 For example, if a hypothetical muscle protein requires phenylalanine (an essential amino acid), then this must be provided in the diet for the muscle protein to be produced.If the current protein source in the diet has no phenylalanine in it the muscle protein cannot be produced, giving a low usability and BV of the protein source.In a related way if amino acids are missing from the protein source which are particularly slow or energy consuming to synthesise this can result in a low BV.