N.P.U. or “Net Protein Utilization” is a rating system that evaluates the useable protein levels of different food sources. Just because your turtle food claims to be “high protein” does not mean that the proteins are “useable” or high-quality proteins. The higher the NPU rating is, the better the protein source, based on its amino acid profile. Classically, most animal-based proteins have a better variety of amino acids and so, have higher NPU scores then plant-based proteins.