Tarnopolsky has published some of those papers himself. In 2011, he and a team studied mice with a terrible genetic disease that caused them to age prematurely. Over the course of five months, half of the mice were sedentary. The other half were coaxed to run three times a week on a miniature treadmill.

By the end of the study, the sedentary mice were barely hanging on. The fur that had yet to fall out had grown coarse and gray, muscles shriveled, hearts weakened, skin thinned–even the mice’s hearing got worse. “They were shivering in the corner, about to die,” Tarnopolsky says.

But the group of mice that exercised, genetically compromised though they were, were nearly indistinguishable from healthy mice. Their coats were sleek and black, they ran around their cages, they could even reproduce. “We almost completely prevented the premature aging in the animals,” Tarnopolsky says.

That’s remarkable news, if you’re a mouse. And though there are obvious differences between rodents and humans, Tarnopolsky has seen something similar happen in his ill patients. “I’ve seen all the hype about gene therapy for people with genetic disease”–Tarnopolsky treats kids with severe genetic diseases like muscular dystrophy–“but it hasn’t delivered in the 25 years I’ve been doing this,” he says. “The most effective therapy available to my patients right now is exercise.”

Tarnopolsky now thinks he knows why. In studies where blood is drawn immediately after people exercised, researchers have found that many positive changes occur throughout the body during and right after a workout. “Going for a run is going to improve your skin health, your eye health, your gonadal health,” he says. “It’s unbelievable.” If there were a drug that could do for human health everything that exercise can, it would likely be the most valuable pharmaceutical ever developed.

The consequences of a sedentary life are as well documented as they are dire. People with low levels of physical activity are at higher risk for many different kinds of cancer, heart disease, Alzheimer’s disease and early death by any cause. That’s at the end of life. Long before that, inactivity can worsen arthritis symptoms, increase lower-back pain and lead to depression and anxiety–not to mention cause a sallow complexion.

Despite public-awareness campaigns, the health benefits of exercise have not been effectively communicated to the average American. Humans are notoriously bad at assessing the long-term benefits–and risks–of their lifestyle choices. And vague promises that exercise is “good for you” or even “good for the heart” aren’t powerful enough to motivate most people to do something they think of as a chore. Humans are, however, motivated by rewards. That is why experts like Tarnopolsky are so focused on proving that the scientific benefits of exercise–slower aging, better mood, less chronic pain, stronger vision, the list goes on–are real, measurable and almost immediate.

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