The headline in the New York Times — “Can Exercise Keep You Young?” — hedged its bets but the scientists would produced a groundbreaking study were much more certain. And so, too, was the writer of the story, Gretchen Reynolds, with whom I once rode mountain bikes from Telluride to Moab: “Exercise reduced or eliminated almost every detrimental effect of aging in mice that had been genetically programmed to grow old an an accelerated pace,” she wrote.

The study doesn’t just show that exercise suspends aging, but how it works.

The effect takes place at the cellular level. (That’s body cells, people, not iPhones.) Within each cell are mitochondria, which create the energy on which the cell runs. Mitochondria has its own DNA, which naturally mutates and breaks down. As we get older, the cells lose their ability to repair those snippets of wayward code, and scientists believe that’s what leads to hair loss, graying, brain deterioration, shrunken sex glands, and the other not-so-fun manifestations of leaving your youthful indiscretions in the rearview mirror.

In the study by McMaster University in Ontario, mice were genetically reprogrammed to block their mitochondrial repair systems and thus age preternaturally fast, then placed in several groups. There was a control group and then there were the über athletes, a selection of mice that were subjected to reasonably challenging exercise three days a week: 45 minutes per session at the equivalent human pace of running an 8.5-minute mile.

The results were astonishing. The control mice were nearly crippled by old age by eight months, about 60 or so in human years. None lived past one year. The exercising mice, on the other hand, were robust and muscular, with little to no deterioration in brains, follicles, or gonads. They acted liked teenagers, balancing on little rods, and by the end of the year none of them had died of natural causes.

“But perhaps most remarkable,” Reynolds writes, “although they still harbored the mutation that should have affected mitochondrial repair, they had more mitochondria over all and far fewer with mutations than the sedentary mice had.”

Put more succinctly by Dr. Mark Tarnopolsky, the author of the study, “Exercise alters the course of aging,” he said.

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