Moving up or down a weight class? Switching positions within your sport? Competing in a new league or level? Are you big enough, quick enough, and strong enough?

Elite athletes understand the impact that body weight and composition have on performance. Gain too much, and lose that all-important first step. Drop too much, and risk being overpowered by the opponent.

In The Athlete's Guide to Making Weight, renowned sport dietitians Michele Macedonio and Marie Dunford have analyzed today's top athletes, competitive trends, and positional demands across 21 sports to help...