Time course of plasma and brain Phe and examples for LNAAs. (a) Steep increase of plasma Phe levels after an oral dose of L-Phe (time 0 h) in the Phe_only (dotted line) and Phe_+LNAA series (solid line). (b) Plasma concentrations of valine and (c) tryptophan are given as examples of LNAA increase during LNAA supplementation in the Phe_+LNAA series (solid line), compared with the Phe_only series (dotted line). (d) K_m-corrected Phe/LNAA ratio versus time after load. During Phe_only, this ratio increased because of the increase in plasma Phe; it decreased during LNAA treatment. (e) Brain Phe concentrations as determined by in vivo ¹H-MRS demonstrating a continuous increase during Phe_only and a block of Phe influx during LNAA treatment. (f) The ratio of plasma/brain Phe increased much more with LNAA treatment than without. All curves represent the average values from all six patients.