Is Sitting Down Killing Us All?

Whether the new study about sitting is a reason to get up off your chair

Pictured: Deadly, according to the headlines. Photo by LinkedIn Sales Solutions on Unsplash

Smoking was a remarkable public health success. A single, simple behaviour that on its own increased your risk of dying by a truly staggering amount. Despite the addictive nature of nicotine it’s still relatively straightforward to get people to change a single facet of their lives.

These days, everyone is trying to find the new smoking. We all want to identify another simple trait that we could fix to save millions of lives. One thing that pops up regularly is the idea that sitting is the new smoking—that spending long periods of time sitting down is a simple, easy-to-change behaviour that makes us sick.

The newest study seems to support this idea. Sitting down is, it seems, causing us to get cancer and die more often. If you just replace an hour of sitting with some movement every day, you’d live a lot longer, according to the headlines.

There are many issues with this idea. Yes, sitting down a lot probably isn’t ideal for your health, but there’s no reason to buy a standing desk just yet. Let’s look at the science.

The Data

The new paper that’s making headlines is a large observational study based on the large UK Biobank cohort. The authors had data on about 100,000 people who wore accelerometers—things that measure your movement, similar to a Fitbit—for 7 days between 2013 and 2015. In a model correcting for a range of common confounding factors, they found that sedentary behaviour—sitting down—was associated with an increased risk of dying from various cancers. They also found that people who had more “interrupted sedentary time”—defined as getting up frequently during periods of sitting—were less likely to die from cancer.

The numbers were, however, a bit unimpressive. People who spent the most time sitting down—on average 13+ hours per day—had a 2.8% risk of dying from cancer in the decade of the study. People who spent the least time sitting down—less than 10.7 hours per day—had a 1.3% risk of dying. That’s not nothing, but it’s not quite the sort of health impact that you’d see from smoking.

The findings were also quite complicated. The authors ran what’s called a replacement analysis where they looked at what might happen if people were to replace one activity with another. They found that replacing prolonged sedentary time with interrupted sedentary time actually had no impact unless the interrupted sedentary time included exercise. That is, standing up was not associated with better health unless you went for a walk or did some star jumps.

Unfortunately for science communication, “Vigorous exercise is better for your health than sitting at a desk” doesn’t quite make headlines. Photo by Vultar Bahr on Unsplash

This sort of study also has all of the usual issues that you see in observational research. For one thing, the authors only had movement data at one time point. They got 7 days of physical activity for each person sometime between 2013-2015, and had to use that to define their movement for the entire study.

This creates some obvious issues. Reverse causality, for example, is when your outcome variable—in this case, cancer—is causing the intervention variable. In other words, people who were experiencing the early stages of cancer or other health problems that would eventually lead to cancer might have already been spending more time sitting down at the start of this study. That would make the results here largely meaningless, and it’s a possibility the authors do not appear to have looked into.

There’s also the ever-present issue of unmeasured confounding. People who spend less time sitting were are different in lots of ways to people who don’t sit down much at all. In this study, the people who spent the least time sitting down were younger, smoked less, drank less alcohol, exercise far more, were thinner, and were generally much healthier than people who sat down a lot. It’s likely that there were some things that the authors simply could not measure which also influenced the cancer risk, which makes it hard to know if the increased risk of cancer related to sedentary time was caused by the sitting or by something else that the study couldn’t control for.

Exercise Is…Good?

The bottom line here is that the study doesn’t really tell us anything new. People who get more exercise are healthier than people who get less. There’s probably some causal relationship there, because we know that exercise improves your health in a multitude of ways.

Whether this is really related to sitting down or not is up for debate. I think you could argue that the results from the replacement analysis prove that sitting has very little to do with the issue—instead, it’s how much exercise people are getting and how much of that exercise is vigorous vs light.

At best, the study shows that if you sit down a lot it’s probably better to go for a walk once every 90 minutes or so than to stay sitting down the entire time. That is, I think, not really news to anyone. Yes, sitting down for your entire day probably isn’t ideal for your health, but there’s still no strong evidence that standing up for the same period of time is good for you. The real trick is to get more exercise, but unfortunately that sort of boring health advice rarely makes headlines.