Sitting for more than three hours a day may shorten your life by two years, even if you are physically active and don’t smoke, according to a new study by researchers at Harvard School of Public Health (HSPH) and Pennington Biomedical Research Center (PBRC) in Baton Rouge, La. Watching TV for more than two hours a day may also reduce life expectancy by another 1.4 years, they found.

The study, published July 10, 2012 in the online journal BMJ Open, is one of a growing number of recent studies pointing to the health hazards of a sedentary lifestyle. Several previous studies have linked extended periods spent sitting down and/or watching TV to conditions such as type 2 diabetes, heart disease, and stroke.

I-Min Lee, professor of epidemiology at HSPH, and Peter T. Katzmarzyk of PBRC analyzed data from the National Health and Nutrition Examination Survey (NHANES) to calculate the amount of time U.S. adults spent watching TV and sitting. They reviewed the research database MEDLINE for published studies on sitting time and deaths from all causes, pooled the data from the five relevant studies involving almost 167,000 adults, and reanalyzed it, taking account of age and sex. They combined this data and the NHANES figures to come up with an estimate of the theoretical effects of a risk factor at a population.

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