Physical inactivity: an easily modified risk factor?

The classic modifiable risk factors for the development of coronary heart disease (CHD), derived from the Framingham Heart Study and other long-term epidemiological studies, are increased blood pressure, elevated plasma cholesterol, and cigarette smoking. However, evidence has been accumulating for many years suggesting that physical inactivity or the lack of exercise is also a potent force in this field. The pioneering studies by Morris et al from England1,2 and Paffenbarger et al from the United States3,4 were the first to strongly suggest that increased physical activity, either at work or during leisure time, was a deterrent to the development of CHD. In 1953, Morris et al1 studied transportation workers in London, England, and found that sedentary bus drivers had a greater incidence of CHD than the more active conductors on double-decker buses. Later, this group showed that vigorous leisure time activity also decreased the incidence of CHD.2 In 1970, Paffenbarger et al3 examined the prevalence of CHD in San Francisco, Calif, longshoremen according to their levels of physical activity. They found that coronary death rates were lower in the middleand high-activity groups than in the low-activity group.3 Subsequently, Paffenbarger et al4 used a questionnaire to determine the activity index of a large sample of Harvard University alumni and found a progressive decline in both fatal and nonfatal coronary events with an increasing activity index up to 2000 kcal/wk. Further activity had little additional effect on the incidence of CHD. For their similar independent studies, these 2 investigators were awarded the first International Olympic Committee (IOC) Olympic Prize in 1996. Since their seminal findings, numerous epidemiological studies comparing groups of physically inactive subjects with active subjects have been published. Most of these studies have been reviewed5 and subjected to meta-analysis.6 Powell et al5 examined all studies published in the English language that contained adequate data to determine a relative risk or odds ratio for CHD at different amounts of physical activity. From this analysis, they concluded that “physical activity is inversely and causally related to the incidence of CHD.” Berlin and Colditz6 performed a meta-analysis on the studies reviewed by Powell et al5 and included results from studies published after that article. They found a summary relative risk of death from CHD of 1.9 (95% CI 1.6 to 2.2) for physically inactive compared with active occupations.