Nutrition has always been high on my list of important topics, and yes, they do teach the subject in medical school. It used to be that it was an elective set of courses, so true, not all medical students became well-versed in it. Not sure how much it's changed, though I suspect nutrition is still not widely enough taught.

Interestingly, when I was a kid in the 8th grade it was standard practice to have the boys and girls switch shop/home ec classes a few times during the school year. It was certainly educational for me. Actually the students rather liked doing this, and no doubt quite an enlightened policy for the time.

As for behavior modification, no reason a doctor wouldn't be able to be skilled and assist patients needing help with dietary management, etc. Doctors have to do a lot of that sort of thing anyway since many conditions and treatments require "lifestyle" changes. Of course, some doctors will be better at it, and others not so adept. But that's pretty much the case re: certain skill sets among those in any given occupation.