he word marathon comes from a Greek legend. The story goes that Philippides, a messenger, ran from the Battle of Marathon all the way to Athens, some 26 miles away, to deliver news of the Athenians victory over the invading Persians.

Joy, he announced. We have won! And then he dropped dead.

Most marathoners I know recognize that running 26.2 miles is not good for their body. Its a punishing physical ordeal, which is kind of the point; if it were easy, why do it? Most have also heard (and, frankly, are probably tired of hearing) about the links between marathon running and heart problems from mid-race cardiac arrest to lasting structural deformations.

That marathon research raises some important questions about the longevity consequences of various exercise volumes. How much physical activity is too much? And what amount is optimal?

Those are questions some recent studies have attempted to answer.

Last year, research in the journal Mayo Clinic Proceedings examined the links between weekly exercise and mortality outcomes. Using long-term data from nearly 9,000 adults, the study found a U-shaped association between total weekly exercise time and the risk for cardiovascular and all-cause mortality.

While higher volumes of weekly sports training (cardio exercise, but also ball sports, weight-lifting, and anything that makes you sweaty or short of breath) initially led to a big drop in mortality risk, those benefits began to regress among people who trained hard for more than 4.5 hours per week. Their mortality risks were still much lower than those of non-exercisers, but they didnt fare quite as well as the people who took a more measured approach to physical activity.

Participation in sport activities should be promoted, but the potential risk of very high weekly hours of sport participation should be considered, the studys authors concluded.

Excessive strenuous exercise can actually erase some longevity benefits seen with moderate exercise.

Back in 2015, the same research group found a similar U-shaped risk curve among runners.

Those who ran between 60 minutes and 2.4 hours per week had the best mortality outcomes, but the benefits started to diminish among those who ran more. In fact, the most strenuous runners defined as running at a pace of at least 7 miles-per-hour for four or more hours per week had mortality rates on par with sedentary adults who didnt run at all. Long-term strenuous endurance exercise may induce pathological structural remodeling of the heart and large arteries, that study team wrote.

After that study was published, I spoke with one of its authors for a piece about the perfect amount of running.

James OKeefe, MD, is a cardiologist and medical director of the Duboc Cardio Health & Wellness Center at Saint Lukes Mid America Heart Institute in Kansas City. He told me that running and other forms of intense exercise are healthy, and most people arent moving enough. But he also said that fitness and longevity dont always go hand in hand. Excessive strenuous exercise can actually erase some longevity benefits seen with moderate exercise, he said.

Not everyone is convinced that heavy endurance training is risky. Some expert commentators have pointed out that its difficult to parse long-term exercise data, and there are a hundred confounding variables that could skew these sorts of findings.

On the other hand, it makes sense that, at a certain point, more exercise especially more of the same exercise is just added wear and tear. Even if your heart isnt harmed, your joints will eventually pay a price.

If youre aiming for optimal, the best heart and mortality outcomes are associated with roughly 150 minutes of weekly vigorous exercise, and no more than 60 minutes of weekly weight training.

New research on rodents supports the theory that too much exercise can damage the heart in ways that reduce lifespan.

That work, which appeared earlier this year in the Journal of the American College of Cardiology: Basic to Translation Science, found that intense exercise the human equivalent of running 60 minutes a day, five days a week for 1012 years promoted several forms of arterial stiffening and thickening, as well as imbalances in the enzymes that control the contraction and relaxation of the heart.

SEO tip accompanying that JACC piece argued that its not feasible to perform a randomized controlled trial to look at the effects of different exercise volumes on long-term health outcomes in humans; issues with costs, monitoring, ethics, and adherence make this sort of study practicably DOA.

The best we can do is to look at real-world human data (like the observational studies above), and combine those with mechanistic evidence derived from animal models. These findings indicate a possible upper limit for the health benefits of exercise, the editorial argued.

So what is that upper limit?

To find out, I got back in touch with OKeefe, the Kansas City cardiologist, who has probably published more research on this topic than any other active investigator. (More than a dozen peer-reviewed papers, and counting).

For runners, he cautions not more than 4.5 hours per week or 30 miles per week. Thats the max, not the ideal.

Middle-aged or older adults should probably run much less. I tell people that, after 40 or 45, their heart is much less resilient and thus more prone to cardiac overuse injury, he says. Once youve reached your fifth decade of life, his work indicates that swapping out some of your heavy cardio training for other, less-strenuous forms of physical activity walking, yoga, light hiking, easy cycling, gardening, etc. is a good idea.

Yes, throttling back will cause your fitness capacity to decline. But what you lose in pace youre likely to gain in lifespan. Also, by mixing things up, youre more likely to avoid the kinds of repetitive-use injuries that are so common among middle-aged and older athletes.

Unfortunately, a lot of us take just the opposite approach. We seldom exercise, taking our health for granted. But then 35 or 40 rolls around, bringing with it lifes first gray hairs or significant health scares, and thats when we decide to start running marathons. (The average age for women finishing the New York City Marathon is 40, and for men its 42.)

If youre aiming for optimal at any age, or what OKeefe calls the Goldilocks zone for exercise and health, he has new research, slated for publication later this year, that finds the best heart and mortality outcomes are associated with roughly 150 minutes of weekly vigorous exercise, and no more than 60 minutes of weekly weight training. (When it comes to light exercise, such as walking or gardening, theres no cap.)