hmm...try swimming as a cross-training. It's great to keep some substance in the upper body, to keep it from losing tone, that whole "wasting" thing. Multisport is great for balancing the body.
But, yes, many Americans have a culturally skewed idea of what's too skinny. After a year abroad in my mid 20s, my folks were concerned upon my return about being too skinny, even though I was healthy. Of course, now at 41, I'm working to get back down to "fighting weight", and we all chuckle that we were ever concerned about my being too thin.