Depends on who the Dr is, we have had a gym rat told to lose weight here in our corner of the sandpit. Has anyone ever been threatened with grounding based on being skin and bones? Not that I know of and no, I am not a big guy I am one of the boney ones just at the bottom end of the acceptable range. The gym rat is in better shape than I am but....BMI says otherwise.
BMI is just a guide, ridiculous to use it as more than that. The Canadian Forces tried it in the 1980s and it did not work very well.