Australasian Science: Australia's authority on science since 1938

The Science of Sitting

By Tim Olds

Sitting for extended periods increases mortality, but is it worth working at a treadmill desk?

The average adult Australian spends about 9.5 hours each day on his or her derrière, about 7.5 hours asleep, about 6.5 hours standing or moving around slowly, and about half an hour walking briskly or exercising.

What do we do in those 9.5 hours of sitting? We spend 3 hours watching TV, 1.5 hours on the computer, an hour eating and another hour travelling, we read for an hour, sit and talk for an hour, plus spend an hour just chilling.

Since the 1930s we’ve been paying a lot of attention to exercise, which occupies scarcely 2% of our day, yet almost no attention to sitting, which takes up 40% of our day. Now there’s a new science of sitting, and it’s spearheaded internationally by Australian researchers like Genevieve Healy and Wendy Brown at the University of Queensland, Adrian Bauman at Sydney University, and David Dunstan at the Baker IDI.

The epidemiological evidence is fairly straightforward: sitting is bad for you. People who sit more, and who sit for prolonged periods with fewer breaks, are more likely to have larger waists, higher blood fats, higher levels of inflammation, and higher all-cause mortality – no matter how much exercise they do.

This is not to say that sitting negates the benefits of exercise. Exercise reduces risk, but at any given level of exercise the risk is greater if you sit more.

In a study of 123,116 US adults, the lowest all-cause mortality was among those who exercised the most and sat the least. But even among those who exercised the most, all-cause mortality was 10–25% higher for those who also sat the most. Among those who exercised the least and sat the most, mortality was 50–100% higher.

At-risk groups spend even more time sitting: our recent studies with people with chronic obstructive pulmonary disease and stroke survivors show they spend about 2 hours more each day sitting than healthy people of the same age.

The science of sitting is reaching into everyday life. Those attending physical activity conferences will notice the anti-sitting mafia standing at the back of lectures trying to look unselfconscious. Standing ovations between presentations are not designed to applaud the speaker but to get the audience moving.

In offices around Australia people are standing with their computers perched on adjustable-height desks, or even trying the new treadmill desks that allow the sufficiently dextrous to type as they walk. Avant-garde firms encourage walking meetings, forbid employees to email the person in the office next to them and move printers to inaccessible places to make employees walk to pick up their printing.

While the epidemiological evidence shows consistent, but relatively modest, associations between sitting and health risk, the mechanisms are less clear. There appear to be three contenders.

  1. There’s something about just being on our backsides that is bad for us. Marc Hamilton’s research with mice (no, they don’t make little armchairs and TVs for them – they have other methods), and more recently with humans, shows that prolonged sitting reduces levels of lipoprotein lipase, the enzyme responsible for hoovering up fats from the bloodstream, and increases insulin resistance.
  2. We do bad things when we’re sitting. Like watching TV, which leads to snacking, which leads to … well, you know where this is leading. Some recent studies are suggesting that high risk and low risk groups alike spend about the same amount of time sitting — it’s just that the high risk groups spend more time watching TV.
  3. When we sit, we use less energy, and it’s energy expenditure that is the key. It’s devilishly difficult so separate the effects of sitting per se and reduced energy expenditure. Because 95% of our waking day is made up either of sitting or low-level standing activity, an increase in one necessarily means a decrease in the other.

But don’t rush out to buy that treadmill desk quite yet. Bauman warns that the science of sitting is evolving. Most of the data are cross-sectional, and definitive intervention studies remain to be done.

Part of the problem is measurement: big studies rely on self-reported sitting. Try it yourself: how long did you spend sitting yesterday? How confident are you in that number? Would you stake your life on it?

What’s more, the reduction in risk is fairly modest even with large differences in self-reported sitting time. In a Sydney University study that followed up 223,000 Australians for almost 3 years, an increase of more than 5 hours sitting per day was only associated with a modest (15%) increase in the risk of death. That’s enough to reduce an adult’s yearly risk of dying from about 1 in 250 to about 1 in 215. To me, that seems like a lot of standing up for not much return.

Getting people to sit less might seem to be a way of harvesting the low-hanging fruit of health behaviours, given the notoriously high drop-out rate from exercise programs, but if you’re wondering whether you’re better off reducing sitting time or doing some exercise then the trade-off is about six to one: 3 hours less sitting is roughly equivalent (in terms of all-cause mortality) to about 30 minutes of exercise.

So, depending on your tolerance of risk and love of repose, you can get up and move … or rest easy.

Professor Tim Olds leads the Health and Use of Time Group at the Sansom Institute for Health Research, University of South Australia.