Bioavailability is a term more commonly used in the science of pharmacology (drugs).
However, when used to discuss dietary supplements and nutrients, as long as the chemical structure and digestive environment of a particular nutrient is chemically identical to that of another, it would make no difference whether that nutrient is sourced from a plant or an animal.
The bioavailability of any mineral (like magnesium or selenium) would be expected to be nutritionally identical to any other pure form of the same mineral — whether or not it had been derived from a soybean or a chicken. It makes absolutely no difference from where it is sourced.
However, when that same mineral has been chelated — combined with an amino acid, for example — it would become more bio-available for the animal.
So, in our ratings, we do tend to slightly favor recipes that contain chelated minerals over their standard inorganic counterparts.
In the case of whole foods like eggs or corn — not individual nutrients — the biological value (or nutritional completeness) of a food can vary significantly for any particular species.
For clarification, it may help to read this article about biological value published elsewhere on our website.
Or the Wikipedia articles about biological value and bioavailability, too.
Hope this helps.
