HARVARD GAZETTE ARCHIVES

Christine Albert (left) and William Whang followed the exertions of 85,000 women for 24 years and came to the welcome conclusion that heart attack deaths from moderate and severe exercise are extremely rare. (Staff photos Kris Snibbe/Harvard News Office)

Exercise improves your health, but can you kill yourself with too much snow shoveling, yard work, jogging, or playing tennis?

JoAnn Manson: 'Regular exercise is truly a 'magic bullet' for good health.'

"Despite all of the known benefits of exercise, there are also well-documented associations between acute episodes of exertion and sudden cardiac death," notes Christine Albert, an assistant professor of medicine at Harvard Medical School. "Although relatively rare, these deaths commonly occur in an unexpected fashion among those who appear quite healthy."

In the largest study ever done to get a better handle on this question, Albert and her colleagues followed the exertions of almost 85,000 women for 24 years, while keeping track of their hearts. The women, selected from an ongoing study of registered nurses known as the Nurses Health Study, were between 34 and 59 years old in 1986. From then until 2004, the women filled out questionnaires about how much time they spent jogging, running, bicycling, swimming, playing tennis or squash, and undertaking other activities that require moderate to vigorous exertion.

"To our knowledge, this analysis is the first to assess both the transient and long-term risk of sudden cardiac death associated with physical activity among women," says Albert, senior author of the study and also director of the Center for Arrhythmia Prevention at the Harvard-affiliated Brigham and Women's Hospital in Boston. Results of the study appear in the March 22/29 Journal of the American Medical Association's theme issue on Women's Health.

The findings are encouraging. Out of almost 85,000 women, only nine died while doing yard work, housework, swimming, or physical therapy. To put this in numbers, as scientists always like to do: Their investigation covered 1.93 million person years of exercise and recorded only one death for each 36.5 million hours of exertion. In other words: Sudden cardiac death during exertion is an extremely rare event in women.

And there's still more good news. Regular exercise may significantly minimize this small risk, in both the short and long term. "The women who reported exercising four or more hours a week had a 59 percent lower risk of sudden cardiac death over 18 years of follow-up compared to women who reported not exercising at all," notes William Whang, lead author of the study and a fellow at Massachusetts General Hospital. "Also, women who exercised two or more hours a week had no elevated risk of sudden cardiac death during exertion. This risk appeared to be primarily limited to inactive women."

"It was not surprising that the absolute risk of sudden cardiac death with moderate-to-vigorous exercise was extremely low in women," comments JoAnn Manson, a professor of women's health at Harvard who participated in the research. "However, it was surprising that regular exercise had such a powerful role in reducing the long-term risk of sudden death. Regular exercise is truly a 'magic bullet' for good health."

Men at higher risk

Women in this study were more than 34 years old; would the results also apply to younger women? "Most likely," says Manson, who is also head of the Division of Preventive Medicine at Brigham and Women's Hospital. "The biological relationships are likely to hold for younger women as well. However, younger women have exceedingly low risks of sudden cardiac death.

"It's likely that the key factor for all women, regardless of age, would be their habitual exercise level," she continues. "Women who are regularly active would be able to tolerate vigorous exercise much better than women who are habitually sedentary, whatever their age."

No matter how high the risk rises, however, it will probably be less than for men who exercise vigorously. This is likely related to the heart protection women enjoy from their natural hormones during the years before menopause, Manson believes. "After menopause women gradually catch up with men but the gender gap for heart and blood vessel diseases can be substantial," she says.

An earlier analysis of male physicians done by Albert, Manson and colleagues found that men's risk of sudden cardiac death during vigorous exertion was 19 times higher that that of women. However, like women in the study, the most active men enjoyed the lowest risk of sudden death during exertion.

Part of the difference, Albert notes, could be due to including both moderate and intense exercises in the women's study but only vigorous exertion in the male analysis. But that's not the whole story. Similar sex differences show up in smaller studies of men in both the United States and Finland. In these latter studies, the same measures of exercise showed a nine- to 14-fold higher risk for men. To date, this apparent sex difference remains unexplained.

The study of males at Harvard, known as the Health Professional Follow-up, revealed another benefit of regular exercise. "Physical activity is strongly associated with better erectile functioning," reported Eric Rimm of the Harvard School of Public Health in 2000. In an analysis, Rimm and colleagues concluded that male health professionals, ages 51 to 87, who exercised vigorously for 20 to 30 minutes a day, were half as likely to have erectile dysfunction as men with the lowest level of physical activity. (Additionally, the researchers concluded that as a man's waist size increased, so did his chances of erectile dysfunction.)

The final conclusions of the women's study probably apply to men: Moderate and vigorous exercise can be done safely and, if performed regularly, may lower the long-term risk of sudden cardiac death.