HomeCirculationVol. 122, No. 13Letter by Ding and Mekary Regarding Article, “Television Viewing Time and Mortality: The Australian Diabetes, Obesity and Lifestyle Study (AusDiab)” Free AccessLetterPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessLetterPDF/EPUBLetter by Ding and Mekary Regarding Article, “Television Viewing Time and Mortality: The Australian Diabetes, Obesity and Lifestyle Study (AusDiab)” Eric L. Ding and Rania A. Mekary Eric L. DingEric L. Ding Search for more papers by this author and Rania A. MekaryRania A. Mekary Search for more papers by this author Originally published28 Sep 2010https://doi.org/10.1161/CIRCULATIONAHA.110.950030Circulation. 2010;122:e472To the Editor:We read with interest the article by Dunstan and colleagues,1 which described the association between television viewing time and risk of cardiovascular disease and total mortality in an Australian cohort. The authors reported an overall positive association between television viewing time and mortality. However, the analysis of television time was performed via conventional models, which do not fully account for confounding by total activity time and by the nature of the activity displaced by television watching time. Thus, there may be a certain bias and an incomplete picture of the effects of television watching.2Each 24-hour day possesses a fixed quantity of finite time. Therefore, time spent on any activity automatically displaces time spent on another activity in the limited hours of each day. Hence, the direct effects of time spent watching television not only reflect the direct effects of sedentary activity of television watching but also simultaneously depend on the effects of the alternative activity being displaced. For example, 30 minutes of television watching would exhibit substantially heterogeneous effects on cardiometabolic risk depending on whether these 30 minutes displaced another sedentary activity such as 30 minutes of book reading, a moderate activity such as 30 minutes of walking, or an even more intense activity such as 30 minutes of running.This is analogous to the isocaloric replacement analysis of dietary carbohydrate calories for fat calories while holding total energy intake constant,3 in which the effects of carbohydrates will differ depending on substitution by fat or protein. Hence, the basic partition model2,4 currently applied by the study of Dunstan et al may be potentially confounded by total activity time and other displaced activities.In summary, investigative studies of daily discretionary activity time, whether television watching or physical activity, should optimally utilize the isotemporal substitution model2 for examining their relative effects on mortality or other outcomes. All in all, however, the authors should be commended for their careful analysis and important research in this area, although we encourage the use of time substitution analysis in future studies to further improve the interpretation of television watching time and related physical activity risk factors.Eric L. Ding, ScD Department of Nutrition Harvard School of Public Health Channing Laboratory Department of Medicine Brigham and Women's Hospital and Harvard Medical School Boston, Mass Rania A. Mekary, PhD Department of Nutrition Harvard School of Public Health Boston, MassSources of FundingDr Ding was supported by a fellowship from the American Diabetes Association. Dr Mekary was supported by R25 grant CA098566 from the National Institutes of Health.DisclosuresNone.