Watching 3-4 hours of TV per day significantly increases risks for major causes of death.
The study, previously published online, will appear in the December issue of the American Journal of Preventive Medicine.
According to the study authors - who were led by Sarah K. Keadle, PhD, from the National Cancer Institute - on average, 80% of adults in the US watch 3.5 hours of television each day.
Previous studies have shown a link between TV viewing and poorer health; there are already established links between prolonged TV viewing and increased risk of death from cancer and cardiovascular disease, for example.
Because 92% of Americans have a TV at home, and watching it takes up over half of their leisure time, it likely displaces more physical activities, which is why there are associated health risks.
"We know that television viewing is the most prevalent leisure-time sedentary behavior, and our working hypothesis is that it is an indicator of overall physical inactivity," says Keadle.
To further investigate, she and her team studied over 221,000 adults between the ages of 50-71 years old who were chronic disease-free at the start of the study in 1995. Study participants were followed until either death or December 31, 2011.
Death risk increases with 3-4 hours of TV viewing per day
Results of the study confirmed the previous link between television viewing and higher risk of death from cancer and heart disease.
In detail, the study found that compared with those who watched less than an hour of TV per day, those who watched 3-4 hours were 15% more likely to die from any cause. And those who watched 7 or more hours were 47% more likely to die during the study period.
"In this context," says Keadle, "our results fit within a growing body of research indicating that too much sitting can have many different adverse health effects."
The researchers note that risk began to increase at 3-4 hours of TV viewing per day for most causes they assessed. In addition, after controlling for other factors that might explain the links - such as caloric and alcohol intake, smoking and the population's health status - the team says the associations persisted.
Interestingly, the adverse health effects of TV viewing applied to both active and inactive participants. Keadle says exercise did not completely diminish the risks associated with prolonged TV viewing, but she adds that for those who want to reduce time spent in front of the TV, "exercise should be the first choice to replace that previously inactive time."
TV viewing: an important public health intervention target
The researchers say although the links have "plausible biological mechanisms," many of these associations are reported for the first time in this study, and as such, further research is warranted.
Keadle adds that she hopes their study "will spur additional research," particularly to determine whether the same associations are observed when people are sitting in other contexts, including driving, working or doing other sedentary activities during leisure time.
"Older adults watch the most TV of any demographic group in the US. Given the increasing age of the population, the high prevalence of TV viewing in leisure time, and the broad range of mortality outcomes for which risk appears to be increased, prolonged TV viewing may be a more important target for public health intervention than previously recognized."
Although this study focuses on adults, previous studies have shown that many children watch more TV than what the American Academy of Pediatrics (AAP) recommend. For children under 2, the AAP recommend no TV time, while older children should watch fewer than 2 hours per day.
This latest study contrasts a recent one published in the International Journal of Epidemiology, which suggested prolonged sitting does not damage health, as long as the individual takes part in regular exercise.