Do Our Cells Pay the Price When We Sit Too Much?

Excessive sedentary behaviors, typically characterized as sitting for prolonged periods, are an increasingly important health risk in our current age of televisions and computers. The link between television watching and obesity was first reported more than 30 years ago.¹ Since then, extensive data have confirmed this connection in children and adults. In addition, emerging evidence suggests that, independent of exercise levels, sedentary behaviors are associated with higher risk of weight-related chronic diseases—including cardiometabolic conditions and certain cancers—as well as overall mortality (reviewed in de Rezende et al.²). What remains unclear, however, is the influence of such behaviors on the function of critical physiological systems in the body. In this issue of AJPH, the report by Xue et al. (p. 1425) helps shed light on this important question.

MAGNITUDE OF THE PROBLEM

To place such knowledge in context, it is important to highlight the prevalence of sedentary behaviors in the United States. According to nationally representative data from the most recently available cycle of the National Health and Nutrition Examination Survey (NHANES), almost one third of US adults aged 20 years or older reported sitting nine or more hours a day in 2013 to 2014 (Figure 1). Television viewing in particular is the most commonly reported regular activity separate from work or sleep, and accounts for a large proportion of total sitting time. Recent Nielsen data indicate that US adults watch an average of more than five hours of television a day.³ These figures are troublingly high when compared with the Centers for Disease Control and Prevention guidelines to avoid all inactivity,⁴ and underscore the critical need to better understand the long-term biological consequences of televison watching and other types of sedentary behaviors. In recognition, this question was deemed a priority area for future research by a recent Sedentary Behavior Workshop sponsored by the National Institutes of Health.⁵

FIGURE 1—

Percentage of US Adults Aged 20 Years or Older Reporting Sitting or Reclining, by Hours Each Day, in 2013 to 2014: National Health and Nutrition Examination Survey

Note. Data were weighted to be nationally representative. Error bars indicate 95% confidence intervals that incorporate these sample weights.

RELATIONSHIP WITH TELOMERE LENGTH

To this end, the report by Xue et al. marks an important contribution to the literature. The authors investigated the relationships between sedentary behaviors, physical activities, and leukocyte telomere length in a cross-sectional study of 518 Chinese adults aged 20 to 70 years. Telomeres are DNA–protein structures that cap and protect the ends of chromosomes.⁶ Because telomeres shorten with cell division or damage, telomere length can act as a quantifiable proxy of biological aging and damage accumulated across a lifespan. Telomere length has been linked to a number of health outcomes in observational studies, and may facilitate chronic disease development.⁶ Thus, understanding the associations between sedentary behaviors, physical activities, and telomere length not only helps assess the degree to which these modifiable behaviors may affect cellular damage, but can also help uncover if altered cellular aging is a potential mechanism driving previously observed associations between these behaviors and health outcomes.

In the study by Xue et al. conducted in 518 adults from Southwest China, independent of physical activity and energy intake, each one-hour increase in television watching was associated with a 72-base-pair decrease in mean telomere length. This translated to a difference of approximately 1.2 to 1.8 years in biological age. Although this association is modest, the difference in telomere length for a three-hour increase in television watching would be roughly comparable to the difference in telomere lengths comparing smokers with nonsmokers.⁷ The authors note no associations with any of the other sedentary behavior or physical activity variables.

STRENGTHS OF THE STUDY

The study by Xue et al. has several important strengths, including the use of Southern Blot, long considered the gold standard of telomere length measurement, as well as the comprehensive assessment of sedentary behaviors, physical activities, and potential predictors of telomere length unavailable in similar studies, such as socioeconomic status and stress levels. As the authors note, levels and types of sedentary behaviors and physical activity vary widely across populations (which they suggest may account in part for the reported null associations with exercise) and it is important to study associations in different geographical regions and racial/ethnic groups.

Importantly, reported estimates were both adjusted and unadjusted for percent body fat, physical activities (or sedentary behaviors for analyses of activity), and total energy intake. Sedentary behavior may increase risk of chronic disease by altering energy expenditure or energy intake.² Here, the observed association with television watching remained after they adjusted for physical activities and energy intake, suggesting that other factors may drive the association between television watching and telomere length. For instance, television watching may be associated with unhealthy eating habits or greater exposure to unhealthy lifestyle choices (via ads), or television may displace time spent on other activities.²

REMAINING CONSIDERATIONS

Despite the strengths, the findings of Xue et al. should be interpreted with caution for several reasons. First, as the authors note, the body of literature examining sedentary behaviors and telomere length is scarce—previously comprising two cross-sectional studies with inconsistent results. Thus, large follow-up studies are needed to confirm the observed association with television watching. In particular, beyond the scope of the current report, these studies are needed to carefully tease apart the role of diet—whether as a confounder or mediator—in the relationship between television watching and telomere length.

Second, telomere length was assessed with a single measure, precluding the evaluation of telomere shortening. Although resource-intensive, a prospective study with at least two measures of telomere length would be required to test if sedentary behaviors or physical activities affect telomere shortening.

Third, in addition to television watching, the authors explored associations with many types of sedentary behaviors (e.g., computer or phone, screen-based, total) and physical activities (e.g., walking, household activities, moderate-to-vigorous activities, calisthenics or aerobics). It is possible given this exploratory design that the association with television watching was attributable to chance, although studies evaluating studies of other health conditions support the observed association with television watching.² Moreover, the authors evaluated non–television-watching sedentary behaviors and physical activities as continuous variables; associations may have been missed if they were U-shaped, as has been suggested for physical activity.⁸

Finally, the authors report a difference by age in the association between television watching and telomere length—with the association present in younger participants (statistically significant in those aged 20–40 years, and suggestive in those aged 41–55 years), but not in participants aged 56 to 70 years. Similar findings have been reported, but data examining age differences are limited and exploratory.² Future large studies should directly examine this hypothesis.

Taken together, Xue et al. contribute compelling data to the emerging body of evidence for the long-term biological consequences of television watching. Many questions remain, however, and these findings warrant follow-up. Confirming the connection between television watching and accelerated aging on a cellular level would provide further proof of principle for telomere length to serve as a proxy that could help identify and compare modifiable behaviors most likely to cause similar types of cellular damage. In addition, these data would provide the impetus for mediation studies to confirm if short telomeres are a possible mechanism that may, in part, underlie published relationships between television watching and health. This could, in turn, help identify individuals most at risk for the detrimental health effects of such behaviors. Although everyone could benefit from minimizing sedentary behaviors, such knowledge can inform more direct targeting of public health interventions aimed at minimizing such behaviors and promoting physical activity.