We read with interest the paper by Maher and colleagues [1] entitled “Reconsidering the Sedentary Behavior Paradigm” that investigated whether the adverse health effects of sedentary behavior are independent of physical activity, or whether they simply arise from displacement of physical activity. The authors showed that after adjustment for total physical activity the association between sedentary behavior and 9 of 11 biomarkers was eliminated and concluded that the failure of previous studies to account for the full spectrum of physical activity has inadvertently led to attribution of the negative health effects to sedentary behavior, rather than positive effects of physical activity. The title of the paper suggests the authors believe their findings indicate the sedentary behavior paradigm should be reconsidered.

We appreciate the author's contribution to the literature and the discussion this paper has generated. However, we believe this report to be emblematic of a common misunderstanding of "sedentary behavior paradigm" [1,2]. The paradigm has three main features: 1) Displacement: sedentary behavior reduces total physical activity leading to increased disease risk; 2) Mechanisms: “some” of the biological mechanisms linking sedentary behavior and exercise (not total physical activity!) to health may be different from one another, and 3) Determinants: the constellation of personal and environmental factors that induce us to exercise are distinct from those which induce us to be sedentary [2, 3]. For simplicity we define sedentary behavior to be sitting/reclining/lying during the waking day and use the terms “exercise” and moderate-vigorous activity interchangeably.

Maher and colleagues conclusion that “sedentary behavior may not have health effects independent of physical activity” is entirely consistent with the displacement hypothesis (on this we seem to agree), but their suggestion that the sedentary behavior paradigm should be reconsidered appears to arise from the belief that the relevance of the paradigm rests entirely on whether the biological mechanisms are distinct. In our experience this is a common misinterpretation of the paradigm and this confusion diminishes the importance of the features of the paradigm that are likely to have the most public health impact (Displacement and Determinants). Maher and colleagues results clearly demonstrate the powerful impact of the tradeoff between sedentary behavior and total physical activity on the cardio-metabolic biomarkers examined, which is expected given the reciprocal relationship between sedentary behavior and total physical activity [4].

From a public health perspective, the question is what to do? The sedentary behavior paradigm, in addition to recommending regular exercise, simply proposes that we take advantage of the displacement phenomenon and leverage the health benefits of replacing sedentary behavior with physical activity. To seize this opportunity we need to understand what types and patterns of physical activity are healthful alternatives to sitting, and how best to encourage people to reduce their sedentary time. The etiologic evidence base is rapidly growing, for example, a provocative new mortality study has suggested that "standing may be a healthier alternative to excessive periods of sitting" [5]. Thus, we urge caution when inferring public health implications based on a single cross-sectional study. Etiologic studies alone cannot identify the "best message" for intervening on sedentary time, but they provide essential insights into the types of physical activity behavior that can reduce risk. To understand how best to change sedentary behavior we need to identify personal, social, and environmental factors that determine the balance between time in sedentary behavior and physical activity, and interesting new studies have begun this critical work [6,7]. To move the field forward, we fully agree with Maher and colleagues [1] that well-designed prospective and intervention studies are needed to inform the development of effective public health messages designed to increase overall physical activity and improve population health.


Authors: Sarah Kozey Keadle and Charles E. Matthews
Affiliations: Cancer Prevention Fellowship Program, Division of Cancer Prevention, National Cancer Institute (SKK)
Nutritional Epidemiology Branch, Division of Cancer Epidemiology & Genetics, National Cancer Institute (SKK, CEM)