Exercise is medicine. That's the slogan of a new initiative being promoted by the American Medical Association and the Canadian Society for Exercise Physiology, and it reflects a growing realization in the medical community that the cheapest and most effective tool to fight dozens of conditions ranging from diabetes to depression might be a jog in the park.

But like all medicines, the devil is in the dosage. We all know the perils of too little exercise, but researchers are just starting to wrestle with the opposite question: How much is too much? And better yet, how much is just enough?

Two years ago, preliminary results from a study presented at the annual meeting of the American College of Sports Medicine produced headlines that made joggers blanch and couch potatoes smile: "Running can shorten your life," The Globe's website warned, reporting that as few as three or four hours a week might be enough to damage your heart and wipe out the benefits of lesser amounts of exercise.

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The ensuing debate forced runners and non-runners alike to reconsider long-held assumptions about exercise and health, and try to distinguish between what the evidence showed and what they wished it showed. There's just one problem: The original analysis was deeply flawed, as the publication of the full study last month reveals. Instead of warning about the dangers of too much, the study now highlights the powerful benefits of as few as five to 10 minutes of running a day to cut the risk of death from heart disease in half.

The researchers, from Iowa State and several other universities, followed 53,000 patients at the Cooper Clinic in Texas for an average of 15 years after their initial checkup. In the original analysis, runners were 19 per cent less likely to die during the study than non-runners – but that mortality advantage disappeared for those who reported running more than 32 kilometres a week or faster than about five minutes a kilometre.

Even at the time, the findings spurred a backlash. Dr. Larry Creswell, a triathlete and heart surgeon at the University of Mississippi School of Medicine, whose Athlete's Heart blog discusses cardiac health for athletes, pointed out that conference presentations, unlike medical journal articles, haven't yet gone through peer review. "Essentially, if you're invited to speak at a meeting you can say what you want – whether it's scientifically correct or not," he wrote when the findings were first presented. Others questioned the statistical methods used to analyze the study's data.

"One thing that's striking is that when it comes to factors affecting long-term health, it's really hard to change people's beliefs," admits Dr. Robert Gazzale, a dedicated runner and an economist at the University of Toronto, whose research interests include how people weigh the short-term and long-term consequences of their decisions. "I saw this in myself. When I read about the initial results, I was rather skeptical … I want to believe that running – a lot – is good for my health."

So how did the message of the Cooper Clinic study change so dramatically?

In actuality, it's utterly uncontroversial to say that there is such a thing as "too much," whether you're talking about running, exercise in general or pretty much anything health-related. "In the physical and biological worlds, there seems to be a sweet spot for everything," Creswell says. "It must be true for exercise, too."

The only debate is where that point lies. At the far end of the scale, a 2011 study found evidence of fibrosis – scarring, essentially – of the heart in six of 12 extreme ultraendurance athletes who had each completed an average of 178 marathons, 65 ultramarathons of 80 km or longer and four Ironman triathlons. It's not clear that this fibrosis had any negative effects, but it suggests that cardiac wear and tear can indeed accumulate.

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The Cooper Clinic study was noteworthy because it suggested negative effects show up at a much lower level of exercise, not just in serial ultramarathoners. But there was a flaw in the original analysis, as cardiac researcher Thomas Weber of the Icahn School of Medicine at Mount Sinai in New York pointed out in a letter to the editor in the journal Heart.

The results were "adjusted" for body-mass index, blood pressure and cholesterol, meaning that the researchers used statistical techniques to artificially equalize these parameters among the various groups of runners and non-runners. This is standard practice in epidemiological studies to eliminate confounding variables – for example, if one group happens to be older or younger on average than another, statistical adjustment can correct for age.

But in a study of the health effects of running, BMI, blood pressure and cholesterol are not "confounding" variables, Weber pointed out. Running directly lowers all three, so we should expect runners to have lower values on average. Artificially equalizing them hides – and perhaps even reverses – the effects you'd expect from running.

Sure enough, peer reviewers pointed out this flaw when the results were submitted for journal publication – and it no longer appears in the final analysis. That changes the conclusions considerably, since the benefits of running no longer seem to lessen with increasing mileage.

As a result, the take-home message is dramatically different and communicates that you don't need to train like a marathoner to be healthy – but neither is it unhealthy to train like a marathoner.

That's the conclusion Catrin Jones, a top ultra and trail runner from Victoria, has taken. The debate about heart health and running didn't dent her enthusiasm (or her mileage accumulation), but a prolonged struggle with an injured hip and a possible stress fracture over the winter has reminded her that more isn't always better.

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"I'm still figuring out how much training in one week is ideal for me," she admits. "By gauging how I feel both physically and mentally, I can alter my training load on a day-to-day basis."

Creswell, too, continues to swim, bike and run enthusiastically. But he worries that the original message about too much exercise may linger much longer than the corrected one. "I have this uneasy sense that primary-care providers – the professionals that relatively healthy people visit – are subject to the headlines," he says.

Still, if excessive exercise does cause harm, he'd like to know, so he hopes research into the question continues.

"Is the debate worthwhile?" he asks. "Sure. In the right context. In general, though, we have a problem with too little exercise, not too much."

Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com.