Let me start with a simple truth: there is such a thing as "too much" running. This is just common sense. If your goal is to optimize health, there will come a point where running an extra 5 miles per week doesn't make you healthier in any meaningful way, and eventually there will come a second threshold where running an extra five miles will actually make you less healthy. I don't know anyone, including Runner's World shills like me, who would disagree with this obvious statement.

The debate is about where these thresholds lie. Over the last few years, a lot of attention has been paid to claims that running as little as 20 miles a week could be harmful to your health. This is interesting and worthwhile research, but I think it's worth looking closely at the data that supposedly leads to these conclusions before throwing away your running shoes. Obviously I'm not a completely neutral observer here, since I enjoy running -- so by all means take my observations with a grain of salt. My goal here is to provide you with enough information that you can make up your own mind.

The most recent story to make the rounds (forwarded to me yesterday, and so ridiculous that it's what spurred me to write this!) is coverage of an abstract presented at the annual meeting of the American College of Cardiology earlier this week. The media headlines: "Too much running tied to shorter lifespan, studies find," and many similar variations:

The researchers behind the newest study on the issue say people who get either no exercise or high-mileage runners both tend to have shorter lifespans than moderate runners. But the reasons why remain unclear, they added.

This makes it sound like the new study found evidence that high-mileage runners have shorter lifespans. It didn't. The study is simply an online questionnaire called the MASTERS Running Study that anyone can fill out -- in fact, you can go there now and fill it out if you like. It asks a series of questions about how old you are, how much you run, whether you have high blood pressure, how often you take painkillers, and so on. That's it. That's all. It has no follow-up and no data on how long you live.

The results the researchers presented at the conference (here's the abstract) divided the data into two groups: those who ran more than 20 miles per week, and those who ran less than 20 miles per week. And they found... no differences. Actually, that's not quite true -- the low mileage runners were actually more likely to use NSAID anti-inflammatories. The conclusion is that this data offers no explanation about why high mileage runners might die earlier than low mileage runners. I don't know where the strange media spin on this study came from -- maybe there's a poorly worded press release somewhere? -- but it has nothing to say about mortality and mileage.

So the next question is: why were these researchers even considering the idea that runners who do more than 20 miles a week die earlier than those who do less than 20 miles a week? There are three main pieces of evidence that people cite:

(1) A series of editorials and opinion pieces by a Missouri cardiologist named James O'Keefe and his colleagues over the past few years;

(2) A conference abstract presented at the ACSM annual meeting in 2012;

(3) Recent results from the Copenhagen Heart Study.

I don't have much to say about the O'Keefe pieces, because there's no actual data associated with them. I've blogged before about some of the critiques about the selective presentation of results in those pieces -- as a subsequent letter to the editor of one of the journals put it, "The interpretation of the data provided in the review by O'Keefe et al is misleading, particularly given the response of the authors of the original data." But in the end, it's not all that fruitful to bicker about interpretations. The data should tell the story.

(It's not that O'Keefe's hypothesis is totally crazy or impossible; it's just that it hasn't yet been demonstrated. One of O'Keefe's reviews in 2012, in the Mayo Clinic Proceedings, was titled "Potential adverse cardiovascular effects from excessive endurance exercise." I thought a subsequent response in the journal summed it up quite nicely: "Potential, but unobserved, adverse cardiovascular effects from endurance exercise.")

The ACSM conference abstract is what O'Keefe views as the smoking gun. This drew on data from 52,000 adults in the Aerobics Center Longitudinal Study between 1971 and 2002, of which 27% reported doing some running. While all runners had a lower risk of dying during the study than non-runners, the headline result was that runners who ran less than 20 miles per week had better mortality rates than those who ran more than 20 miles per week. Uh oh.

The problem is that this result is not what it seems. In fact, it suffers from a serious error in statistical analysis: the failure to distinguish between confounding variables and mediating variables. Imagine you're trying to study whether smokers die sooner than non-smokers. You assemble two groups -- smokers and non-smokers -- that are as identical as possible, with roughly the same age, income, and so on. But there may still be some differences. For example, you may find that the smokers are also less likely to wear seatbelts when they drive. This is a problem, because it may lead you to falsely conclude that smoking causes early death, when in fact it's the lack of seatbelt use that's killing them. To handle this, researchers use statistical adjustment to "control" for seatbelt use: essentially they use mathematical techniques to artificially adjust the results so that it's as if both groups had the same rate of seatbelt use. This is entirely appropriate.

