Former Cleveland Browns offensive lineman Joe Thomas fueled his NFL workouts with 10,000 calories a day. Now, he would need about five days to eat that much. After retiring in March 2018, Mr. Thomas radically overhauled his diet and exercise.

During his 11-year NFL career, the 10-time Pro Bowler tipped the scales at 325 pounds. “When you’re practicing three hours a day in pads and a helmet in the heat, your muscles need fuel,” he said. “I spent years training myself to turn off my stomach-brain connection. When my stomach...