I worked with a physician whose entire practice consisted of lipid and weight loss/nutrition management. He was an internist, who just had a special interest in that area. He didn't particularly receive any extra formal training in the area of nutrition, but just happened to be very well read in that field. He didn't practice any "regular" internal medicine. His business was booming, and his patients were very motivated. I've also heard of other internists who only deal with one disease (ex. HTN, diabetes), without any extra training, and are still considered consultants.