If you are sitting at your computer as you read this, you may want to stand up.

A study published Monday at bmj.com finds that limiting sedentary time to less than 3 hours a day would add about 2 years to life expectancy. And limiting TV-viewing time to less than 2 hours a day would add about 1.4 years to life.

The findings call to mind research published a year ago by the American College of Sports Medicine. That study was surprising in that it concluded sitting at a desk all day shortens your life, even if you get the recommended 30 minutes of exercise at least 5 days a week.

For the new study, a meta-analysis of existing research, the researchers searched the scientific literature related to sedentary lifestyle, television viewing time and life expectancy. Out of 460 studies they reviewed, only five met their criteria for inclusion.

The estimated impact of sitting and TV viewing on life expectancy were arrived at after adjusting the collected data for gender and age.

For perspective, the study cites earlier research that associated smoking with a loss of 2.5 years of life expectancy for men and 1.8 years for women.

The authors point out that life expectancy numbers apply to populations, not to individuals, and that they estimate the number of years a newborn baby will live, not how many years an adult can be expected to live, under certain circumstances. As the study notes, its findings “should not be interpreted to mean that people who are more sedentary can expect to live 1.4 or 2.0 years less than someone who does not engage in these behaviours as much.”

The authors note that their paper assumes cause-and-effect relationships between sitting/TV time and life expectancy; they allow that more, differently designed research is needed to determine for sure whether such a cause-and-effect relationship is in fact in play.

Still, the authors suggest people with desk jobs take frequent breaks to stand up and stretch; they also say we should get up and walk to co-workers’ desks rather than communicate via e-mail or phone.

That’s all well and good. But I don’t see how anyone whose job requires working at a computer can manage less than three hours of sitting down per day – unless we invest in one of those exorbitantly expensive treadmill desks.

Do you sit at a desk for most of the day? Do you worry about how that might affect your health? And do you take any steps to counter all that sitting-down time?