But now imagine that you notice another difference between the two groups: the smokers seem to be more likely to have lung cancer, which is also contributing to their earlier deaths. Now, you could use statistical adjustment to control for lung cancer -- i.e. artificially equalize lung cancer rates between the smoking and the non-smoking groups. But this would be an error, because smoking actually causes lung cancer, which in turn causes death. Statistically speaking, lung cancer isn't a confounding variable in the relationship between smoking and death; it's a mediating variable. Controlling for it not only hides the real relationship between smoking and death; it may actually produce the opposite result and suggest that smoking extends lifespan!

So what does this have to do with running? In the ACSM abstract, the results they present are adjusted for "baseline age, sex, examination year, body mass index, current smoking, heavy alcohol drinking, hypertension, hypercholesterolemia, parental CVD, and levels of other physical activities." Some of these factors are clearly confounders, like baseline age, sex, smoking, drinking, and family history. Some of them are debatable, like other physical activity. But some are inarguably mediators: there's no doubt whatsoever that running affects BMI, blood pressure and cholesterol levels, and those factors in turn affect mortality. In fact, those "confounders" are in large part the reason running is thought to extend lifespan -- so statistically eliminating them is akin to asking "If we ignore the health benefits of running, does running have any health benefits?"

I pointed this out in 2012 when the abstract was first presented, and again when O'Keefe relied on those results in an opinion piece in the journal Heart. And I'm not the only one: in a 2013 response in Heart, cardiovascular researcher Thomas Weber wrote:

One possible explanation for the U-shaped curve observed by Lavie and colleagues [4] is that the authors adjust for body mass index, hypertension and hypercholesterolaemia. Running has been shown to lower those risk factors in a dose-dependent fashion with no sign of negative returns until at least 50 miles/week.[5] Arguably, adjusting for all these factors is akin to adjusting for low-density lipoprotein (LDL) values in a study analysing the survival benefit of taking statins to treat hypercholesterolaemia. Put simply, this editorial represents a selective interpretation of the available data, at the best.

In a response to the response, the authors basically said "It doesn't matter, trust us." That may be true, but no one can assess that until the data is actually published in a peer-reviewed journal, in unadjusted form -- which, at this point, it still hasn't been. As Weber wrote: "[T]he authors provide no evidence that is published in a peer-reviewed journal supporting this sweeping claim. Instead, their conclusion is mostly based on two unpublished studies, in one of which Dr Lavie is the senior author. The published studies that they quote [2,3] show just the opposite."

That brings us to the third, and best, piece of evidence for the dangers of too much running, which comes from the Copenhagen City Heart Study. This is a well-run study that started in 1976 and now includes more than 17,000 subjects; you can read the full text of their analysis of the effects of jogging here. The first thing you'll notice if you read the abstract is that it doesn't say anything about "too much running" -- in fact, its main conclusion is "The age-adjusted increase in survival with jogging was 6.2 years in men and 5.6 years in women. This long-term study of joggers showed that jogging was associated with significantly lower all-cause mortality and a substantial increase in survival for both men and women."

However, if you dig a little deeper, you'll find a small sub-analysis where they looked at the effects of how long and how fast the subjects ran. And sure enough, they found a U-shaped relationship, in which the best outcomes accrued to those who ran less than 2.5 hours a week at a slow pace. But there's a catch: only about 10% of the subjects were joggers, and they didn't start asking questions about quantity and pace of running until the later years of the study. Why does this matter? Well, take a look at the numbers for mortality versus jogging pace: in the group that reported their subjective jogging pace as "slow," there were 3 deaths out of 178 people; in the "medium" group, there were 12 deaths out of 704; in the "fast" group, there were 5 deaths out of 201. These are very small numbers. It may turn out that the effect was real -- but if one slow jogger had been hit by a bus sometime in the last few decades, the mortality rate for that group would have jumped by 33% and we'd be talking about the "dangers" of slow jogging!

