"It is now well known that spending too much time sitting down is bad for the heart, even if one takes regular exercise."
That's what the Daily Telegraph told us on 10 July this year. Behind this piece of insight is a study published by Katzmarzyk and colleagues a few days earlier. The authors investigated the question what effect the daily time we spend sitting down has on health and life expectancy. In the USA, that is.
Maybe the Daily Telegraph lives in a different knowledge universe, but in the one where biomedical research counts, the association between sitting and heart disease is not as clear as the reporters make it out to be. Call me a fusspot, but the only study design, which allows us to draw conclusions about causality, are those where we expose a randomly assembled group of individuals to a certain intervention (in this case: sitting down for extended hours every day) and then we compare the outcome in that group with the outcome of another randomly assembled group which didn't get our intervention. Assuming the two groups didn't differ in any meaningful way from each other at the outset of our experiment, we can, at the end of it, ascribe a possible difference in outcome between the groups to our intervention. That's what I want you to keep in mind while I walk you through the study which had prompted the Daily Telegraph to tell you that sitting too long will cut your life expectancy.
Let's first look at the background to the authors' research question, which was "To determine the impact of sitting and television viewing on life expectancy in the USA" . Over the past 60 years we have accumulated a vast body of evidence for the benefits of physical activity on health. The results of this research are reflected in every guideline on how and how much we should exercise. You could say "made to move" is written all over our genes. Only very recently are we discovering a sub-clause, written in small-print, saying "man is not made to sit", which we interpret to mean that cramming movement into a brief period of time every day doesn't help us much if sitting around is what we do for the rest of the day. Katzmarzik and Lee simply wanted to extract from the available evidence how a violation of this newly discovered sub-clause impacts our health and longevity.
So, they set out to identify all the studies from which reliable data could be gleaned about the effects of sitting and television time on the risk of dying. Only 5 studies matched those criteria. From these they pooled the relative risk results into a meta-analysis. Then they looked at the sedentary behaviors of the U.S. population. For that purpose they consulted the data of the National Health And Nutrition Surveys (NHANES), and they also looked at the latest life tables for this population as published by the World Health Organization (WHO). We don't need to go into the statistic intricacies of the procedure. They are a very thorough and methodical attempt at coming up with an educated guess about the impact of extended sitting on the life expectancy of a population. By way of analogy: the authors threw all those data into the statistics blender and came up with what we call the population attributable fraction, or PAF, which tells you how many deaths (or disease cases) could be avoided in a population if the risk factor or exposure were eliminated, in our case, the exposure being extended sitting time.
Fast forward to the results, which the authors comment as follows: "The results of this study indicate that limiting sitting to less than 3 hrs/day and limiting television viewing to less than 2 hrs/day may increase life expectancy at birth in the USA by approximately 2.0 and 1.4 years respectively, assuming a causal relationship." That's what I like about the authors, whose work I have been following for quite some time. They point out that this conclusion is only valid UNDER THE ASSUMPTION that sitting and dying early are causally related. They also go on to emphasize that this is "...a theoretical estimate..." (emphasis in italics by the authors) and that "This should not be interpreted to mean that people who are more sedentary can expect to live 1.4 or 2.0 years less than someone who does not engage in these behaviours as much." That's obviously addressed at those media types who, of course find it far more sexy to tell you that spending too much time on your butt cuts down your life expectancy.
Now, instead of picking the raisins out of this nicely done study, I want to walk you briefly through the 5 studies from which the authors extracted their results. After that, you can still judge for yourself how much trust you want to put into the Daily Telegraph interpretation.
The first of the 5 studies was conducted by the same lead author, Dr Katzmarzyk. It was a study of 17,013 adults of the 1981 Canada Fitness Survey (CFS) who had been followed for up for 12 years . At baseline, the survey participants had been asked, among other things, about their time spent sitting. Death from cardiovascular and other causes were the outcome measure. In such a study it wouldn't make sense to simply correlate sitting time with death. After all, there are a lot of other factors which determine our demise. Age being one of them. My chances to die in the next 12 years are quite a bit greater if I'm 70 than if I'm 35. So, believe me when I say that the authors adjusted as much as possible for such factors. And it is this "as much as possible" where we begin to find hairs in the soup.
First of all, cardiovascular disease (CVD) is a main cause of death today. So, we should account for all those people who already had CVD when they entered the study. But that's not as simple as it sounds. CVD has a mean streak in that it remains asymptomatic for years, often decades, before it hits you with a heart attack or stroke. So, eliminating those cases who had reported such events at baseline, doesn't mean our survey participants had a clean bill of cardiovascular health. At the average age of over 40, there will certainly have been quite a number of people who had such silent stages of CVD. The principal manifestation of "silent" cardiovascular diseases are those atherosclerotic plaques which narrow the arteries and arterioles. While the authors used the PAR-Q (physical activity readiness questionnaire) which asks, in five questions, about symptoms of CVD, silent CVD would have flown below that radar. So, not accounting for those silent cases may, in all likelihood, have biased the results. Think about it, if those with silent CVD don't move as much, simply because exercising causes them discomfort (which happens when narrowed arteries don't supply enough blood to a working muscle, or heart), it is not the sitting time, but the silent CVD which correlates with an earlier death.
On to the 2nd study: Author Patel and colleagues looked at 123,216 adults, aged 60+, of the CPS-II nutrition cohort, who had been followed up for 14 years . Again the results support an association between sitting time and CVD mortality, but, again, silent asymptomatic disease had not been assessed. Interestingly, in this study the association was far stronger in women than in men. Tellingly, age 60+ is also the age at which women start to "catch up" with their male peers in respect to CVD risk.
