Experiment of 1 - for the last few weeks dog has been on kibble (Skinners, then co-op own). We have not been doing much work recently. She got wired, hyper, barked at nothing in the morning, wouldn't settle, and howled when left alone.
Two days on beef heart/chicken thigh/fleshy scraps of pig trotter and she's calm, quiet, fur is lying flat and smooth, and she doesn't have that one-too-many-cups-of-coffee look.
Interestingly - after the other thread on low protein being calming - chicken is allegedly 30% protein, so protein level is not the full story.
I do think there's something to the psychology of chewing too - isn't it said that horses need long-length hay to chew for mental satisfaction, rather than grass cubes or short chopped stuff?