That, to my knowledge, is the sum total of the evidence that running modest amounts (say an hour a day or less) might have negative effects on your health. But you have to weigh these findings against the avalanche of literature that finds health benefits far beyond the thresholds proposed by O'Keefe and his colleagues. For example, here's a graph from the study I mentioned above, a longitudinal study of more than 400,000 people, that O'Keefe strangely cited as support for his hypothesis:

No matter what pace you're running, there's no way you'll be maxing out of the health benefits observed in this graph on just 20 miles per week. Similarly, Paul Williams' National Runners' Health Study has consistently found that the health benefits of running keep increasing, even for people running more than 39 miles per week. Here's a graph showing cardiovascular benefits at different mileage levels, from an analysis of 35,000 runners (for more details on what this graph shows, see my earlier blog entry):

Running 20 miles per week would put you in the second group from the left; the two higher mileage groups continue to see improved health markers.

Okay, this post is reaching novel-like length, but there's one more study I'd like to briefly mention. Last week, O'Keefe and his colleagues published a slightly different study, looking at the build-up of arterial plaque in 50 runners who had run 25 consecutive Twin Cities marathons. This is a topic that runners should be paying attention to, and I think Kevin Helliker in the Wall Street Journal did a good job extracting the right message from the findings: "Why Runners Can't Eat Whatever They Want." Sure enough, 30 of the 50 runners (average age: 59) had plaque build-up, which is about what you'd expect with or without running. The old Quenton Cassidy credo -- "If the furnace is hot enough, anything will burn, even Big Macs" -- may be true for staying thin, but it's not true for arterial health. Runners have to worry just as much as anyone else about risk factors like family history and diet.

So far, so good. The more controversial part of the study is that they compared the runners to a "matched" sedentary control group, and found that, while the prevalence of arterial plaques was the same, the runners seemed to have larger plaques. So, in addition to not making you immune to heart disease, does running (or at least running marathons for 25 straight years) actually cause an increase in plaque size? It's possible, and worth exploring further. But again, this study has same flaw as the previous one.

The control group was explicitly chosen to have "similar coronary risk factors" as the runners -- despite the fact that, as the paper itself notes, running "confers advantages in many traditional coronary risk factors including favorable changes in lipid levels, glucose metabolism, and blood pressure." To understand why this is a problem, imagine you were doing an interventional trial: you start with two evenly matched groups, and ask one group to run marathons for the next 25 years while the other stays sedentary. After 25 years, the marathon group will have significantly better cardiovascular risk scores, as noted above. And in that context, you can reasonably ask what effect running had on arterial plaque.

But that's not what we have here. Instead, the researchers selected controls who somehow managed to have similar coronary risk factors as the runners without running. This means that they were an abnormally healthy group in other respects, perhaps because of family history or other health-related habits. In his piece on this study, Amby Burfoot noted the odd fact that 52% of the running group were current or former smokers, while just 39 percent of the non-running controls were. This isn't just an odd coincidence -- it's an inevitable consequence of trying to artificially "match" runners and non-runners for traits that are directly affected by running, and it produces exactly the same problems as the faulty statistical adjustment in the earlier study.

This doesn't mean that I'm dismissing the possibility that running could have a negative effect on arterial plaques. It's certainly true that runners can have blocked arteries -- and so far the best way to predict whether you're likely to suffer from them is using the exact same risk factors (like family history) that apply to non-runners. But it will take more than this study to make the case that runners are more likely to have these problems.

So, did I convince anyone who didn't already agree with me? ... Rats, I didn't think so. :) As I said at the top, the question isn't whether there's such a thing as "too much"; it's where that threshold is. Right now I think it's fair to say that nobody knows the answer -- or perhaps that the answers we come up with depend on the subtle nuances of exactly how we ask the question.

There's one thing I'll concede: if you really hate exercise and you're looking for an excuse to do as little as possible, the results discussed above offer some pretty good evidence that doing the equivalent of 20 miles of running per week will give you most of the health benefits available from running. On the other hand, if you love exercise and are wondering whether you should cut back to prolong your life, the results above make it pretty clear that running an hour a day is certainly no less healthy, and probably a bit healthier, than running less. But if you choose to follow that path, make sure you're doing it for the right reasons: because you enjoy it.

***

Read , and follow the latest posts via Twitter, Facebook, or RSS.

This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses. You may be able to find more information about this and similar content at piano.io