enough workout to help prevent premature heart disease.

"This study is important because it demonstrates that even moderate levels of exercise, something as simple as walking, can be equally protective against heart disease as more vigorous exercise," said Teri Manolio, director of the epidemiology biometry program at the National Heart, Lung and Blood Institute, which sponsored the study.

The study, published in today's New England Journal of Medicine, included 72,488 female nurses age 40 to 65. All were part of the larger Nurses' Health Study, a long-term research project that began in 1976, involving nearly 122,000 nurses.

Participants filled out questionnaires about their physical activity. They detailed the average amount of time spent jogging, bicycling, hiking, swimming and engaging in other common exercises, such as aerobics. They tallied the time they spent in daily activities, such as climbing stairs, and were asked to report how often, how long and at what pace they walked. Blood tests, fitness tests and other physical measures were used to verify information from the questionnaires among randomly selected participants.

Women who walked briskly at least three hours per week--about 30 minutes per day--had a 30 to 40 percent lower risk of suffering a heart attack than sedentary women, the study found. The more women walked, the lower their risk. Women who walked briskly five hours per week cut their risk of heart disease in half compared with their less active counterparts.

Even women who smoked, were overweight or had other risk factors for heart disease, such as elevated blood cholesterol levels, high blood pressure or a family history of premature heart disease, significantly reduced their risk of premature heart disease by regular, brisk walking, the study found.

"The important caveat is that the heart protection does not apply to casual strolling and window shopping at the mall," said JoAnn E. Manson, professor of medicine at Brigham and Women's Hospital in Boston and lead author of the study.

Women who walked at 3 mph or faster showed the greatest reduction in heart disease risk. The study found only moderate health benefits from a pace of 2 mph to 2.9 mph, and there was virtually no heart protection from walking more slowly than 2 mph, Manson said.

The study also underscores the heavy toll cigarettes take on the heart. Whereas smokers cut their heart disease risk with exercise, even the most physically active smokers had at least twice the coronary risk as the most sedentary nonsmokers. "It shows that it is much better to be a nonsmoker and a couch potato," said Meir Stampfer, a co-author of the study. "The number one priority is to quit smoking and then worry about physical activity."

Walking appears to cut the risk of heart disease by reducing blood pressure, blood sugar and blood cholesterol levels. Walking also improves insulin sensitivity and reduces blood clot formation. "All have favorable effects on heart disease risk," Manson said.

Combining brisk walking with other vigorous physical benefits had strong additive effects, the study found. "If you are already going to the gym every day, keep doing it," said Noel Baireymerz, chairman of the prevention committee for the American College of Cardiology. "But for all those people who say they can't exercise, this study is saying, 'Yes, you can, by doing brisk walking.' "

Nor is it ever too late to start exercising, the study found. Sedentary women who began brisk walking showed the same heart disease risk reductions as those who had been working out all along. "It just suggests that you don't need to be a marathon runner," Manson said. "If every woman were to adopt three hours a week of brisk walking, it would substantially reduce heart attack risk."

Staying Fit

One hour of brisk walking at 3 mph or more has roughly the same health benefit of:

Roller skating 45 minutes

Aerobics 40 minutes

Jogging or Basketball 30 minutes

Running, Squash or Swimming 20 minutes

SOURCE: JoAnn E. Manson, MD, Brigham and Women's Hospital, Boston