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Jog On

by on 2015/02/17

Keen runners might have been alarmed by recent headlines warning of potential health risks associated with relatively intense exercise regimes. These included: “Fast running is as deadly as sitting on the couch” (Telegraph, 2 February); “Too much jogging ‘as bad as no exercise at all’” (BBC, 3 February); and “Stop that binge jogging! Three times a week is best for you… and too much is as bad as doing nothing” (Daily Mail, 5th February).

What did the media articles say?

Reading further into the articles was unlikely to put minds at rest. The Daily Telegraph went on to say, “Too much exercise really can kill you, scientists have discovered, after finding runners who speed along at 7mph or more are doing themselves more harm than good”. The BBC reported: “People who jogged more intensively – particularly those who jogged more than three times a week or at a pace of more than 7mph – were as likely to die as those who did no exercise”.

Well, it might be a bit of a spoiler for where this is going, but I feel obliged to mention that the original BBC article from 3rd February has actually since been amended (BBC, 6th February). In addition, the media coverage has already been criticised in other blogs, including NHS Behind the Headlines, and the Runners World Sweat Science column.

What did the original source say?

The articles referred to a study published in the Journal of the American College of Cardiology, which used data from the Copenhagen City Heart Study (a large-scale long-term survey) to analyse the likelihood of death over a 12 year period, in 1,098 regular runners and 413 sedentary non-runners. The group of runners was subdivided according to their own reports of the pace, frequency, and weekly duration of their running, and these measures were combined to give a classification of light/moderate/strenuous running.

Given the study limitations that had been discussed in the NHS and Runner’s World websites (and also the updated BBC article), I was expecting to find that the authors of the original article would be relatively restrained in their claims, compared with the headlines. However, on reading the study, I had some sympathy for the headline writers. The sensationalist spin appears to have come direct from the authors themselves. In the abstract, where the authors present their overall summary of the article, they conclude that, “the findings suggest a U-shaped association between all-cause mortality and dose of jogging” (in other words, they are claiming that there is an optimal upper limit, and that doing more than this is not only fails to contribute additional benefits, but actually reduces these benefits).

Is strenuous running really bad for us?

Looking at the data upon which these claims are based it is hard to see how such a strong conclusion could be drawn. The result that appeared to have generated most of the headlines was one that used the categories of light, moderate and strenuous running, based on the combination of pace, frequency and weekly duration of running. Separate analyses for these three measures were also reported, and I won’t go into those here except to note that all show a similar pattern, and all suffer from the same flaws.

There were 413 sedentary participants, of whom 128 died (31%) over the twelve year period. There were 40 runners who were classified as “strenuous”, of whom two died (5%). The statistics for “light” and “moderate” runners were 7/576 (1%), and 8/262 (3%) respectively. I hope that you can already see a couple of problems with the conclusion that strenuous running is as risky as not exercising at all. Firstly, we should be extremely cautious about how much we can conclude from just two deaths in the strenuous group, especially when we know nothing about the cause of death (because these figures represent all-cause mortality). Secondly, it’s pretty clear that the death rate in the sedentary group was actually much higher than the death rates of all groups of runners.

Now, it’s important to explain that the authors’ conclusion doesn’t come directly from these figures, but from adjusted risk ratios which took into account other factors which differed between the groups (particularly age). The average age of the sedentary group (61) was much higher than that of the runners (which varied between 37 and 46, depending on analysis subgroup). Put in this context, the much higher death rate of the sedentary group seems less striking. However, the two deaths in the strenuous running group are not really rendered any more alarming by the knowledge that this group was mainly comprised of relatively young people (who also tended to score low on other risk factors, like heavy smoking, and symptoms of diabetes). And in fact, the finer details of the risk-adjusted statistics confirm this intuition, because although the estimated mortality risk for this group was higher than that of the light and moderate groups, the confidence interval on that estimate is enormous (much bigger than for any other group), as a consequence of the small sample size. Confidence intervals are statistics which specify a range of values within which a particular parameter is estimated to lie. So the large interval tells us that (as you might expect) we simply can’t generate a precise estimate of typical mortality risk from these numbers.

However, it’s this large confidence interval (indicating a very uncertain estimate, remember) that allows the authors to conclude that “strenuous joggers have a mortality rate not statistically different from that of the sedentary group”. But being unable to confidently conclude that there is a difference is not at all the same thing as being able to confidently conclude that there is no difference. In fact, going by the reported confidence intervals, the mortality rate of the strenuous running group is also not statistically different from the moderate running group, although the authors never mention this. So although it’s true that the study finds no evidence that more is better (although other, much larger, studies of physical activity have done, e.g. Samitz et al., 2011), there is also no evidence to suggest that more is worse.

Where did the 7mph figure come from?

And what about the very specific figure of 7mph, which appeared in the press reports? For me, this called to mind a sinister (and very implausible) medical equivalent of Back to the Future, with runners’ hearts exploding when they exceeded a critical speed. Once again, this detail appears to stem directly from the original article, despite the fact the researchers never actually asked their respondents about their specific running speed. The authors report that, “we included questions on the weekly quantity of jogging, frequency of jogging, and the subject’s own perception of pace (slow, average, fast). We found that a relative scale of pace (intensity) is more appropriate than an absolute scale when the age span is very wide (20 to 95 years) and when the participants have wide differences in levels of physical fitness.” Now, that’s perfectly fine of course, but it means there is no way that these data can be used to draw conclusions about optimal running speeds as measured in miles per hour. Despite this, the authors later describe slow/average pace as “approximately 5 miles per hour”, and fast pace as “more than 7 miles per hour”. So it’s no wonder the press reports latched onto this figure, even though it appears to be completely spurious in relation to this particular study.

Why were the findings reported in such a misleading way?

A counter-intuitive finding always makes for an eye-catching story, certainly much more so than a result that provides further evidence for things we already know (e.g. exercise is good for us!), but when it comes to informing the public about how best to look after themselves, accurate and responsible reporting is particularly important. The NHS blog piece on the news coverage of this study emphasised that encouraging people to engage in physical activity is a much greater problem than the possibility that a very small minority might be pushing themselves a little too hard. By playing up potential (and indeed unproven) risks of physical activity, the media reports did little to inspire healthy lifestyle choices. However, in this instance, the scientists also seemed to play a role in concocting a newsworthy angle on their data.

The BBC deserve credit for improving the accuracy of their article, emphasising the demonstrated benefits of moderate running, and highlighting the limitations associated with the conclusions regarding strenuous running. The piece now draws attention to the very small sample sizes involved, even though this was never mentioned in the text of the original source. I was also pleased to see that the BBC removed the spurious figure of 7mph from their story. The article had probably been shared many times over social media before the changes were made, but it’s definitely a case of better late than never, and if someone follows the link now they will find a much more considered article.

The bottom line

Don’t throw your trainers out just yet. The only really clear conclusion that we can draw from the study is that, overall, running is good for your life expectancy (and yes, I know that’s not exactly news).

Schnohr, P. et al. (2015). Dose of jogging and long-term mortality: the Copenhagen City Heart Study. Journal of the American College of Cardiology. DOI: 10.1016/j.jacc.2014.11.023

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