Dietary Phenylalanine and Tyrosine Requirements in Healthy Human Pregnancy

Abstract Objectives Phenylalanine (PHE), an indispensable amino acid, is required for protein synthesis and is fundamental for proper fetal development. PHE is the precursor for tyrosine (TYR), a conditionally indispensable amino acid that is crucial for synthesis of dopamine, norepinephrine and epinephrine. The current recommendation for PHE + TYR during pregnancy is 36 mg/kg/d, which is factorially determined and not based on experimental studies. Our objective was to experimentally determine PHE + TYR requirements during early (13–19 wks) and late (33–39 wks) gestation, accounting for potential changes in requirement since metabolism and rate of growth are not static throughout pregnancy. Methods 20 women (22–39 y) were studied using the non-invasive stable isotope based indicator amino acid oxidation technique (with L-[1–,13C]Leucine as indicator) for a total of 51 study days. A range of PHE intakes (5 to 100 mg/kg/d) were provided in early and late pregnancy as isonitrogenous and isocaloric meals on separate study days, in the absence of dietary tyrosine. Breath samples were collected at baseline and isotopic steady state. Samples were analyzed for ,13C enrichment on an Isotope Ratio Mass Spectrometer, and the PHE + TYR requirement was determined using two-phase linear regression crossover analysis to identify a breakpoint in ,13CO2 production (represents the minimum dietary requirement) in response to changes in PHE intake. Results Results suggest the PHE + TYR requirement is 43 and 55 mg/kg/d in early and late pregnancy respectively. Conclusions Our preliminary results suggest that PHE + TYR requirements in pregnancy are higher compared to current recommendations, and are different between gestational stages. These results are similar to our previous findings of increased requirements for dietary protein, lysine and PHE (in the presence of excess TYR) in late compared to early pregnancy. They also highlight the need to investigate the dietary requirement of the remaining indispensable amino acids during this life stage. Funding Sources Canadian Institute of Health Research.