In the third study, Dunstan and colleagues had looked at the correlation between television viewing time and death among 8,800 adults aged 50+ with a median follow-up period of 6.6 years. In contrast to the previous 2 studies, the authors were able to adjust for known CVD risk factors such as hypertension, blood lipids, blood glucose and diabetic status. Those who reported sitting in front of the TV for more than 4 hours per day, had a 50% higher risk of dying from any cause and an 80% higher risk of dying from CVD causes. But adjusting for risk factors of CVD is not the same as adjusting for CVD.
In the fourth study, Stamatakis and colleagues had looked at the data of 4512 people, aged 57+, of the Scottish Health Survey, who had been interviewed in 2003 and followed up until 2007. Those who had reported watching more than 2 hours of TV per day had an increased risk of CVD events (not of CVD death), and only those who had reported watching TV for more than 4 hours per day had a statistically significant risk increase of dying from any cause.
In the fifth and final study Wijndaele investigated the data of 13,197 adults aged 60+ of the EPIC study cohort. Those people had been assessed at the 1998-2000 baseline and followed up for 9.5 years. Like in the other 4 studies, the association between increased TV viewing time and all-cause and CVD death was evident. This observation prompted the authors to say that: "Given the high prevalence of excessive TV watching, ... these results indicate the importance of public health recommendations aimed at decreasing TV time and possibly overall sedentary behaviour." So, will throwing away your TV make you live longer?
You'll probably appreciate the difference between Wijndaele's and Katzmarzyk's way of interpreting essentially similar results. I personally go with Katzmarzyk's more careful interpretation. It does not outright assume a causal correlation to exist. There are still too many question marks. For example: We know that self-reported physical activity, self-reported screen time, well, self reported anything, is inherently fraud with over- and under-reporting of facts. Dunstan and colleagues were adamant at pointing out that this couldn't have affected their results. But when you look at how well, or how poorly, their questionnaire really performs, you will be forgiven to be less enthusiastic than the authors. Use that questionnaire twice on the same person to assess same-level PA, and chances are you'll get two different answers. That's not just me being the party pooper, it has been confirmed in validation studies which have shown, at best, only a moderate level of agreement between two rounds of questioning (the parameter is the intraclass correlation coefficient, or ICC) . If repeated questioning is already fraud with inconsistencies, how large, do you think, such inconsistencies will be between the answers of any given respondent and his actual physical activity level?
So, what are we to make of all this? I can only give you my personal opinion. I tend to believe that there is a threshold volume and intensity of DAILY physical activity, which protects you against the effects of extended sitting time. Only we can't see this level in the 5 discussed studies for obvious reasons. Their ways of assessing PA were not accurate enough.
I have to admit, that my belief is biased: I don't know about you, but less than 3 hours of sitting time appears unachievable for most of us today. And while I'm working at a desk, which allows me to alternate between standing and sitting, seen through the lenses of these 5 studies, I still have what those studies proclaim to be a risk factor for premature death: extended sitting time. But I also do exercise on a daily basis at an intensity and with a volume which far exceeds what 90% of the population is doing. That's why I love to think of this effort as being CVD-protective. This belief is founded in a large body evidence which essentially says: exercise triggers biochemical reactions and mechanisms with a vast array of protective effects. In a dose-dependent way.
Fortunately, I'm able to measure the effects of my personal dose of exercise in my health lab. And from doing the same thing for our clients, I happen to know that everyone is unique in his response to intervention, be that exercise or diet or a pharmacological treatment. Which is why I am quite confident when I tell you not to lose any sleep over those attention grabbing headlines. Especially, when they suggest cause-effect relationships from studies which simply can't establish such relationships. In the case at hand, none of the 5 studies could have adjusted for pre-existing silent CVD. CVD is a cause of premature death and, as I have argued, it can be a reason for people to avoid exercise and spend more time sitting, simply because exercise causes them discomfort. So, here is my question: Are people dying early because they sit too long, or are they sitting so long because they'll die earlier? Stay skeptic! [tweet this].
1. Katzmarzyk PT, Lee IM: Sedentary behaviour and life expectancy in the USA: a cause-deleted life table analysis. BMJ Open 2012, 2(4).
2. Katzmarzyk PT, Church TS, Craig CL, Bouchard C: Sitting time and mortality from all causes, cardiovascular disease, and cancer. Med Sci Sports Exerc 2009, 41(5):998-1005.
3. Patel AV, Bernstein L, Deka A, Feigelson HS, Campbell PT, Gapstur SM, Colditz GA, Thun MJ: Leisure Time Spent Sitting in Relation to Total Mortality in a Prospective Cohort of US Adults. Am J Epidemiol 2010:kwq155.
4. Brown WJ, Trost SG, Bauman A, Mummery K, Owen N: Test-retest reliability of four physical activity measures used in population surveys. J Sci Med Sport 2004, 7(2):205-215.
Katzmarzyk PT, & Lee IM (2012). Sedentary behaviour and life expectancy in the USA: a cause-deleted life table analysis. BMJ open, 2 (4) PMID: 22777603
Katzmarzyk PT, Church TS, Craig CL, & Bouchard C (2009). Sitting time and mortality from all causes, cardiovascular disease, and cancer. Medicine and science in sports and exercise, 41 (5), 998-1005 PMID: 19346988
Patel AV, Bernstein L, Deka A, Feigelson HS, Campbell PT, Gapstur SM, Colditz GA, & Thun MJ (2010). Leisure time spent sitting in relation to total mortality in a prospective cohort of US adults. American journal of epidemiology, 172 (4), 419-29 PMID: 20650954
Brown WJ, Trost SG, Bauman A, Mummery K, & Owen N (2004). Test-retest reliability of four physical activity measures used in population surveys. Journal of science and medicine in sport / Sports Medicine Australia, 7 (2), 205-15 PMID: 15362316