I don't think it's childish. I don't think it's unprofessional either, unless it interferes with the normal stuff you do at a job. If you delay a meeting to go get a can of tuna, then that might not be the best idea. If you make a big fuss over travel accommodations because there's not a gym nearby and annoy whoever it is making the arrangements, that's probably not a good idea either. But if you're not annoying people and it's not affecting the way you do your job, then I don't think it's anything to worry about. At worst, the people you work with might think you're a little bit obsessive about your body, but that's not necessarily a bad thing. Like JJ said, typically people who are in good shape and take care of their bodies are thought of more highly.