I couldnt for the life of me think of the name of this stuff, but Boots used to sell a product called Brands Essence. It was a tiny jar, and quite expensive, but it was a very high protein food for invalids. 1 teaspoon equalled something like a large steak. You could get beef or chicken, and it was made for very sick human invalids, who couldnt eat.
I used it for my father in law, and i also used it on a very sick puppy that couldnt keep anything down. I administered it via a small syringe.
Ive just googled it, and it does appear to still be available in the chicken version.