Physical Activity, Clinical Medicine, and Public Health

Introduction The principal thesis of this commentary is that physical inactivity is one of the most important public health problems of the 21st century. I review some of the evidence linking inactivity to various health outcomes, briefly discuss current levels of physical activity in the United States, and make a few suggestions for a course of action to address the issue. Systematic research on the relationship of physical inactivity to health only began in the middle of the past century. Morris et al. 1 studied the relationship of job-related physical activity to coronary heart disease. Their most well-known early study compared coronary attack rates for drivers and conductors who operated the double-decker buses for the London Transport Company. The authors observed substantially lower rates of coronary heart disease in the conductors than in the drivers. They reasoned that this might be due to the increased physical activity on the job for the conductors, who repeatedly climbed the stairs to the top deck in order to collect tickets. Another early evaluation of occupational physical activity was done by Paffenbarger and Hale 2, who studied longshoremen in different job categories. Their results were consistent with those of Morris et al. 1 in that men working in jobs requiring greater energy expenditure were far less likely to develop heart disease during follow-up. Both Paffenbarger and Morris later turned their attention to physical activity during leisure time and its association with health outcomes. Morris et al. 3 studied executive-grade British Civil Servants and Lee and Paffenbarger 4 followed male graduates of Harvard College. These two studies have made substantial contributions to our understanding of the benefits of physical activity, and they confirm the results observed in occupational studies: that being regularly physically active protects against coronary heart disease and early mortality. Assessment of physical activity by job category or self-report of leisure time physical activity is a relatively crude process that leads to misclassification, which likely leads to an underestimate of the association of activity to health outcomes. A more objective measurement of exposure is possible by assessing cardiorespiratory fitness by an exercise test. Of course, there is a genetic component of fitness, as there is for anything else we can measure in human beings, but the primary determinant of fitness is a person's physical activity level during the weeks and months prior to the assessment. The Aerobics Center Longitudinal Study (ACLS) is a prospective observational study of women and men examined at the Cooper Clinic (Dallas, TX) over the past 35 years. Numerous reports from the ACLS show a strong inverse gradient for several health outcomes across categories of cardiorespiratory fitness. In these analyses low fitness is defined as the least fit 20% of participants in each age and sex group, moderate fitness is the next 40% of the distribution, and high fitness is the most fit 40%. The clinical evaluation includes extensive measurements of clinical status and medical history, which allows for good control of possible confounding variables. In these analyses, the relative risk for all-cause mortality in low-fit women and men in the ACLS is greater than 2.0 and is comparable with the risk of smoking, obesity, or having elevated cholesterol or blood pressure 5. Low fitness is not only related to mortality risk, but is also a strong predictor of loss of function with aging. Moderately fit individuals have a 50% to 60% lower risk, and high-fit individuals have a 70% lower risk of loss of function when compared with low-fit persons during 6 years of follow-up after the clinical examination 6. Prevalence of Inactivity and Low Fitness in the United States Americans are quite sedentary in their leisure time. Data from the Behavioral Risk Factor Surveillance System (BRFSS) indicate that nearly 25% of adults report no leisure time physical activity in the past month 7. Using different criteria to define inactivity, estimates from another BRFSS report show the prevalence of inactivity ranges from 10.3% in 18- to 29-year-olds to 29.7% for those 75 years or older 8. Data from another representative population survey, the National Health and Nutrition Examination Survey (NHANES), show that approximately 26% of adults from 20 to 49 years of age report no moderate or vigorous activity in the previous 30 days. The current NHANES includes a submaximal treadmill exercise test that provides information on cardiorespiratory fitness levels of adults in that age group. The prevalence of low fitness, using ACLS criteria, ranges from about 10% to as much as 30% across male/female and racial/ethnic groups 9. Thus, US population estimates vary depending on the measurement technique and survey, but the total number of sedentary and unfit individuals is as many as 50 million adults. Population-attributable Risk for Mortality The health risks associated with physical inactivity and the percent of the adult population that is inactive/low-fit can be combined to provide estimates of population-attributable risk, or the number of deaths that can be considered to be due to the hazardous exposure to a sedentary lifestyle. Population-attributable risk estimates have not been published for the entire US population, but there are estimates available from specific cohorts such as the ACLS. In these analyses, 19% of the deaths in normal-weight men can be attributed to prevalent cardiovascular disease and 10% to low fitness. The attributable risks for these two exposures are comparable for overweight men, but low fitness accounts for more deaths than prevalent cardiovascular disease in obese men: 44% and 27%, respectively. In most body mass index categories, low fitness and prevalent cardiovascular disease had higher population-attributable risk estimates, in many cases much higher, than the other exposures considered in the analyses, such as diabetes mellitus, high cholesterol, hypertension, and smoking. These data and similar data from other reports do not indicate that the other mortality predictors are unimportant or should be ignored. However, the data do suggest that low fitness, which is the result of sedentary habits, should receive more attention in clinical medicine and public health programs. Risk stratification is incomplete if health professionals only assess current chronic disease and risk factors such as lipid profile, blood pressure, and smoking. Physical activity, and ideally, cardiorespiratory fitness, also should be taken into account. Recommendations for Action Over the past decade increased attention has been given to physical inactivity as a clinical and public health problem 10. However, current efforts fall short of the coordinated and focused action required because of the magnitude of the problem. In my opinion, every primary care physician should inquire about each patient's physical activity habits and obtain data on fitness, if only from submaximal tests at a local health club. This is as important as asking about smoking habits and measuring lipids, blood pressure, and body mass index. In addition, public health authorities and agencies must focus more attention on inactivity. The United States is one of the few industrialized countries in the world that does not have a national physical activity plan, and this deficit should be rectified. There is a bright spot on the horizon. Secretary Mike Leavitt recently announced that the Department of Health and Human Services will develop comprehensive and science-based physical activity guidelines. This process will be similar to the US Dietary Guidelines activities that have been in place for many years. The physical activity guidelines will be issued in late 2008, and should give a welcome boost to the prominence of physical inactivity as a public health problem. Hopefully, the process will lead to the development of a national plan to address the issue. I hope that each of you will do your part to address the major problem of physical inactivity in your practices, communities, and personal lives.