Last winter, JaMarcus Crews forced his feet, however numb, to walk a paved public track in the town of Centreville, Alabama, until his calves cramped and sweat bloomed across his T-shirt. He knew the route well, from Library Street to Hospital Drive. He’d walked it as a kid, when he was diagnosed with Type 2 diabetes and determined to shed weight. With freckled cheeks and soft eyes, JaMarcus was built big: 6-foot-1, wide shoulders, a round torso on skinny legs. Now, at the age of 36, he was back again. The diabetes had destroyed his kidneys, and he was trying to slim down so that he could get a transplant.