You should have any skinny GSD, Doberman Pinscher, Labrador Retriever, or other breed known to be born with EPI checked to make sure their pancreatic acid is working. EPI is exocrine pancreatic insufficiency, which means they don't manufacture enough pancreatin, hence the food is improperly broken down and doesn't digest, causing diarrhea and weight loss.
I have a beautiful German Shepherd named Bullett, he is 2, but to look at him, he looks like I do not feed him. He goes to the vet for his check ups and does not have worms, parasites, etc. He eats like a pig but will not put any weight on.