A very quick post today to point out an interesting new paper in the journal BMJ Open. Written by Peter Katzmarzyk (Peter J and I took his epi course while at Queen’s University) and I-Min Lee, the paper estimates the impact of both sedentary behaviour (e.g. all sitting) and television viewing on the life expectancy of Americans.
As you might expect, the paper is extremely stats-based. The authors first performed a meta-analysis of available studies (see below) to estimate the impact of various levels of sedentary behaviour on mortality (e.g. do people who sit >3 hours per day die sooner than those who sit < 3 hours per day?). As the figures below demonstrate, the meta analyses suggest that people who accumulate more sedentary time die sooner than their peers who get less sedentary time. The authors then estimated the likely impact of sitting in the US population by combining the results of these meta-analyses with data on the amount of time that Americans spend sitting (using the nationally representative NHANES dataset).
The results? The authors estimate that “population life expectancy in the USA would be 2.00 years higher if adults reduced their time spent sitting to <3 h/day and 1.38 years higher if they reduced television viewing to <2 h/day.” They go out of their way to state that this does not mean that someone who watches >2 hours/day of TV is definitely going to live 1.38 years less than someone who watches <2 hours/day. Rather, this is a “theoretical estimate of the effects of a risk factor on an outcome at the population level”.
These sorts of theoretical studies obviously need to be taken with a large dollop of salt (just like the recent Australian study which estimated that every hour of TV viewing shortens your life by 22 minutes). The point is simply that there is a non-negligible impact of sitting/TV viewing on mortality, and given the extremely high prevalence of these behaviours at the population level, they can have noticeable impact on the lifespan of the population as a whole.
The paper is open access, so I’d encourage everyone to check it out here. I’m curious to hear what others think of this – and similar studies – in the comments. I will be taking a break from my computer (reducing my screen time!) for the next week or so, so if I don’t reply to a relevant comment please don’t take that to mean I think it unimportant; I’ll catch up as best I can upon my return to digital life!
Katzmarzyk, P, & Lee, I-Min (2012). Sedentary behaviour and life expectancy in the USA: a cause-deleted life table analysis BMJ Open DOI: 10.1136/bmjopen-2012-000828