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May 13, 2011

Sitting Won’t Kill You, Except on Train Tracks

http://www.jewishjournal.com/blog/item/sitting_wont_kill_you_except_on_train_tracks_20110513/

In the last couple of weeks the media has published stories making it sound like your Ikea chair is a death trap waiting to assist your suicide through the dangerous activity of sitting down. Stories with sensational titles like “Is Sitting a Lethal Activity?” (see link below) make you think that you’re better off walking outside for a smoke. Let’s spend a few minutes sifting the solid science from the wacky conjecture. You might as well sit down for this.

The media interest in the idea that sitting might kill you started with the publication last month of a paper entitled “Sedentary Behaviors Increase Risk of Cardiovascular Disease Mortality in Men” in the journal Medicine and Science in Sports and Exercise. (The link to the paper is below, but I’ve read it so you don’t have to.) The study followed thousands of men who in the 1980s were surveyed about their activity levels. They were asked how many hours per week they spent riding in a car and watching TV. They were also asked about how physically active they were. The study then followed these men for over twenty years and counted those who died of cardiovascular disease. The men who spent the most time watching TV and riding a car were at highest risk of dying of cardiovascular disease. So the authors conclude that time spent sedentary, regardless of how much exercise is done the rest of the day, is dangerous.

My regular readers will know that this doesn’t mean that sedentary activity causes cardiovascular disease, because the study isn’t randomized. This conclusion is just as wrong as guessing that there’s something specifically about televisions or cars that cause cardiovascular disease. The only way to sort out whether sedentary time in itself causes cardiovascular disease is to force one group to spend a lot of time sitting and to force another group not to sit (which I think is an enhanced interrogation technique). We can assume that sitting is perfectly safe and still imagine lots of factors that would lead men who sit a lot to die sooner. Sick people, for example, may feel too poorly to be active. They will therefore sit a lot more and die sooner than their healthier counterparts. The authors took some precautions to avoid such confounding factors, but these factors can’t be eliminated entirely without randomization.

The NY Times article about this issue (link below) is an interesting jumble of good science and unfounded conclusions. The article cites a study by James Levine in which subjects were instructed not to exercise and were carefully fed a diet containing 1,000 more calories than needed to maintain their weight. Some subjects gained weight and others didn’t. The ones who didn’t gain weight subconsciously increased their activity level when their caloric intake increased. They fidgeted, paced, stood, stretched and generally moved enough to burn the excess calories. That’s a fascinating discovery which teaches us that even small repeated movements can burn a lot of calories. But this has nothing to do with the article’s main claim which is that being sedentary poses a hazard that is not compensated by exercise – that sitting for 8 hours is dangerous even if you’re going to jog for 30 minutes later that day. The article supports that claim only by a lot of non-randomized epidemiology and metabolic studies, nothing persuasive.

The NPR story and The Dish graphic (links below) also commit the very common error of arguing from design. Arguing from design happens whenever someone asserts what our bodies are “built for” or “meant to do”. The stories state that “we just aren’t really structured to be sitting for such long periods of time” and “a hundred years ago, when we were all out toiling in the fields and factories, obesity was basically nonexistent.” Yes, and a hundred years ago life expectancy was much shorter. We are more sedentary now and living longer than ever. The problem with arguing from what nature “meant” us to do is that for most of human history most humans lived on the edge of starvation, fleeing from predators, and dying young. All of human progress, from wearing glasses to modern medicines, has been marked by rebellion against what nature intended for us. What we were “built” to do can’t help us figure out what we should do. Only a randomized study can.

The articles do have some good common-sense suggestions. If you’re overweight, or have poorly controlled diabetes, high blood pressure or high cholesterol, then more physical activity would certainly help you. You can get that activity by spending more time exercising or less time sitting. There’s certainly no harm in getting up from your chair periodically to stretch or pace around the office, and every calorie burned is a good thing. But if your weight, blood pressure, cholesterol and blood sugar are normal and you exercise regularly, there is nothing in these articles that should convince us that sitting in a chair is bad for you. But you should probably take your feet off the desk before your boss walks in.

Learn more:

NY Times article: Is Sitting a Lethal Activity?

NPR story: Sitting All Day: Worse For You Than You Might Think

The Dish graphic: Sitting Is Deadly?

Medicine & Science in Sports & Exercise article: Sedentary Behaviors Increase Risk of Cardiovascular Disease Mortality in Men

Important legal mumbo jumbo:
Anything you read on the web should be used to supplement, not replace, your doctor’s advice.  Anything that I write is no exception.  I’m a doctor, but I’m not your doctor despite the fact that you read or comment on my posts.

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