STANFORD, Calif. — For some runners, a marathon is not enough.
Participation in so-called ultramarathons — defined as any distance beyond the standard 26.2-mile marathon — has grown exponentially in recent years. The number of runners who finished ultra-length races in North America increased from 15,500 in 1998 to 63,530 in 2012, according to UltraRunning Magazine. Despite its popularity, however, little is known about the health effects of this intense form of exercise.