Research on physical activity and health has pointed clearly to increasing the time that adults spend doing moderate to vigorous intensity activities: 30 minutes a day is generally recommended. However, recent evidence underlines the importance of also focusing on sedentary behaviours—the high volumes of time that adults spend sitting in their remaining “non-exercise” waking hours. We provide a brief overview of recent evidence for the distinct relationships between ‘too much sitting’ and biomarkers of metabolic health and, thus, with increased risk of type 2 diabetes, cardiovascular disease and other prevalent chronic health problems. Particular concerns for this new field include the challenges of changing sedentary behaviours in the context of ubiquitous environmental and social drivers of sitting time; examining the effects of interventions for reducing or breaking-up sitting time and identifying the most relevant implications for clinical and public health practice.

Increasing participation in leisure time physical activity (LTPA) in adult populations is a central tenet of strategies for preventing major chronic diseases (type 2 diabetes, cardiovascular disease, breast and colon cancer) and obesity in developed and developing nations.1 2 To date, clinical practice, community programmes, mass-media campaigns and population strategies have focused mainly on encouraging and supporting individuals to be more active, largely during discretionary or leisure time, but more recently (and to a lesser extent) also in travel time.2 While these approaches have met with some success, our recent body of work has identified sedentary behaviour (time spent sitting) as a novel and potentially important risk factor for the development of chronic disease. Even if people meet the current recommendation of 30 minutes of physical activity on most days each week, there may be significant adverse metabolic and health effects from prolonged sitting—the activity that dominates most people’s remaining “non-exercise” waking hours.

As population levels of overweight and obesity continue to increase, and chronic health problems from inactivity become increasingly prevalent, there is now an imperative to increase population levels of overall total daily energy expenditure. However, discretionary participation in moderate to vigorous physical activity is insufficient to raise energy expenditure as much as is necessary for population-level obesity prevention.3 Objective data derived from a recent accelerometer study,4 in which physical activity was measured objectively (as opposed to being self-reported), show that adults, on average, spend more than half their waking hours in sedentary activities (primarily prolonged sitting). The remainder of the time is spent in light intensity physical activity (LIPA; predominantly standing with some ambulation) and only about 4–5% of the day is spent in moderate to vigorous physical activity (MVPA) (fig 1).4 It is clear that if sedentary time decreases, then time spent in LIPA or MVPA will increase. While the metabolic and health consequences of actual shifts in sedentary time relative to LIPA and MVPA are currently unknown, new evidence now suggests that such challenges need to be addressed.

Compelling recent evidence emphasises the need to influence sedentary behaviour—to limit excessive sitting—and to reduce its likely health consequences. This will involve very different approaches from those needed to increase LTPA, such as exercise prescription. The behaviour of sitting time occurs in different contexts, including sitting for transport, at work, at home and in leisure time. It also requires that researchers determine the quantum of the increase in total physical activity (LIPA or MVPA) that would be required to achieve better health outcomes and prevent obesity and identify how best to go about achieving such changes.

TOO MUCH SITTING—THE HEALTH CONSEQUENCES OF SEDENTARY BEHAVIOUR

New evidence suggests that contemporary changes in transport, occupations, domestic tasks and leisure activities have had negative effects on daily energy expenditure.5^–8 Sedentary behaviour (from the Latin sedere—“to sit”) is the term now used to characterise those behaviours for which energy expenditure is low, including prolonged sitting time in transit, at work, at home and in leisure time. In this context, the metabolic equivalent (MET) is used to define body mass neutral energy expenditure of activities, as the ratio of the metabolic rate of the activity and resting metabolic rate, which is defined as 1 MET.9 Running has a MET value of at least 8 METS, moderate-pace walking has a value of about 3–4 METs and sedentary behaviours are in the range of 1–1.5 METs.10 11

A recently published prospective study showed that self-reported sitting time (as a marker of sedentary behaviour) was a predictor of weight gain in Australian women, even after adjustment for energy intake and leisure time physical activity.11 This was followed by observational studies using objective measurement of sedentary time, which showed that not only is total sedentary time important for blood glucose control but also that a larger number of breaks in sedentary time are associated with more favourable metabolic profiles.4 12 Additionally, these relationships have been found to be consistently stronger for women than for men.11 12

Recent findings also suggest that leisure time MVPA, in the context of otherwise sedentary lifestyles (fig 1),4 is unlikely to be sufficient to prevent increasing population levels of overweight, obesity and chronic disease.3

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Figure 1 Objectively measured distributions of moderate to vigorous physical activity (MVPA), light intensity physical activity (LIPA) and sedentary time during adults' waking hours.

More than two-thirds of the mid-age population in developed countries like the UK, the USA and Australia is now overweight or obese, which poses additional significant health risks for this generation.13 Also, about 6% of children are also now obese14 and this obesity is likely to track into adulthood.15 Both overweight/obesity and physical inactivity in mid-age are strong markers for the development of non-communicable disease over time.16 17 Thus, reducing sitting time may have at least as important a role as promoting physical activity in maintaining healthy weight and in preventing further weight gain and improving chronic disease outcomes in mid-age adults; it is crucial for the future health of ageing populations.

What is now needed is a broader, innovative approach to understanding and influencing sedentary behaviour in addition to increasing physical activity. This requires a paradigm shift, so that one thinks about the balance of sedentary behaviour and activity in all aspects of daily life, including transport, occupational settings, domestic work and leisure, especially around obesity management and the role of total physical activity in the prevention of further weight gain. This involves some re-orienting of the physical activity and health field from its well-established focus on LTPA to a comprehensive programme of research to understand the determinants of sedentary behaviour and both LIPA and MVPA. It will also involve studying the effects on health outcomes of shifting the balance of these behaviours towards more activity in all domains of daily life.

CLINICAL IMPLICATIONS

Given the recent recognition of this phenomenon of too much sitting, there are not yet any recommended clinical guidelines. Commonsense might suggest that it may be prudent to try to minimise prolonged sitting with 5 minute breaks every hour. However, more specific advice will require dose-response relationships between sitting and health outcomes to be defined using controlled studies. Many of the possible interventions that encourage movement may well be undertaken in settings in which prolonged periods of sedentary behaviours are the norm.

CONCLUSIONS

Research, policy and practice on physical activity and population health has focussed largely on increasing the time that adults spend doing moderate to vigorous intensity activities; 30 minutes a day is generally the target. However, recent evidence from biomarker studies and objective-measurement studies (and also from some prospective epidemiological studies) highlights the importance of focusing on the balance of light-intensity activities and sedentary behaviours—particularly the high volumes of time that adults in industrialised and developing countries spend sitting in their 15.5 “non-exercise” waking hours. A particular concern for this new research agenda is how to approach reducing or breaking-up prolonged sitting time, and how this might relate to increasing light intensity and moderate to vigorous intensity physical activities. Other research opportunities include carrying out studies on how best to promote higher volumes of overall physical activity (light intensity activities in addition to moderate to vigorous intensity activities), in the context of the ubiquitous environmental and social drivers of sitting time in occupational, transportation, recreational and domestic settings.

Particular concerns for exercise science research agenda include identifying why sedentary behaviour and the associated health relationships seem to be particularly strong for women and examining the effects of interventions for reducing or breaking-up sitting time. The issue of too much sitting has challenging implications for future healthcare practice and will require development of new kinds of clinical and public health guidelines.18