MONDAY, March 26 (HealthDay News) -- For better health, try standing up more, a new study suggests. Those who spend 11 or more hours a day sitting are 40 percent more likely to die over the next three years regardless of how physically active they are otherwise, researchers say.

Analyzing self-reported data from more than 222,000 people aged 45 and older, Australian researchers found that mortality risks spike after 11 hours of total daily sitting but are still 15 percent higher for those sitting between 8 and 11 hours compared to those sitting fewer than 4 hours per day.

"The evidence on the detrimental health effects of prolonged sitting has been building over the last few years," said study author Hidde van der Ploeg, a senior research fellow at the University of Sydney. "The study stands out because of its large number of participants and the fact that it was one of the first that was able to look at total sitting time. Most of the evidence to date had been on the health risks of prolonged television viewing."

The study is published in the March 26 issue of the Archives of Internal Medicine.

Average adults spend 90 percent of their leisure time sitting down, van der Ploeg said, and fewer than half meet World Health Organization recommendations for 150 minutes of at least moderate-intensity physical activity each week.

The data was collected as part of Australia's 45 and Up Study, a large, ongoing study of healthy aging. Strikingly, the elevated risks for dying from all causes remained even after taking into account participants' physical activity, weight and health status.

Sixty-two percent of participants said they were overweight or obese (a similar proportion to Americans), while nearly 87 percent said they were in good to excellent health, and one-quarter said they spent at least 8 hours each day sitting.

Inactive participants who sat the most had double the risk of dying within three years compared to active people who sat least, van der Ploeg said, and among physically inactive adults, those who sat the most had nearly one-third higher odds of dying than those who sat least.

Because many people must sit for long hours at their jobs, they should make sure a greater portion of their leisure time is spent standing, walking or engaging in other movement, said Dr. Suzanne Steinbaum, director of Women and Heart Disease at Lenox Hill Hospital in New York City and a spokesperson for the American Heart Association.

"Yes, you have to work, but when you go home it's so important you don't go back to sitting in front of the computer or television," Steinbaum said. "After the 8-hour mark, the risks go up exponentially. It's really about what you're doing in your leisure time and making the decision to move."

Several workplaces in Australia are testing sit-stand work stations, van der Ploeg said -- a generally well-received initiative that may be a future option for other offices. "Try ways to break up your sitting and add in more standing or walking where possible," she suggested.

While the study uncovered an association between total sitting hours and death risk, it did not prove a cause-and-effect relationship.

The study was limited by the relatively short follow-up period of less than three years, experts said, which may have obscured undiagnosed health problems among participants that could have led to earlier death. Dr. David Friedman, chief of heart failure services at North Shore Plainview Hospital in Plainview, N.Y., said those who sit longer "tend to be sicker, have obesity issues and cardiovascular problems. Perhaps they're less ambulatory in the first place."

Van der Ploeg acknowledged these limitations and said more studies will need to replicate the findings and focus more on sitting's influence on developing conditions such as diabetes, cancer and heart disease.

"Studies that measure sitting time with activity monitors instead of questionnaires will also help build the evidence base," she said. "All these studies will further inform us of the exact relationship between sitting and health conditions, which ultimately will result in public health recommendations like we already have for physical activity."

More information

Read more about the World Health Organization's physical activity recommendations.