Disclosed are methods for treating a subject suffering from phenylketonuria and/or phenylalanemia. The methods include, in part, enterally administering to the subject a LNAA supplement in which the weight ratio of Leu to Val is greater than 2:1 in which the weight ratio of Leu to iLeu is greater than 3:1 or which includes one or more LNAAs and which further includes Lys. LNAA supplements are also disclosed. Also disclosed are methods for treating a subject suffering from a condition involving a metabolic disorder involving the metabolism of a first amino acid X. The method includes enterally administering to the subject a composition which (i) is substantially free from the first amino acid X and (ii) which includes a second amino acid Y that competes with amino acid X at a gastrointestinal tract transporter.
