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Phys Ed Gretchen Reynolds on the science of fitness.

The ideal amount of running for someone who wants to live a long and healthy life is less than most of us might expect, according to a new study, which also suggests that people can overdo strenuous exercise and potentially shorten their lives.

There is increasing consensus among physicians and exercise scientists that people should exercise intensely at least sometimes. Past studies have found, for instance, that walkers who move at a brisk pace tend to live longer than those who stroll, even if they cover about the same distance.

Similarly, a 2012 study of cyclists in Denmark concluded that those who regularly rode hard tended to live longer than those who rode gently, even if the easy riders put in more hours on the road each week.

But that result, while intriguing, felt unsatisfyingly vague to the Danish researchers. It did not delineate just how much intense exercise might be most protective against premature death. It also didn’t address whether there could be a ceiling to the benefits from vigorous exercise and, in terms of lifespan, whether someone might work out too much.

So for the new study, which was published this month in The Journal of the American College of Cardiology, the researchers, most of them affiliated with the University of Copenhagen, turned to the enormous database about health habits among Danes known as the Copenhagen City Heart Study.

In this case, instead of focusing on cycling, the researchers decided to look at jogging, since it is the most popular strenuous activity worldwide.

The researchers culled data for 1,098 adult men and women of varying ages who, upon their entry into the study in 2001, had identified themselves as joggers. They also had provided information about how often they ran per week, at what pace, and for how long.

The researchers also pulled records for 3,950 age-matched volunteers who had said in 2001 that they did not engage in any type of vigorous exercise or, in fact, any exercise at all.

All of the volunteers were generally healthy, however, without evidence at the time of disease or obesity.

Then last year, the researchers compared the names of the volunteers in both groups against death records. They also determined whether, based on average life expectancies, the volunteers were living longer, or had shortened lifespans.

As it turned out, and as expected, joggers consistently tended to live longer than people who did not exercise.

But when the researchers closely parsed the data about how much and how intensely people jogged, some surprises emerged.

The ideal amount of jogging for prolonged life, this nuanced analysis showed, was between 1 hour and 2.4 hours each week. And the ideal pace was slow. (The researchers did not specify exact paces in their study, using instead the broad categories of slow, average and fast, based on the volunteers’ self-reported usual pace.)

Plodding joggers tended to live longer than those who ran faster. In fact, the people who jogged most often and at the fastest pace — who were, in effect, runners rather than joggers — did not enjoy much benefit in terms of mortality. In fact, their lifespans tended to be about the same as those who did not exercise at all.

The results suggest that the “optimal dose of jogging is light, and strenuous joggers and sedentary non-joggers have similar mortality rates,” said Jacob Louis Marott, a researcher for the Copenhagen City Heart Study and a co-author of the study.

You can, in other words, potentially run too much.

Of course, there are caveats. The number of hardcore runners in the study was quite small, for one thing, consisting of barely 80 men and women. So any statistical information about death rates among that group must be viewed cautiously, as the scientists acknowledge.

And perhaps most important, the researchers did not determine how and why the runners and nonrunners had died. So it is impossible to draw any conclusions about what deleterious effects, if any, hard and prolonged exercise might have on our bodies. There could be scarring or other impacts on the heart muscle after years of strenuous exercise, the Danish scientists suggest, though that possibility remains completely speculative at the moment.

So the message of this study remains that sweaty exercise is generally healthy and desirable – but a little sweat goes a long ways. Even slow jogging counts as “vigorous exercise,” Mr. Marott said and, as this study showed, can lengthen lifespans.

Related Stories: Read John Schwartz’s “Happy to Be the Tortoise in the Race to Fitness” and Tara Parker-Pope’s “A Marathon Run in the Slow Lane.”