The (hyperbolic, fear-mongering) idea of the phrase “sitting is the new smoking” is that an inactive lifestyle is allegedly just as bad for your health as smoking. In particular, the analogy also ominously suggests that, just like smoking, a lot of sitting may be harmful even if we are otherwise healthy and active.
Excessive inactivity is almost certainly unhealthy, but is it anywhere near as dangerous as smoking? Unlikely: smoking is crazy bad for you. (And it's always seem unlikely the risks of inactivity, whatever they are, cannot be at least mitigated by exercise breaks!)
The ubiquity of cheap, accurate accelerometers has made it possible to start putting this ridiculous analogy to the test more directly than in the past.
What if you stuck those gadgets on 8000 middle-aged people for four years? A new paper reports on just such an experiment (Diaz et al). It’s probably the biggest of its kind so far, though much bigger and better ones are surely on the horizon as the technology gets ever cheaper. Using accelerometers has advantages that past studies cannot match. (Not that it’s a perfect study, of course — they never are.)
The results did affirm a link, which has already been widely reported — and almost all of the reporting assume that the link means that inactivity caused early deaths. But this link is not a causal link.+They did not prove that inactivity causes a shorter life expectancy. No one has yet proved that. It’s plausible, but not established. There are other explanations for the link, as there always are in epidemiology. Some of the study subjects died: 340 of them, and those people spent more time overall not moving, and had longer periods of not moving.
While everyone else is busy reporting that “inactivity will kill you,” my take on this study is different: my main interest in it is that it helps us to see that the danger of inactivity is not in the same league as smoking. Not even close. The hazard ratio for greater total sedentary time was 1.6. A hazard ratio of 2 means “twice the chance.” Hazard ratios for smoking-related diseases are dramatically higher, and even higher still for not-so-obviously related problems like coronary heart disease.
And the data also showed that taking breaks mitigated the risk. Which also contradicts the smoking analogy. Phew!
So this study is a good FUD-fighter.
Bottom line: inactivity is almost certainly unhealthy, and is associated with a measurably higher risk of death, but comparing it to smoking is %@!^% ridiculous.
Criticism of the study
Ars Technica has some harsh criticism of Diaz et al.: “funded by Big Soda & riddled with weaknesses.” Noted, but meh: methinks their concerns are overstated. Most of what they point out is par for the course in epidemiology, the kind of limitations you can find in any study — noteworthy, but they don’t nuke the value of the data.
Their hand-wringing about the “Big Soda” angle seems especially uninteresting to me: this may be “exactly the kind of research” Big Soda allegedly likes to see (to take the heat off the unhealthiness of their product), but it isn’t exactly an isolated result. We have piles of evidence that inactivity is unhealthy, so if that’s useful PR for Big Soda, it’s a benefit in addition to it being, y’know, true.
I’ve updated the main PainScience.com article about chairs and sedentariness with this study and its implications. The reporting there is much more thorough than this summary, covering every aspect of the topic and more relevant research …