In 1999, Peggy Bryant, a fifty-year-old oncology nurse in Boston, received a postcard asking whether she’d like to take part in a clinical trial aimed at preventing diabetes. Well, this is fitting, she thought. How many patients have I asked to enroll in trials? Bryant, who’d long struggled with her weight, told me that she had cared for people dealing with grave complications of diabetes—vision loss, kidney failure, limb amputations—and had worried that “full-blown diabetes might be in my future.”