In an analysis of data from several studies, watching television for 2-3 hours per day or more was associated with a higher risk of type 2 diabetes¹, fatal and nonfatal cardiovascular disease（心血管疾病） and all-cause death, according to a study in the June 15 issue of JAMA. Television (TV) viewing is the most commonly reported daily activity apart from working and sleeping in many populations around the world. In the United States, the average number of daily hours of TV viewing has recently been reported to be 5 hours. "Beyond altering energy expenditure²（支出，花费） by displacing time spent on physical activities, TV viewing is associated with unhealthy eating (e.g., higher intake³ of fried foods, processed meat, and sugar-sweetened beverages⁴ and lower intake of fruits, vegetables, and whole grains) in both children and adults," according to background information in the article. "Physical inactivity, various dietary factors, and smoking are well-established independent risk factors of type 2 diabetes, cardiovascular disease, and all-cause mortality. Because TV viewing is the most prevalent（流行的，普遍的） and pervasive⁵ sedentary behavior, there is a great deal of interest in quantifying its independent association with health outcomes. However, a systematic⁶ and quantitative⁷ assessment⁸ of published studies is not available."

Anders Grontved, M.P.H., M.Sc., of the University of Southern Denmark, Odense, and Frank B. Hu, M.D., Ph.D., of the Harvard School of Public Health, Boston, conducted a meta-analysis to summarize data from published prospective⁹ cohort studies on the association between TV viewing and the incidence of type 2 diabetes, nonfatal or fatal cardiovascular disease and all-cause mortality. The researchers performed a search of the medical literature for relevant studies from 1970 to March 2011 and identified 8 studies that met criteria¹⁰ for inclusion in the analysis.

For type 2 diabetes (4 studies), the total number of individuals was 175,938 with 6,428 incident cases during an average follow-up of 8.5 years. For fatal or nonfatal cardiovascular disease (4 studies), the total number of individuals was 34,253 with 1,052 incident cases during an average follow-up of 10.4 years; and for all-cause mortality (3 studies), the total number of individuals was 26,509 with 1,879 deaths during an average follow-up duration of 6.8 years.

An analysis of data indicated that per 2 hours of TV viewing time per day was associated with a 20 percent higher risk for type 2 diabetes; a 15 percent increased risk for fatal or nonfatal cardiovascular disease; and a 13 percent higher risk for all-cause mortality. "While the associations between time spent viewing TV and risk of type 2 diabetes and cardiovascular disease were linear, the risk of all-cause mortality appeared to increase with TV viewing duration of greater than 3 hours per day," the authors write.

Based on incidence rates in the United States, the researchers estimated that the absolute risk difference (cases per 100,000 individuals per year) per 2 hours of TV viewing per day was 176 for type 2 diabetes, 38 for fatal cardiovascular disease, and 104 for all-cause mortality.

"It is biologically plausible¹¹（似合理的） that prolonged TV viewing is associated with type 2 diabetes, cardiovascular disease, and all-cause mortality. Numerous prospective studies have reported associations of TV viewing with biological risk factors for these outcomes including obesity¹², adverse¹³ lipid levels, and clustered cardiovascular risk; however, some studies did not report these associations. Furthermore, associations of sedentary（久坐的） behaviors analogous¹⁴ to TV viewing (e.g., sitting during work or while driving) with type 2 diabetes, fatal or nonfatal cardiovascular disease, and all-cause mortality have been reported in cohort studies," the authors write.

"Additional research quantifying the mediating¹⁵ influence of diet and physical inactivity is warranted. Future research also should assess the association of prolonged daily use of new media devices on energy balance and chronic¹⁶ disease risk."