You new comment created this question - what makes a protein undigestable? Is it a coating on that protein or inability of the consumer's digestive fluids to separate the protein into the basic amino acids?
Lack of a complete or even near complete amino acid profile in the protein of the ingredient being ingested. It all gets broken down but all of the building blocks if you will are there to put it back together in a form the animal can use.
Eric may be have a better or more complete answer.
