As a caretaker of a turtle, I sometimes struggle understanding the logic behind the inclusion of corn, soybean, and wheat meals in reptile food, such ingredients seem truly aberrant from what a reptile would naturally be exposed to. Nevertheless I am aware of the biological and chemical similarity between most organisms, so I wonder whether that will help reptiles, especially omnivores with an eclectic digestive ability, to digest these compounds.
I'm looking for legitimate studies on reptile digestibility; anecdotal experience or advice is supplementary if present.