Exercise is good, but when does it go past the line into over exercise? It's a fuzzy line for many marathoners, triathletes and people with eating disorders

We've been told for decades that exercise is good, yet how much is too much? When does exercise start to be too much exercise? My story on exercise addiction for the Union-Tribune Health section: http://www.sandiegouniontribune.com/news/2016/jun/27/exercising-to-excess/