Physical Activity and Mortality: The Potential Impact of Sitting

Over the past few years, numerous studies of sedentary behavior (sitting) have documented significant associations with chronic disease and mortality outcomes (6). These results have significant implications for workplace health and associated policies, because long-term trends in occupational data suggest that sedentary occupations have increased significantly in the United States between 1960 and 2008 (1). Self-reported levels of sitting in the United States average approximately 4.7 h·d (3); however, this estimate is likely conservative because individuals tend to underreport their sitting behavior. For example, a study among Australian adults using an objective assessment (inclinometer) reported an average of 8.8 h of sitting per day (4). Furthermore, objective data collected by accelerometry in the U.S. National Health and Nutrition Examination Survey indicate that children and adults spend approximately 7.7 h·d being sedentary (5). Thus, a large proportion of the U.S. population is exposed to excessive levels of sedentary behavior and is at increased health risk because of this exposure. A topic of interest that has been addressed in this arena is whether the observed associations of sedentary behavior and increased chronic disease and mortality risk are modified or moderated by physical activity (light, moderate, vigorous, or total). Studies typically statistically “adjust” for a measure of physical activity by including it in a statistical model as a covariate or by stratifying the analysis by level of physical activity (i.e., active vs inactive). Using this approach, Ekelund et al. (2) recently reported the results of a meta-analysis of data from 1,005,791 participants from 13 studies who were followed for all-cause mortality between 2 and 18 yr. A total of 8.4% of the participants died during follow-up, and the hazardous