"It feels a bit like you might feel if you're on a crash diet," Rapoport said. "Except that when you diet, it happens over the course of a few days, whereas a runner experiences it in the course of a few minutes.
Maggie Pietila, an accountant for a Seattle cellphone company, says pressure from colleagues led her and 19 co-workers last year to go onacrashdiet of fruit and vegetables that left her sick and shaky.