Earlier this year I wrote about a fascinating new study by Dr David Dunstan and colleagues at the Baker IDI in Melbourne, Australia. They reported that sitting for just 5 hours resulted in dramatic increases in the insulin and glucose responses to a standardized meal. What’s more, simply taking a short walk break every 20 minutes was enough to prevent these increases, and was equally effective whether the walk was at a high or low intensity.
As regular readers of the blog will know, the acute (e.g. short-term) impact of sedentary behaviour is something that I find particularly fascinating. We’ve known for a long time that a short bout of intense exercise has immediate health benefits, but studies like Dr Dunstan’s suggest that sitting for a few hours may have an immediate negative health impact.
But of course it’s never a good idea to read too far into any one study, no matter how interesting the results may be. For every study showing that X causes an increase in Y, it’s possible (and actually very likely) that there are several other studies showing no relationship between the two. So when I began my PhD in this area 3 years ago my supervisor and I decided that it would be a good idea to perform a systematic review to see what evidence was out there regarding the short-term impact of uninterrupted sedentary behaviour on health. Systematic reviews are considered the highest form of research evidence, so they are a good way to figure out the “truth” when it comes to any research topic. The results of our review were recently published (available here) so I thought this would be a good time to share them here on the blog.
This next section goes over the uber-nerdy details of our review methodology, so feel free to skip straight to the “What did we find?” section if you’d rather cut to the chase.
What we did
We used Medline and Embase to systematically search for intervention studies that forced people to engage in sedentary behaviour (e.g. sitting or lying down) for 7 days or less (our search algorithm is available here). Not only that, but the studies also had to assess at least one key metabolic risk factor (insulin, glucose, cholesterol, or triglyceride levels) before and after the bout of sedentary behaviour.
As shown in the figure below, our initial search identified 7,770 potentially relevant papers. My labmate Richard and I each read the abstract of all 7,770 papers, selecting out any that had the slightest chance of meeting our search criteria. This cut it down to just 85 papers, which both of us then read in their entirety (if anyone tells you that a systematic review is easier than doing an original research paper, that person is lying). After reading the full papers, we identified just 29 that met all of our search criteria (a few studies were discussed in more than one paper, so we actually only identified 25 interventions).
What did we find?
Our search identified some very cool papers that we never would have come across otherwise (the table of included studies is too big to include in this post, but you can see all the details on our included studies by clicking here). Lots of studies looking at bed rest, a few looking at sitting, and a couple looking at other models of sedentary behaviour such as casting (e.g. put one leg in a cast, then compare the insulin sensitivity in that leg to the other non-casted leg at the end of the week).
Across all of these models of sedentary behaviour we found a wealth of evidence suggesting that Dr Dunstan’s study was not an outlier – even short bouts of sedentary behaviour seem to consistently result in reductions in insulin sensitivity and glucose tolerance. In other words, an afternoon on the couch makes you measurably closer to having type 2 diabetes. The figure below synthesizes the data for insulin sensitivity and glucose tolerance from all the identified studies, and illustrates that the majority of studies looking at these endpoints – whether they lasted a few days or just a few hours – found that uninterrupted sitting resulted in reduced insulin sensitivity and glucose tolerance (any datapoints to the left of the vertical line mean that values were lower following the intervention).
Further, we also found consistent evidence that short bouts of sedentary behaviour increase the amount triglycerides (e.g. fat) circulating in your blood – a big risk factor for cardiovascular disease. However,we found little consistent evidence that sedentary behaviour lasting anywhere from a few hours to 7 days has any impact of fasting insulin, glucose, HDL- or LDL-cholesterol levels. A few studies suggested a relationship, but the majority did not (although to be fair, there were very few studies looking at HDL- and LDL-cholesterol).
In addition to the metabolic outcomes, we were also able to see what/who hasn’t been studied in this area so far. What we noticed is that very few studies have looked at acute bouts of sedentary behaviour in anyone other than adult white males. Very few studies included female participants (although the newer and higher quality studies did include female participants), and no studies have looked at acute sedentary behaviour in kids (this is what I am focusing on with my thesis project). Not surprisingly, we also found that just 6 of 25 interventions had any sort of control group. This led us to conclude that even for insulin sensitivity, glucose tolerance, and triglyceride levels, the current evidence is only of moderate quality (that will hopefully improve as more people perform randomized interventions – this is what I’m doing for my PhD study).
Oh, and in case you were wondering, we also concluded that there is no health benefit to bouts of uninterrupted sitting.
What is the take-home message?
From the paper:
This study demonstrates that, at present, there is moderate quality evidence that acute bouts of uninterrupted sedentary behaviour result in significant and deleterious changes in insulin sensitivity, glucose tolerance, and plasma triglyceride levels. There is currently very low-quality evidence linking uninterrupted sedentary behaviour with changes in circulating insulin, glucose, and HDL- and LDL-cholesterol levels. There is no evidence that acute bouts of uninterrupted sedentary behaviour provide any positive changes in markers of cardiometabolic risk. However, the majority of studies identified by this paper focused on healthy young men, and it is therefore unclear whether these results will generalize to females or to other age groups. These findings suggest that uninterrupted bouts of sedentary behaviour should be avoided in order to prevent transient increases in metabolic risk.
The full text of our systematic review is available for free through Pubmed Central.
Saunders TJ, Larouche R, Colley RC, Tremblay MS. Acute Sedentary Behaviour and Markers of Cardiometabolic Risk: A Systematic Review of Intervention Studies. Journal of Nutrition and Metabolism. doi: 10.1155/2012/712435.