In the months leading up to the Olympic games around 444 B.C., Ikkos of Tarentum, a legendary Pentathlon champion, began routine preparations for competition. He consumed large quantities of wild boar, cheese and goat meat. He hit the gymnasium to practice javelin throwing and long jumps. He coated himself in olive oil to make his rippled body gleam. But there was one thing that made Ikkos's training regimen, which Plato described in a dialogue around 347 B.C., particularly noteworthy: He gave up sex. Ikkos, who went on to win the Olympic Pentathlon, believed that abstinence before competition was essential for...