Among more than 120,000 adults, those who sat more than six hours a day in their leisure time were significantly more likely to die in a 14-year period than those who sat less than three hours, according to Alpa Patel of the American Cancer Society in Atlanta, and colleagues.
The association was stronger in women than in men, the researchers reported online in the American Journal of Epidemiology.
The findings were independent of physical activity levels, body mass index, smoking and several other factors contributing to mortality risk.
According to the researchers, sedentary time may be related to mortality risk because sitting may be associated with other unhealthy behaviors, like excessive eating. In addition, sitting too much could have adverse metabolic effects.
"Public health messages and guidelines should be refined to include reducing time spent sitting in addition to promoting physical activity," Patel and colleagues wrote.
Numerous studies have identified a relationship between high levels of physical activity and reduced risks of death and a multitude of health problems. Few studies, however, have examined the association between time spent sitting and mortality.
Patel and her colleagues evaluated data from the ACS's Cancer Prevention Study II Nutrition Cohort, a large, prospective study of cancer incidence and mortality.
At baseline, the participants completed a questionnaire detailing -- among other things -- the amount of time they spent sitting and exercising in their leisure time.
The current analysis included 53,440 men and 69,776 women who were healthy at the beginning of the study.
During the 14-year follow-up, there were 11,307 deaths in men and 7,923 in women.
Even though the link between sitting and risk of death was greatest for those participants who sat at least six hours a day, there was a significant association among both men and women who sat three to five hours as well.
As expected from previous studies, there was a significant inverse relationship between physical activity levels and mortality risk. The risk of dying during follow-up was greatest among participants who sat the most and exercised the least.
Broken down by cause, death from cardiovascular disease was significantly associated with both sitting time and physical activity levels in men and women. Cancer mortality, on the other hand, was only related to sedentary time and physical activity among women.
Patel and her colleagues acknowledged that the study was limited by the lack of data on occupational physical activity and time spent sitting. In addition, the analysis relied on self-reported data, which could be subject to bias. And finally, the researchers could not differentiate between types of sitting (while driving, watching TV, reading); energy expenditure and behavior might vary with the type of sitting.