I had a girlfriend with this - she knew it was wrong, it wasn't that she didn't eat, she just over-exercised - she just had no control over it, she tried to stop, heck she is a nurse, she knew what damage she could be doing her to baby but it was beyond anything she could control - heck she went on a 5km run the day before she had a c/s for her son (they couldn't turn him - he was breech). She cried when she saw him after he was born, knowing that his lack of bubba fat was probably due to her over-exercise, both mum and bub are perfectly fine now - he was within "normal" percentiles within a few weeks of breastfeeding and he is now a happy healthy 18 month old (who can happily wake a household of 10 people at 5am with his lungs)

So don't dismiss it as something trivial, something that is all the rage, it is as bad as anorexia, where it become a mental issue not a physical one.