Before last year’s World Half Marathon Championships in Gdynia, Poland, a young British distance runner named Jake Smith had a call with scientists from a small company his agent had connected him with. They’d crunched the data from his performance two weeks earlier at the London Marathon, where he’d struggled in his assigned role as a pacer, and had a simple message for him: “They literally said, ‘You need to eat more,’” he recalls.
On the back of his upper right arm, the 22-year-old was wearing a circular adhesive patch about an inch across, with a tiny filament embedded into his flesh. It was a continuous glucose monitor, or CGM—a device designed to track real-time blood sugar (also known as glucose) levels in diabetics, repurposed for athletes by an Atlanta-based start-up called Supersapiens in collaboration with the medical device giant Abbott. The data Smith uploaded after London showed that