Too Much Sitting: The Population Health Science of Sedentary Behavior

Abstract and Introduction

Abstract

Even when adults meet physical activity guidelines, sitting for prolonged periods can compromise metabolic health. Television (TV) time and objective measurement studies show deleterious associations, and breaking up sedentary time is beneficial. Sitting time, TV time, and time sitting in automobiles increase premature mortality risk. Further evidence from prospective studies, intervention trials, and population-based behavioral studies is required.

Introduction

The physical, economic, and social environments in which modern humans sit or move within the contexts of their daily lives have been changing rapidly, and particularly so since the middle of the last century. These changes — in transportation, communications, workplace, and domestic entertainment technologies — have been associated with significantly reduced demands for physical activity. However, these reductions in the environmental demands for being physically active are associated with another class of health-related behaviors.

Sedentary behaviors (typically in the contexts of television (TV) viewing, computer and game console use, workplace sitting, and time spent in automobiles) have emerged as a new focus for research on physical activity and health.^{[18,27,31–33]} Put simply, the perspective that we propose is that too much sitting is distinct from too little exercise. Research findings on sedentary behavior and health have proliferated in the 10 yr after publication of our first Exercise and Sport Sciences Reviews article on this topic.^[32] As we will demonstrate, initial findings on the metabolic correlates of prolonged TV viewing time have since been confirmed by recent objective measurement studies, which also show that breaking up sedentary time can be beneficial. Furthermore, we describe recent studies from Canada, Australia, and the United States, which show prospective relationships of sedentary behaviors with premature mortality. Importantly, adults can meet public health guidelines on physical activity, but if they sit for prolonged periods, their metabolic health is compromised. This is a new and challenging area for exercise science, behavioral science, and population health research. However, many scientific questions remain to be answered before it can be concluded with a high degree of certainty that these adverse health consequences are uniquely caused by too much sitting, or if what has been observed so far can be accounted for by too little light, moderate, and/or vigorous activity.

The updated recommendation for adults on Physical Activity and Public Health from the American College of Sports Medicine and the American Heart Association (ACSM/AHA) clearly states that "the recommended amount of aerobic activity (whether of moderate or vigorous intensity) is in addition to routine activities of daily living, which are of light intensity, such as self-care, casual walking, or grocery shopping, or less than 10 min of duration such as walking to the parking lot or taking out the trash" (,^[20] p. 1426). Logically, doing such daily activities differently could involve reductions in sitting time, but sitting per se is not addressed specifically in the recommendations. In this context, the key question to be asked about the strength of the evidence on sedentary behavior and health that we present in this article is: Would one expect to see a statement on reducing sitting time included in future physical activity recommendations?

Sedentary Behavior

Sedentary behaviors (from the Latin sedere, "to sit") include sitting during commuting, in the workplace, the domestic environment, and during leisure time. Sedentary behaviors such as TV viewing, computer use, or sitting in an automobile typically are in the energy expenditure range of 1.0–1.5 METs (multiples of the basal metabolic rate).^[1] Thus, sedentary behaviors are those that involve sitting and low levels of energy expenditure. In contrast, moderate- to vigorous-intensity physical activities such as bicycling, swimming, walking, or running may be done in a variety of body positions, but require an energy expenditure of 3–8 METs.^[1] In this perspective, light-intensity activity behaviors are those done while standing that require an expenditure of no more than 2.9 METs.

Addressing research on the health consequences of sedentary behavior requires some initial clarification of terminology. We refer to sedentary behaviors (different activities for different purposes in different contexts; see previous description). We also refer to sitting time, a generic descriptor covering what these sedentary behaviors primarily involve. As we demonstrate later, adults spend most of their waking hours either sitting or in light-intensity activity (predominantly standing with some gentle ambulation).

Time in sedentary behaviors is significant, if only because it displaces time spent in higher-intensity physical activity — contributing to a reduction in overall physical activity energy expenditure. For example, displacement of 2 h·d⁻¹ of light-intensity activity (2.5 METs) by sedentary behaviors (1.5 METs) would be predicted to reduce physical activity energy expenditure by about 2 MET-h·d⁻¹ or approximately the level of expenditure associated with walking for 30 min·d⁻¹ (0.5 h * 3.5 METs = 1.75 MET-h).

Research on physical activity and health has concentrated largely on quantifying the amount of time spent in activities involving levels of energy expenditure of 3 METs or more, characterizing those with no participation at this level as "sedentary".^[33] However, this definition neglects the substantial contribution that light-intensity activities make to overall daily energy expenditure^[8] and also the potential health benefits of participating in these light-intensity activities, rather than sitting. Furthermore, although individuals can be both sedentary and physically inactive, there is also the potential for high sedentary time and physical activity to coexist (the Active Couch Potato phenomenon, which we will explain later). An example would be an office worker who jogs or bikes to and from work, but who then sits all day at a desk and spends several hours watching TV in the evening.

Common behaviors in which humans now spend so much time — TV viewing, computer use and electronic games, sitting in automobiles — involve prolonged periods of low-level metabolic energy expenditure. It is our contention that sedentary behavior is not simply the absence of moderate- to vigorous-intensity physical activities, but rather, is a unique set of behaviors with unique environmental determinants and a range of potentially unique health consequences.^[43] Our population health research perspective is on the distinct role of sedentary behavior, as it may influence obesity and other metabolic precursors of major chronic diseases (type 2 diabetes, cardiovascular disease, and breast and colon cancers).

Sedentary Behavior and Health: A Unique Underlying Biology?

Physiologically, distinct effects are observed between prolonged sedentary time and too little physical activity.^[17] There are broad consistencies between the patterns of findings from epidemiological studies on the cardiometabolic correlates of prolonged sitting that we will describe, and recent evidence on biological mechanisms ("inactivity physiology") identified in animal models. It seems likely that there is a unique physiology of sedentary time, within which biological processes that are distinct from traditionally understood exercise physiology are operating. The groundbreaking work of Hamilton and colleagues^[3,16] provides a compelling body of evidence that the chronic unbroken periods of muscular unloading associated with prolonged sedentary time may have deleterious biological consequences. Physiologically, it has been suggested that the loss of local contractile stimulation induced through sitting leads to both the suppression of skeletal muscle lipoprotein lipase (LPL) activity (which is necessary for triglyceride uptake and high-density lipoprotein (HDL) cholesterol production) and reduced glucose uptake.^[3,16] A detailed account of findings and implications from the studies of Hamilton et al.^[17,18] has been provided in recent reviews.

Findings by Hamilton et al.^[17,18] suggest that standing, which involves isometric contraction of the antigravity (postural) muscles and only low levels of energy expenditure, elicits electromyographic and skeletal muscle LPL changes. However, in the past, this form of standing would be construed as a "sedentary behavior" because of the limited amount of bodily movement and energy expenditure entailed. This highlights the need for an evolution of the definitions used for sedentary behavior research. Within this perspective, standing would not be a sedentary activity, and our approach (subject to revision as further findings accumulate) is to equate "sedentary" with "sitting."