Thanks for all the replies. Freewill: to answer your question, I usually throw up because I feel guilty for eating, or I think that I'm going to become fatter because I ate, or I think "I didn't run today so I need to lost some of the calories another way". And I usually feel relief afterwards. I threw up for the first time in about a week today. I went to the dentist and he said I have a small decay on one of my teeth which could be associated with acids from my stomach when throwing up.

And Candika, I'm not sure if I'm underweight. I'm 5'5 and weight about 112. I believe that's in the normal range or slightly under.

And thanks for the article link Sherryanne, it was really interesting!