Similar amounts of time spent sitting while driving or using a computer did not have the same associations, according to the researchers, who focused on people with an average age of 37 to eliminate old age as a factor.
We did this study because there were some previous reports of a higher mortality among subjects with a higher TV viewing time, but they had been done in elderly people in other countries, Dr. Miguel Martinez-Gonzalez told Reuters Health in an email.
We were interested in knowing whether or not this association also was present in younger subjects, said Martinez-Gonzalez, a researcher with the Department of Public Health at the University of Navarra in Pamplona, who is the study's senior author.
His teams results are in line with past studies that found a 13 percent higher risk of death for each additional two hours per day of television-watching time, and a sharper risk increase when daily TV time went above three hours, he said.
And no study has ever reported lower mortality among those with higher TV viewing time, he noted.
The researchers analyzed data gathered from 13,284 adults who had graduated from the university beginning in 1999. They wanted to examine any associations between three types of sedentary behavior and risk of death from any cause.
The study team followed the participants for an average of eight years and found 97 deaths, 19 of them from cardiovascular causes, 46 from cancer and 32 from other causes.
Compared to the people who sat watching an hour or less of television a day, those who watched two hours a day had a 40 percent higher risk of death. For those who watched three or more hours, the risk was two-fold higher.
For drivers, two hours a day came with a 14 percent higher risk of death compared to one hour or less. And two hours of computer use daily was linked to a 4 percent lower risk than one hour or less. Spending three or more hours at either task was not linked to any further risk changes.
These results took into account other lifestyle factors like diet, age, weight, smoking and other physical activity, the researchers note in the Journal of the American Heart Association.
The study doesnt prove that watching TV directly leads to premature death, it only shows a correlation, Martinez-Gonzalez and his colleagues point out, adding that more research is necessary to learn more about the possible mechanisms behind the links.
Its also possible that other factors, such as illness, could explain both the higher mortality and greater amounts of sedentary time watching TV, for example.
However, Martinez-Gonzalez said that inactivity and a sedentary lifestyle increase resistance to insulin, reduce lean body mass and increase fat mass. These mechanisms are related to a higher risk of diabetes, cardiovascular disease and some cancers, such as colon, rectum, and breast.
Martinez-Gonzalez added that links between a sedentary lifestyle and a greater risk of depression and suicidal ideation may also be involved.
What we realize is that the study stresses the importance of decreasing our sedentary time and then also increasing our moderate intensity physical activity as recommended by the American Heart Association guidelines, Dr. Heather Johnson told Reuters Health.
There have been multiple very large studies demonstrating an association with higher levels of physical activity and lower rates of mortality and lower rates of cardiovascular disease, said Johnson, a cardiologist at the University of Wisconsin School of Medicine and Public Health. She was not involved in the study.
Johnson said that aerobic physical activity helps to lower so-called bad cholesterol, helps to lower blood pressure and improves the health of blood vessels.
The American heart association stresses 150 minutes per week of moderate intensity aerobic exercise, she said. So about 30 minutes five days a week or some people prefer 40 minutes four days per week.
Johnson added that people who dont currently have an exercise regimen should start slowly about 15 minutes per day and should check with their physician before starting a new exercise program.