Less Sitting for Preventing Type 2 Diabetes

The findings reported by Li et al. (1) in this issue of Diabetes Care are a welcome contribution to the body of evidence informing type 2 diabetes prevention. They focus on sedentary behavior (put simply, too much sitting), which has adverse health consequences that are distinct from those of too little exercise (2). These new findings, from a prospective study with long-term follow-up of 475,502 adults with 18,619 type 2 diabetes cases, represent strong evidence pointing to causality in the relationships of sedentary behavior with the development of type 2 diabetes. To this end, the study by Li et al. has strengths of method and analytical approach, employing measures of both physical activity and sedentary behavior as exposures, and examining their interrelationships as predictors of the diagnosis of type 2 diabetes over an 11-year median follow-up interval. Importantly, their analyses used iso-temporal substitution methods, which allow statistical estimation of the benefits of reallocating time in one behavior for time in another behavior. Although iso-temporal substitution has previously been used to model the putative impacts of substituting physical activity for sedentary time, such evidence has come predominantly from cross-sectional studies (3).

Sedentary behaviors—primarily television (TV) viewing and other screen use, time sitting in cars, and desk-based working—occupy the largest proportion of adults’ waking hours. Over the past 10 years, sedentary time, characterized by a sitting posture in the absence of skeletal muscle contractile activity (4,5), has increased by ∼1 h/day (6). The coronavirus disease 2019 pandemic has further exacerbated the pervasiveness of prolonged periods of time spent sitting in people’s lives (7). The widespread implementation of infection-risk mitigation strategies has seen daily time spent sitting increase by a staggering 3 h/day (8). Because sedentary behavior displaces time spent engaging in physical activity, the only countermeasure to addressing the health hazards of sedentary behavior must be some form of physical activity or movement behaviors (2).

This new evidence from Li et al. strengthens the case made by the American Diabetes Association in formally acknowledging the importance of reducing and breaking up sedentary time for diabetes prevention. The 2016 American Diabetes Association Position Statement on physical activity/exercise and diabetes (9) recommends reducing the amount of daily time spent engaging in sedentary behavior and frequently interrupting prolonged periods of sedentary time with bouts of light-intensity activity. Other authoritative bodies have since made similar recommendations. Sedentary behavior has been identified as a key element of the 2018 Physical Activity Guidelines for Americans (10), the World Health Organization 2020 guidelines on physical activity and sedentary behavior (11), Australia’s Physical Activity and Sedentary Behaviour Guidelines (12), and elsewhere. A consistent theme has been the increased emphasis given to achieving a healthier “balance” between time spent engaging in sedentary behavior and total physical activity time (see Fig. 1) (2,13,14), although the “how to” achieve a more desirable balance has received less attention (2).

Figure 1

Addressing the sedentary time/physical activity “balance” for improved health through the dual consideration of daily time spent sedentary and daily time spent engaging in physical activity (of all intensities). Balance: equivalent daily time spent sedentary and engaged in physical activity (orange/yellow color in middle), least desirable (red), and most desirable (green).

The figure illustrates the key principle that sedentary time displaces time spent engaging in physical activity: that this “balance” can be altered through spending a greater proportion of daily time sedentary, relative to time spent engaging in physical activity or, alternatively, providing strong underpinnings for good metabolic health, through reduced time spent sedentary as a result of greater amounts of time engaged in physical activity. As the findings of Li et al. highlight, reductions in sedentary time achieved through the replacement with daily-life activities and structured exercise are associated with reductions in type 2 diabetes risk, particularly among those with a lower T2D genetic risk. They underline the basic importance of addressing the “balance” between too little exercise and too much sitting through simple lifestyle-focused strategies.

This new evidence is compelling, but important research questions remain, particularly on the quantities, intensities, and patterns of physical activity that will be protective for those at risk for developing 2 diabetes, who spend more of their waking hours sitting and less time moving than those who are not at risk (15). This has particular relevance for the prevention of type 2 diabetes, with estimates suggesting that >40% of those with prediabetes are physically inactive (16).

It is pertinent to note that the exposure measures used by Li et al. are self-reported time spent engaging in three common sedentary behaviors: TV viewing time, computer use, and car use. Nowadays, the baseline measures used in epidemiological studies with physical activity and sedentary behavior as exposures include device-based assessment tools—small wearable devices (accelerometers) that provide moment-to-moment data at a fine resolution, identifying the frequency and intensity of movement (which may then be classified as light-intensity and moderate-intensity physical activity) plus nonmovement time (or sedentary behavior) (14).

TV viewing time is well established as having a strong relationship with biomarkers of diabetes risk (from multiple cross-sectional studies). From the seminal findings reported in 2001 by Hu et al. (17), and considering subsequent evidence, TV time has been a strong and consistent prospective predictor of incident type 2 diabetes. There are now good reasons to reconsider the relevance of TV viewing alone, given the proliferation of screen media such as smartphones, tablets, and streaming services (18). The findings from a recent study involving a nationally representative survey of the U.S. population from 2001 through 2016 showed that prevalence of sitting watching TV or videos for at least 2 h/day has generally remained high and stable among all ages (19), with other screen-based sedentary behaviors no doubt increasing.

Considering computer use and car use, there is recent evidence that time spent engaging in these common sedentary behaviors may have distinct and potentially less deleterious consequences than does TV time. Here, a key distinction is emerging between mentally passive versus mentally active sedentary behaviors, with TV time being the obvious instance of the former (20). Recent evidence from prospective observational studies shows clear relationships of mentally passive sedentary time with incident depression and with markers of depression risk (21,22). Moreover, a recent analysis from the 1970 British Cohort Study showed that watching TV or videos for >4 h/day at age 16 years was associated with risk of type 2 diabetes in adulthood (23). Considering the interrelationships of depression and type 2 diabetes, and the strengths of the TV time evidence, the mentally active versus mentally passive sedentary behavior distinction is a key topic for future research studies to better understand the nuances of this key domain of diabetes risk. The contributions of Li et al. in providing some new evidence relevant to the consideration of these emerging issues are welcome and appreciated.

Yet another element of these intriguing findings from Li et al. is their consideration of the potential role of genetic risk. By incorporating variables derived from genomic sequencing data, they set out to understand the confluence of genetic risk in the relationships of sedentary and physically active time in the development of type 2 diabetes. The finding that these two key behavioral influences on diabetes risk were not significant in those with higher genetic risk of diabetes is intriguing. Several questions arise, including whether it is possible that lifestyle modification is less of a predictor of T2D in the face of a genetic predisposition for insulin resistance and resulting β-cell dysfunction.

Considering this new evidence from Li et al. and the body of previous evidence, the question then arises about what might be done to make a difference to diabetes risk? Type 2 diabetes prevention is a global public health challenge. Too little physical activity and too much sedentary time are proliferating problems, driven by rapid social and environmental change. Environment and policy initiatives will thus be needed for a meaningful impact on the sedentary behavior/physical activity balance. Effective large-scale diabetes prevention programs thus cannot rely only on educational and individually focused motivational and behavioral change strategies (14). There nevertheless are important opportunities for diabetes specialists and other clinicians to address sedentary behavior, particularly through addressing sedentary behavior change as a precursor to increasing patients’ levels of physical activity.

• © 2021 by the American Diabetes Association