Physical fitness and activity as separate heart disease risk factors: a meta-analysis

Abstract

Recognition of physical inactivity as a major public health problem was an important recent milestone in exercise science. The Centers for Disease Control and Prevention/American College of Sports Medicine (7), National Institutes of Health (4), U.S. Surgeon General (8), and American Heart Association (3) are among leading organizations that have published reports on physical activity and public health. A consensus recommendation derived from these reports is that all adults should accumulate at least 30 minutes of moderate intensity physical activity each day. An additional feature of these reports is that the recommendations are based on a presumed curvilinear, inverse dose-response relation between physical activity and health outcomes. The shape of this dose-response curve indicates that a given dose of physical activity will have relatively greater health benefits at the lower end than at the higher end of the physical activity spectrum. Dr. Williams disagrees with this conclusion and states that “This presumption is based largely on studies of cardiorespiratory fitness.” We have been involved in the development of the recent reports on physical activity and health cited above, and it is not true that the recommendations are “based largely on studies of cardiorespiratory fitness.” The various committees and groups making the recommendations considered evidence from pathophysiological investigations, randomized clinical trials, and prospective epidemiological studies of both physical activity and cardiorespiratory fitness in their reviews and deliberations. However, we certainly agree that studies on cardiorespiratory fitness and health outcomes were considered in making the recommendations. Dr. Williams’ fundamental approach to evaluate the dose-response relation between activity and health is to compare data from separate meta analyses of recent epidemiological studies on physical activity or cardiorespiratory fitness and the outcomes of coronary heart disease (CHD) or cardiovascular disease (CVD). We agree that it is important to investigate more thoroughly the shape of the dose-response association between physical activity and health outcomes, and compliment Dr. Williams for addressing this important and difficult issue. There are a few additional issues that should be considered when interpreting the results of the meta-analyses. One of the main points emphasized by Dr. Williams is that his Figures 2–4 illustrate that there is a precipitous drop in risk beyond the 20th or 25th percentile for the fitness studies and that there is a linear decline in risk for physical activity. It is important to remember that the data points plotted in these figures result from simulations and estimates from data published in the respective reports. There were widely differing methods used in the various studies to assess either fitness or activity; however, fitness tests yield ratio data and activity assessments generally provide categorical data. Dr. Williams refers to this issue in his discussion on how the samples “were partitioned into intervals.” He discounts this problem by assuming that the physical activity referent categories were defined broadly because of no trend in the interval. We question this assumption because for the studies that used categorical data it is impossible to evaluate any trend within an interval. In fact, even studies with presumed ratio scale data have limitations. For example, suppose that the physical activity measure is miles walked per day. The referent category might be <1 mile per day; however, this does not mean that all individuals in that category did no physical activity. It is unlikely that all of these individuals were confined to bed; therefore, there could be considerable variation in actual daily energy expenditure that would be totally missed because of limitations in the physical activity questionnaire. Depending on the activity questionnaire, it is possible to have a sizable proportion of the study population classified as sedentary yet still have much misclassification because of limitations in the questionnaire used. We are not aware of any research that defines a questionnaire’s sensitivity and specificity to classify participants correctly as active or sedentary. These limitations generally do not hold for cardiorespiratory fitness assessments. Cardiorespiratory fitness test data are typically expressed as V̇O2max or maximal METs attained, which are ratio scales. The reference category for these measures is not interpreted as an absence of fitness, and there is a large volume of data from cardiovascular fitness tests that is available to interpret reference categories. Few investigators present detailed analyses on dose-response issues. This may be because the principal focus of earlier studies has been on determining whether there was any significant association of activity or fitness to health rather than attempting to characterize the shape of a dose-respons