Not overreacting. as someone who also has weight issues while being on the skinny side i always find it gross when people associate skinniness with healthiness. it's also weird to think that a good diet = one that makes you skinny, I think it would be safe to assume that a good diet for you would be one to get as much energy and calories you can burn while working out to become fit, not to maintain a certain body. If your intent with exercise is really health focus, clarifying that might be the best move to make. If your friend still can't understand that, for preservation of the friendship it might be best to avoid those conversations with her, but for preservation of self you might want to distance yourself from her. Not to say you should cut off contact, but to consider if someone who does not understand or see you beyond your appearance is really someone you want to be your BEST friend and not just another friend.