10 July 2012

Life Expectancy Can Be Extended More Than 3 Years By Adjusting Sedentary Behavior

According to the the journal, Applied Physiology, Nutrition and Metabolism, Sedentary behavior is defined as "any waking activity characterized by an energy expenditure ≤ 1.5 metabolic equivalents and a sitting or reclining posture..."

This generally means that sedentary behavior involves any activity that the person does sitting or lying down. This includes watching television, playing video games, computer use, driving, and reading.

A study indicates that life expectancy in the USA would be 2 years higher if adults cut their sitting time to 3 hours a day and 1.38 years higher if television viewing time is cut to less than 2 hours a day.

This analysis is detailed in an article to be published in the online BMJ Open, Peter T Katzmarzyk of Pennington Biomedical Research Center, Louisiana State University System and I-Min Lee of the Division of Preventive Medicine, Brigham and Women’s Hospital and Harvard Medical School,

Several previous studies have linked extended periods spent sitting down and/or watching TV to poor health, such as diabetes and death from heart disease/stroke.

The researchers used data collected for the National Health and Nutrition Examination Survey (NHANES) for 2005/6 and 2009/10, to calculate the amount of time US adults spent watching TV and sitting down on a daily basis.

NHANES regularly surveys a large representative sample of the US population on various aspects of their health and lifestyle.

They trawled the research database MEDLINE, looking for published studies on sitting time and deaths from all causes, and pooled the different relative risk data from the five relevant studies, involving almost 167,000 adults. The database was then reanalysed, taking account of age and sex.

Video: Sedentary Behavior

They combined these data and the NHANES figures to come up with a population attributable fraction (PAF) - an estimate of the theoretical effects of a risk factor at a population, rather than an individual level - to calculate the number of deaths associated with time spent sitting down.

The PAFs for deaths from all causes linked to sitting time and TV viewing were 27% and 19%, respectively.

The results of life table analyses indicates that cutting the amount of time spent sitting down every day to under three hours would add an extra two years to life expectancy.

Similarly, restricting time spent watching TV to under two hours daily would extend life expectancy by an extra 1.38 years.

The authors emphasise that their analysis assumes a causal association rather than proving that there is one. But they point to the evidence showing the detrimental effect of a sedentary lifestyle on health.

And they caution that their findings should not be interpreted as meaning that someone who leads a more sedentary lifestyle can expect to live two or 1.4 years less than someone who is more active.

"The results of this study indicate that extended sitting time and TV viewing may have the potential to reduce life expectancy in the USA," they write.

"Given that the results from objective monitoring of sedentary time in NHANES has indicated that adults spend an average of 55% of their day engaged in sedentary pursuits, a significant shift in behaviour change at the population level is required to make demonstrable improvements in life expectancy," they conclude.

Further research will be required before recommendations on safe levels of sedentary behaviour can be made, they add.


BMJ-British Medical Journal
Pennington Biomedical Research Center
Division of Preventive Medicine, Brigham and Women’s Hospital
Sedentary Behaviour and Life Expectancy in the USA: A Cause-deleted Life Table Analysis
Standardized use of the terms “sedentary” and “sedentary behaviours”
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