Can Running Kill You? (part 4)

Second Thoughts on "Too Much Running"
By
Alex Hutchinson
Published
June 05, 2012

News outlets are yet again reporting about the potential dangers of too much exercise, thanks to a couple of new studies. Over at the Peak Performance blog, Amby Burfoot has done a nice job of putting these findings into context; I'd recommend heading over there to read it. A couple of additional thoughts:
(1) Here's Amby's closing line:

    Many aspects of exercise and running also follow a U-curve. This is why many people believe the moderate approach is the smartest path to follow. Of course, you’ll never qualify for the Boston Marathon that way. We all have to make our choices.

I think that pretty much sums it up. There's not much doubt that, if your goal is to optimize health, running, say, three hours a day as hard as you can is overkill. This is obvious. That's why I'm always surprised by comments like this (from the press release accompanying one of the new review articles):

    But what this paper points out is that a lot of people do not understand that the lion's share of health benefits accrue at a relatively modest level... Beyond 30-60 minutes per day, you reach a point of diminishing returns.

Really? Do a lot of people not understand that? Maybe I'm naive, but I've never met anyone who chose to lengthen their daily training run from 60 to 70 minutes in pursuit of better health. Once you get past an hour a day, I think the vast majority of people are looking for something beyond health.

(2) Okay, so if we accept that running beyond a certain amount is "sub-optimal" for health, the next question is: How a big a health penalty do we pay if we persist in our foolish ways? Amby's blog reports the "hazard ratios" associated with different levels of mileage in new data being presented at the current ACSM conference. In that study, those who ran 15-20 miles per week were 25% less likely to die during the study period than the control group. Those who ran more than 25 miles per week were only 5% less likely to die. So in this study, "moderate" mileage was healthier than "high" mileage, but high mileage was still healthier than no mileage. [These statistics are misleading, though -- more on that below.]

(3) The trickiest question is: what is the threshold beyond which I get less benefit from more exercise? The data in the new study pegs it at somewhere between 10 and 20 miles per week for this particular cohort. But other studies have found different results. For instance, here's some data from Paul Williams' National Runners' Health Study, looking at measures of heart health (angina and coronary heart disease, in this case) in 35,000 runners:

As you can see, the benefits keep getting better for those running more than 9 km per day (which works out to 39 miles per week). Now, there's a very, very crucial point here. The first (black) bar shows the risk without adjusting for bodyweight: big effect. The second (gray) bar shows the risk adjusting for the bodyweight of the subjects before they started running regularly: again, big effect, showing that the results don't simply reflect a "self-selection" effect with leaner people choosing to run farther. The third (white) bar shows the results adjusting for current bodyweight -- and this time, the effect is smaller. This means that the heart benefits of running stem, in part, from the well-established fact the people who run more tend to maintain lower bodyweight. Not a big surprise.

But here's the big problem with the new study being presented at the ACSM (i.e. the hazards ratios in Amby's blog entry): the results are statistically adjusted to remove the effect of bodyweight. So it's telling us that running more than 25 miles per week doesn't provide extra health benefit if you ignore the effect of bodyweight. (And the results are also adjusted to remove the effect of several other parameters that running strongly affects, like blood pressure.) That seems like a pretty misleading comparison to me -- like saying that being tall doesn't make you a better basketball player as long you as assume that everyone is 6'8".

Bottom line: I agree with the general message that, from a health standpoint, moderation is the way to go. But I'm not at all convinced that these apparent thresholds of less than 20 miles per week have any meaning whatsoever.