This week, the Journal of the American College of Cardiology published a study claiming that, while jogging was associated with lower rates of long-term mortality from all causes, the optimal “dose” of jogging was uncertain. Actually, the study went a little farther than that, strongly suggesting that those who engaged in what the study called “strenuous jogging” had mortality rates similar to sedentary people. In other words, a little jogging was a good thing, but a lot was enough to reverse all the benefits. Here’s how the study put it:
“The findings suggest a U-shaped association between all-cause mortality and dose of jogging as calibrated by pace, quantity, and frequency of jogging. Light and moderate joggers have lower mortality than sedentary non-joggers, whereas strenuous joggers have a mortality rate not statistically different from that of the sedentary group.” (Dose of Jogging and Long Term Mortality – The Copenhagen City Heart Study)
I believe that studies like this one, and the glib articles that report on them, are guilty of contributing to scientific illiteracy, and to the large numbers of people who say that they don’t trust “science” or consider a science-based statement as only one of many points of view that deserve equal consideration.
And yet, the study does tackle an interesting question: are there long-term risk factors associated with a lifestyle that includes frequent, intense exercise?
As an aging runner myself, I think it’s important to keep an open mind about this question. Of course, I consider running to be an important part of my life and an activity that brings me a great deal of pleasure as well as benefits to my physical and mental health. But running and training hard won’t make me immortal, and racing well isn’t necessarily the same as being well. All of the things I value about running don’t preclude the possibility that — to borrow a phrase — the good times are killing me.
But this study is so rife with methodological problems, and its conclusions so unsupported by the underlying data that — other than generating sensationalist headlines about the dangers of too much running — it doesn’t add anything to the discussion.
For example, the study looked at 1,098 healthy joggers and 3,950 healthy non-joggers over a 12-year period. Joggers were classified into “light,” “moderate,” and “strenuous” groups based on self-reported jogging activity. There was a large difference in the number of joggers in each of these groups. Furthermore, as far as I can tell from the abstract, other exercise habits did not factor into the classification.
So already, a number of problems arise:
- By limiting the study to healthy people, some potential benefits of jogging (keeping people healthy enough to be in the study) might be obscured.
- By not considering other exercise activities, there’s no reliable way to assess the total amount of cardiovascular exercise for any of the groups.
- By having such a small number of “strenuous joggers,” the study appears “underpowered” — that is the sample size is not large enough to conclude that the observations are due to a real effect and not chance.
Regarding this last point, the study found that of 40 participants categorized as strenuous joggers, after 12 years 2 of them had died. Had even one of them managed to survive, the mortality rate would have been cut in half, and presumably, the study would have reached quite different conclusions.
Also, because the top-line number in the study does not take into account the specific cause of death, there’s little one can say about how or why running specifically contributes to mortality risk. I can’t tell whether this was addressed within the actual study, but media outlets reporting on it made some fairly wild speculative statements about the potential dangers. For example, the BBC of all organizations had an article with the title “Training very hard ‘as bad as no exercise at all’” (notice the use of those cute single-quotes), in which they mentioned “…other work which has suggested long-term strenuous endurance exercise can damage the heart.” The only work cited was a study on mice.
I’m sorry, this might be the BBC and all, but it’s still bad journalism. It reports on a result from a study with numerous problems, and encourages speculation about the reasons for the result that are entirely unjustified by the preponderance of current research. Essentially, it weaves a narrative about (unproven) exercise-induced structural damage to the heart and the (unproven) overall risk that results. It uses hopelessly vague descriptions of activities (“strenuous jogging” “training very hard”) and fails to qualify the conclusions about them.
What is a reader supposed to think?
As a “strenuous jogger” myself, it took me only a few moments to do the calculations. Averaging a measly 40 miles per week and a humble 7:30 per mile, I’m spending fully five hours per week running, a full hour more than the fatal threshold of four hours per week cited in the Danish study. According to the conclusions of the Danish researchers, I’m as likely to die in the next twelve years as my sedentary neighbor who wouldn’t run if his pants were on fire.
I could live with that, no pun intended, since I have no animus towards my sedentary neighbor and no reason to hope he predeceases me, but I resent the implication that it’s not jogging, per se, that’s dragging me down, it’s the excessive amount of time I spend on the habit. If I reformed my habits right away and cut my jogging time in half, would I squeeze out another few years? Would I actually live longer?
Well, as the old joke says, maybe it would only seem longer.