Melbourne, Australia

After his 2014 season ended with a leg injury, Milos Raonic, a 6-foot-5 Canadian tennis star who serves thunderbolts, made a few resolutions. At the top of his list: Stop eating fatty macadamia nuts.

To win Grand Slam titles, Raonic decided that he needed to join the ranks of the ultra-lean. Like Novak Djokovic, who...