Physical Activity, Sedentary Behavior, and Health: Paradigm Paralysis or Paradigm Shift?

Perhaps the greatest barriers to achieving major public health advances in the 21st century will result from pandemic paradigm paralysis or the widespread inability to envision alternative or new models of thinking. One potential example of this phenomenon could turn out to be the continued focus on moderate and vigorous physical activity as the dominant health-related aspect of human movement. The current model of physical activity and health is well supported by over 60 years of scientific inquiry, and the beneficial effects of moderate-to-vigorous physical activity have been more clearly defined in recent years (1–4). However, if we are complacent with the existing paradigm—that increasing levels of moderate and vigorous levels of physical activity will result in the greatest improvements in public health— then we may not obtain the full return on investment with respect to improving quality of life and life expectancy through patterns of human movement. Emerging evidence for the role of sedentary behavior on health, which may be independent of physical activity per se, finds us at a crossroad with respect to prescribing optimal daily human movement patterns for health. Human movement represents a complex behavior that is influenced by personal motivation, health and mobility issues, genetic factors, and the social and physical environments in which people live. These factors undoubtedly exert an influence on the propensity to engage in sedentary behaviors as well as in physical activity. However, the biological, social, and environmental pathways leading to sedentary behavior versus physical activity may be different. Further, the health effects associated with sedentary behavior and physical activity may be the result of different biological mechanisms (5). Humans are designed for movement. Energy balance has been a central selective force throughout human evolutionary history, and humans have evolved to have high levels of energy expenditure, even more so than modern nonhuman primates (6). Obtaining dietary energy and nutrients from the environment traditionally required an expenditure of energy through human movement. Factors related to the expansion of the African grasslands between 2.5 and 1.5 million years ago and the emergence of Homo were major contributors to changes in both brain size and foraging behaviors (6,7). Early Homo (H. habilis and H. erectus) appeared at a time of rapid brain evolution with early Homo having an average brain size of 600–900 cc compared with earlier australopithecines with an average brain size of 400–500 cc (7). The larger brain size of Homo required higher quality diets, which necessitated larger foraging ranges, resulting in greater total energy expenditure. At the same time, the transition from a forest to savanna environment caused changes in resource distribution that would have also resulted in increases in foraging ranges and total energy expenditure (6). Much of human evolution has occurred as hunter-gatherers (3–4 million years), while recent advances in agriculture and technology have occurred over a short time frame ( 10,000 years). Eaton and Eaton (8) have estimated that Stone Age humans had an energy efficiency ratio of 2.25 (i.e., expending 1 kJ of energy to acquire 2.25 kJ of dietary energy) compared with an efficiency ratio of 3.66 for modern humans, which represents more than a 50% increase in efficiency. Modern humans in the Western world have relatively low levels of physical activity compared with contemporary hunter-gatherers. Hayes et al. (9) reported that the total energy expenditure/resting energy expenditure or Physical Activity Level (PAL) among subsistence-level human populations approximates 3.2, while among representative humans living in contemporary society, the PAL is 1.67. The impact of the transition from a semisubsistent existence to a Western lifestyle on physical fitness levels are exemplified by work in an Inuit community (Igloolik, northern Canada) (10,11). Studies in the population from 1970 through 1990 demonstrated marked reductions in average aerobic fitness (ml kg 1 min ) over time in all age-groups (10,11). Recent work among Old Order Amish living a traditional agricultural lifestyle indicates that this population engages in more daily movement than contemporary Americans. The average number of steps per day taken by Amish men and women were 18,425 steps per day and 14,196 steps per day, respectively (12). These values are considerably higher than recent estimates for contemporary U.S. adults (13,14) (Fig. 1). The weighted evidence indicates that humans evolved in environments that required higher levels of human movement than are required today. By becoming more efficient at extracting energy from the environment, there is now a lower level of expenditure required to subsist. Some studies have documented lower levels of physical activity among contemporary humans compared with those living in more primitive societies. A negative consequence to the observed improvements in energetic efficiency is the proliferation of health concerns that are related to low levels of physical activity and/or high levels of sedentary behavior. Physical activity and health. The modern field of physical activity epidemiology arguably began with the studies From the Pennington Biomedical Research Center, Louisiana State University System, Baton Rouge, Louisiana. Corresponding author: Peter T. Katzmarzyk, [email protected]. Received 14 June 2010 and accepted 19 July 2010. DOI: 10.2337/db10-0822 © 2010 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by -nc-nd/3.0/ for details. See accompanying articles, pp. 2715, 2732, and 2790. PERSPECTIVES IN DIABETES