There are a couple of reasons that I can think of, the most obvious is that some runners expend so many calories during training, that they just can't eat enough to replace them.
The other is more sinister: a lot of runners just flat out don't eat. A couple of years ago, my wife was concerned that I wasn't eating enough and forced me to go to a nutritionist. It took the lady about 5 minutes to tell me that I was bordering on anorexic. I was a complete freak about what I was eating, and in the process of trying to be healthy, I was instead starving my body. It's an easy trap to fall into.
|