“Cardio causes heart disease”: misinterpreting science for fun and profit
There’s an article by “paleonutrition” advocate Kurt Harris, posted on the Psychology Today website, that is currently making the rounds, in which he argues that aerobic exercise causes heart disease. The study he’s talking about was published in Radiology back in 2009 by a German group led by Frank Breukmann and Stefan Möhlenkamp. They compared 102 marathon runners between the ages of 50 and 72 with 102 age-matched controls, and used cardiac MRI with late gadolinium enhancement to look for evidence of permanent heart damage. Five of the runners had evidence of past heart attack, and another seven had heart damage not associated with a heart attack, for a total prevalence of 12%. Among the controls, two had evidence of past heart attack, and another two had other damage, for a total prevalence of 4%.
The difference between the two groups wasn’t statistically significant. This doesn’t matter to Harris:
I am obliged to point out that by the conventional arbitrary criteria used in biomedical publishing, the difference was “not statistically significant”… Stop doing what you are told and read the statistics without letting the authors or editors tell you what is “significant”.
That’s certainly a convenient approach: if you already think you know what the right answer is, then you don’t need to worry about little details like statistical significance.
Still, the results are very much worth considering. Harris wonders why the study was ignored by the “nutrition and fitness” blogosphere, and takes a shot at the New York Times:
[I]t’s published in Radiology, which is not exactly Gina Kolata territory.
Actually, the Times ran a Bloomberg article on the early results from this study cohort way back in 2006. And Runner’s World interviewed cardiologist Paul Thompson, a co-author of the most recent results from the study, in January. Thompson points out some important caveats about the study group:
[H]is marathon group includes a number of former smokers and others who might have been quite unhealthy before they began running…
The key issue with Möhlenkamp’s runners is that their cardiac risk scores are compared using their present cholesterol, blood pressure, and other health numbers. They might have had terrible numbers before they started running, so when their coronary calcium is compared with folks who are not athletes, but had good risk numbers all their lives, it looks like the runners had more calcium, ie, more atherosclerosis than predicted by their risk factors… Many of the runners “got religion” when they turned 40 or so.
To his credit, Harris acknowledges that 50% of the runners in the study had a history of smoking, compared to just 42% of the controls — but immediately dismisses the possibility that this (and any other underlying factors that differ between the two groups) could play a role. Again, why worry about statistics and the possibility of confounders when you already know the right answer?
Let me perfectly clear: it’s entirely possible that repeated marathon running (the runners in the study had completed an average of 20 marathons) produces damage to the heart. By all means we should continue to study this, and find out. Despite Harris’s snide comments about aerobic exercise not producing immortality, we’ve known that aerobic exercise doesn’t grant immunity from heart disease for decades. In fact, very first time noted iconoclast Tim Noakes made a splash in scientific circles was way back in 1976, with a paper about heart attacks in veteran marathon runners.
Of course, there’s a difference between running marathons and “aerobic exercise” as practiced (or not) by the vast majority of the population. I’m a lifelong runner, but if someone asked me: “What’s the optimal exercise routine for cardiac health?” I wouldn’t necessarily recommend running marathons. I’d probably suggest something like an hour a day, with two to three days a week devoted to shorter, faster interval workouts. And of course, I would also recommend doing some resistance training — and I wouldn’t take, for example, a study showing that competitors in the World’s Strongest Man competition die early as evidence that “resistance training is bad for you.”
That’s basically what Harris is doing here: concluding, on the basis of this one study of a fairly extreme group of outliers, that aerobic exercise in general is as bad for you as boxing or football. The only sensible exercise, he argues, is resistance training:
I still find no grounds at all to believe that high levels of “cardio” protects your heart or makes you live longer. Certainly not “the more the better” which is what we’ve been led to think since the 1970s running craze.
The reason he finds no grounds, of course, is that he’s never looked. In his article, he waxes philosophic about the fact that “the picture or the test result is not the patient or even the disease.” Then, on the basis of a test result, he concludes that running will kill you. But what if we look at “the thing itself,” and find out whether runners die less from heart attacks — and whether “the more the better” holds up.
Paul Williams did this study and published it back in 2009. He followed 35,402 runners for 7.7 years, during which time 467 men reported heart attacks, angina or had to have bypass surgery, while another 54 died of heart disease. The risks of all these symptoms, including death, decline with every additional kilometre run. Those running more than 9 km/day “produced risks 65% lower for angina, 29% lower for nonfatal CHD, and 26% lower for fatal and nonfatal CHD” compared to those running fewer than 3 km/day (the level corresponding to national physical activity guidelines for adults).
I’ve rambled on a bit here, so let me finish by repeating one simple point. Even if you take the results of Möhlenkamp’s study at face value (which I don’t, as explained above), the conclusion you can draw is that running a large number of marathons may damage your heart. To go from this to arguing that 20 minutes on the elliptical is bad for you requires a Beamon-esque leap of logic, along with a cheerful disregard of literally hundreds of epidemiological studies.