23 4 / 2012

From the Archives of Internal Medicine, via The Atlantic:

A study of more than 200,000 Australians adds to the growing body of evidence that people who sit the most die the soonest. It also found that you can’t exercise this effect away, though exercise does help reduce it greatly…

Its most striking finding was that people who sat more than 11 hours a day had a 40% higher risk of dying in the next three years than people who sat less than four hours a day. This was after adjusting for factors such as age, weight, physical activity and general health status, all of which affect the death risk. It also found a clear dose-response effect: the more people sat, the higher their risk of death.

From reddit’s r/science section, which is surprisingly well-sourced.

Looking at the associated scholarly article the main finding is that the “all-cause mortality hazard ratio” for those who sat 4-8 hours a day is 1.02, and 1.40 for those who sit 11+ hours a day. I will trust that they say they accounted for other confounding factors like physical activity (though it’s hard to imagine a marathon runner who sat 11 hours a day). The studied population was aged 45 and over. So, there is a 40% increased likelihood of death in those who sat for 11+ hours a day, vs 4-8. That’s not the same as a 40% chance of dying.

The all-cause hazard ratio is the chance out of 1000 of dying. So, 1.02 is roughly one in a thousand chance of dying. 1.40 is 1.4 in a thousand, or 0.04% increased chance of death.

(Correction: A hazard ratio is not the chance out of 1000 of dying. It’s a ratio of risks between two conditions, so it’s comparing one exposure (eg, 4-8 daily hours of sitting) to something else (the baseline or control group), and the relative chance of dying in these two conditions is 1.02 to 1. In other words, sitting for 4-8 hours has a 2% (relative, not absolute) higher chance of all-cause mortality compared to whatever the baseline is. Eg, the two risks could be 1.02% vs. 1%, or 5.10% vs. 5%.)

More detail: In the US, the death rate for 45-54 year olds in 2004 was 423.1 per 100,000, or 0.423%. (It’s lower for younger people over 1 year old, and higher for older people. For those 25-34 year old Redditors, it’s 100.5 per 100,000, or almost exactly my erroneous orignal 0.1%. Cool! For 15-24 year olds it is only 0.08%)

40% more likely than 0.423% (age 45-54) is an increased death rate of 0.169%. (Back to 0.04% for 25-34 year olds.) So there’s an estimate of the order of magnitude of the increased likelihood of death. It’s still small for most of us, but age dependent. For those 85+, the US death rate is almost 14%. 40% more than that is 19.6%.. almost 6% higher. There’s a difference worth noting.

From what I’ve seen of people installing standing desks in my office, you could be more likely to give yourself a concussion adjusting the desk height than you are to prevent your death.

I’m sure (some) extreme sports have a higher increase in all-cause mortality than sitting all day.

You should get up, move around, eat right, get exercise and all that, but you shouldn’t stress over sitting on your ass and not having a standing desk unless you have a lot of other risk factors. 0.04% to 0.169% is not a huge increase.

That’s why we didn’t have piles of dead office workers in the streets as we moved away from manufacturing and farm labor a hundred years ago.

Adding a link to Cornell Ergonomics Lab (hat tip to harlows_monkeys), with an actual useful discussion of the ergonomics of the situation. Turns out standing desks may not be so great after all if you just stand there motionless all day. I guess there’s a lot of history with factory line workers standing all day.

tl;dr: I’m trying to convey that the numbers aren’t as bad as the Atlantic article made them look. A 40% relative increase is important, but an increase over what matters, too. The relative increase is large, but the absolute risk is still small. As pointed out elsewhere, an increase in annual murder rate from 1:1,000,000 to 2:1,000,000 is a 100% increase, but still a very low murder rate for a large city. (NYC homicide was 63:1,000,000 in 2008.)

As a reference point to compare to a 1.4 hazard ratio for sitting more than 11 hours a day, here’s some info on cigarettes. Less than 10 cigarettes a day is 1.3 hazard ratio and 1.8 for >10 cigs/day.

(via nearnhart-deactivated20130221)