Prolonged sitting is hazardous to one's health. In 1700 Italy, Ramazini observed that sedentary tailors were not as healthy as active messengers (10). In the 1950s, the first epidemiological study using occupational activity to define sedentary and active behavior was conducted by Morris and colleagues (6). In this study, bus conductors who climbed approximately 600 stairs per day at work had half the number of heart attacks in comparison to bus drivers who spent 90% of their work time sitting. More recently, a wealth of evidence has arisen indicating that postural fixity (either sitting or standing) is undesirable from a health standpoint. A review article by Neville Owen, Ph.D., and colleagues (8) in this issue of Exercise and Sport Sciences Reviews reports on "Too Much Sitting: The Population-Health Science of Sedentary Behavior." Their article expands on information presented in a lecture by Owen at the 2009 American College of Sports Medicine (ACSM) Annual Meeting and helps to strengthen the link between sedentary behavior and ill health.
Their paper draws attention to several new findings:
1) Cross-sectional studies show that TV viewing is associated with obesity, diabetes, impaired glucose uptake, and insulin resistance.
2) These associations remain even after statistically adjusting for moderate-to-vigorous leisure time physical activity and waist circumference.
3) Accelerometer studies indicate that in adults, on average, 60% of waking hours are spent being sedentary (i.e., accelerometer values <100 counts per minute).
4) Individuals who meet the recommended levels of moderate-to-vigorous physical activity and spend the majority of their waking hours in sedentary activities may have compromised health, compared with those who are sufficiently active and sit less.
Owen et al. (8) conclude that sedentary behavior is distinct from a lack of moderate-to-vigorous physical activity and that too much sitting is an independent predictor of disease. While insights into the relationships between health and sedentary time can be extracted from observational and cross-sectional studies, there also is a need for researchers to design studies that, a priori, establish sedentary behavior as an exposure variable and use appropriate measures to assess this behavior.
Self-report measures to quantify sedentary behavior have been the predominant method of choice to examine associations between sedentary behavior and various health outcomes. The sedentary behavior measure often is some form of screen time (e.g., TV watching) that may not capture the complete profile of sedentary time and provides little information about the specific contexts in which this behavior is performed. Therefore, objective measures such as those described by Owen et al. are important advances in sedentary behavior assessment (8). Using a device such as the activPAL, which distinguishes between sitting and standing, may be particularly informative for studies examining specific features of sedentary and light activity. In addition, it would be beneficial to combine these objective measures with information about sedentary behavior context, allowing specific sedentary activities to be identified and targeted by interventions.
In addition to presenting evidence on the detrimental associations between sedentary behavior and health, Owen and colleagues (8) propose that future public health guidelines should target reductions in sitting time in addition to promoting moderate-to-vigorous physical activity. They note that individuals may meet the recommendations for moderate-to-vigorous physical activity and still be at increased risk of disease, if they are sedentary the rest of the day.
In considering whether there is sufficient evidence to include reductions in sitting time in future guidelines, it is prudent to consider the evolution of physical activity recommendations. In the 1970s, researchers gathered evidence of the benefits of vigorous physical activity on maximal aerobic power (V˙O2max). This led to the 1978 ACSM Position Statement that advocated vigorous physical activity at 60% to 90% of maximum heart rate (1). From 1985 to 2000, the health benefits of moderate physical activity on cardiovascular disease risk factors and all-cause and cardiovascular mortality were shown. These findings were highlighted in the U.S. Surgeon General's Report on Physical Activity and Health, which called for adults to perform at least 30 min·d−1 of moderate physical activity on most days of the week (12). Subsequently, the Food and Nutrition Board of the Institute of Medicine recommended 60 min of moderate intensity physical activity daily for effective weight management (3). In 2008, a scientific council appointed by the U.S. government reviewed the breadth and strength of evidence about physical activity to develop specific activity recommendations for populations and diseases (9). Based on this comprehensive review, the recent guidelines recommend 150 min of moderate and/or 75 min of vigorous activity per week (11). There is consensus that this level of physical activity has substantial health benefits (such as lowered risk of early death, cardiovascular disease, diabetes, dyslipidemia, and colon and breast cancers), but there is still controversy over whether it is sufficient to maintain a normal body weight (4).
The suggestion by Owen et al. (8) and Healey et al. (2) that future physical activity recommendations include the health benefits of reducing and breaking up prolonged sitting time is timely. Since 2000, evidence has accumulated linking sedentary behaviors to increased risk of disease and increases in all-cause mortality. In essence, these observational reports provide evidence for the health benefits of light intensity physical activity, in comparison to sitting. Similar to what was reported by Ramazini (10) in 1700, Morris et al. (6) in 1953, and numerous other epidemiological studies, the evidence reported by Owen et al. (8) is based on observational studies comparing people with different occupations. For instance, Australian workers who sit at least 6 h·d−1 are twice as likely to be obese as those who sit less than 45 min·d−1 (7). There is little question that sedentary behavior is becoming more ubiquitous as activity-based occupational demands have diminished rapidly over recent decades. Thus, recommendations targeting nondiscretionary time (i.e., in addition to leisure time activity) and considering light-intensity activity in addition to moderate and vigorous intensity are increasingly important. The practical and policy implications presented by Owen and colleagues (8) emphasize the need for innovative population-level approaches to reduce sitting time in our sedentary modern environment. Their recommendations reinforce the work of Levine et al. (5), who have shown the importance of light-intensity physical activity or nonexercise activity thermogenesis for achieving a healthy body weight. This work led to the creation of a treadmill-based office workstation so that office workers can perform typically sedentary office work while doing light-intensity walking.
As Owen et al. (8) acknowledge, experimental evidence quantifying the health impact of reducing sedentary time and replacing it with light activity (standing and slow ambulation) is not available. The authors present specific research opportunities that will advance the field of sedentary behavior and health. We feel it is important to draw attention to two main topics for future research. First, mechanistic studies to advance our understanding of how small increases in muscle contraction and energy expenditure stimulate positive physiological responses in humans are needed. Second, well-designed interventions to examine the health benefits of reducing sedentary behaviors should be conducted both in individuals who meet the current physical activity recommendations for moderate-to-vigorous activity and in those who do not. The results of trials targeting sedentary behavior must be compared with the benefits of moderate-to-vigorous physical activity across a range of risk factors for chronic diseases.
Owen et al. have noted that these additional studies are necessary to provide more evidence for the health benefits of frequent breaks in sedentary time (8). If the results from well-designed intervention and mechanistic studies show positive effects, then we would agree that future physical activity guidelines should include recommendations to reduce sitting time.
P. Freedson was supported by NIH grant RC1 HL099557.
David R. Bassett, Jr.
Department of Exercise, Sport, and Leisure Studies
The University of Tennessee-Knoxville
Department of Kinesiology
University of Massachusetts-Amherst
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