Can animals use the nitrogen in chitin to build proteins?
This article suggests a certain amount of chitin digestion is possible. 

http://www.bio.unipd.it/agroecology/download/pdf/papers/2009/Chitin-Chitinases-Paoletti-From-Binomium-Chitnchitinase-Recent-Issues-Fp-Version.pdf

I can't quantitatively say how much of any of the myriad potential chitin sources (fungal, crustacean or other arthropods) would be digested within a 'normal' time frame in the human digestive system. That work should probably be undertaken, though.