e study authors joked, a runner could potentially run himself back to couch potato.
That study and others all build evidence for the U-shaped curve of exercise benefits: that greater effort begets greater benefits, but only to a certain point. Then the benefits drop off, suggesting that there's probably a maximum amount of exercise before it starts to become unhealthy.
In another way to look at the U-curve, someone running about 16km a week may actually be healthier than someone running about 96km a week.
While the U-curve is pretty widely accepted, there's disagreement over how much exercise may be too much. Many scientists say, assuming there exists a maximum safe amount of running, it's probably so high that very few runners will ever get there.
"There are probably levels of exercise that are excessive for the average individual," said Dr Anthony Luke, medical director of the San Francisco Marathon. "But we can't arbitrarily say too much is 50 miles (80.45 km) a week, or 100 miles (160.9 km). I don't think everyone has the same limits."
The concern, according to Luke and other sports-medicine experts, is that there does seem to be some small percentage of runners who are at risk for fatal heart attacks when they run at a certain distance or intensity. It's a rare thing - for every 100,000 runners who compete in marathons every year, only one dies - but "no one should be dying at a marathon," Luke said.
But rather than setting safe limits for running, the better approach, many running advocates and sports doctors say, is finding out why those people die, and how their deaths could be prevented.
A middle-aged or older person who's taking up running for the first time should get checked out by a doctor and screened for possible heart conditions.
Plus, doctors and scientists don't yet fully understand exactly what happens to athletes' hearts when they run long distances or at a sustained hard pace. A recent study by the Mayo Clinic, where researchers took care to note the various and many positive health effects from exercise, analysed changes to the heart and blood vessels that are sometimes found in endurance athletes.
The Mayo Clinic scientists found occurrences of irregular heartbeats, enlarged hearts and thickened artery walls, but it was unclear how many, if any, of those effects were potentially harmful or whether they were pre-existing conditions. And it was also not known how many endurance athletes actually experience those effects.
"Any blanket statement about these runners is sure to have exceptions to it," said Dr James Fries, a professor of immunology and rheumatology at Stanford who has studied runners for decades. "We study the mean, and we report what happens to this group or that group. But nobody's actually the average."