PKU requires lifelong management, but the childhood PKU diet does not accommodate for post-adolescent patient variations, including: (1) the diet's effect on blood phenylalanine (Phe) concentrations, and (2) the therapeutic benefits of prescribed blood Phe targets. Most adult patients discontinue the inflexible diet, and are exposed to PKU's adverse effects. Research supports large neutral amino acids (LNAAs) as potential alternative strategy; we propose an additive LNAA strategy. We aim to evaluate the clinical efficacy of a combined treatment strategy: usual care (the PKU diet) with LNAA supplementation.