Dietary Aromatic Amino Acid Requirements During Early and Late Gestation in Healthy Pregnant Women

Phenylalanine and tyrosine (referred to as total aromatic amino acids; TAAs) are essential for protein synthesis, and are precursors for important catecholamines. Current estimated average requirement (EAR) recommendations for TAA during pregnancy are 36 mg·kg–1·d–1, and has not been experimentally determined. The aim was to determine TAA requirements (dietary phenylalanine in the absence of tyrosine) during early and late gestation using the indicator amino acid oxidation (IAAO, with L-[1-13C]leucine) technique. Nineteen healthy pregnant women (age 22–38 y) were studied at a range of phenylalanine intakes (5 to 100 mg·kg–1·d–1) in early (13–19 wk) and/or late (33–39 wk) pregnancy for a total of 51 study days. Graded test intakes were provided as 8 hourly isonitrogenous and isocaloric meals. Breath samples were collected for13C enrichment analysis on an isotope ratio mass spectrometer. A plasma sample was collected and analyzed for phenylalanine and tyrosine concentrations on an amino acid analyzer. The TAA requirement in early and late pregnancy was calculated using 2-phase linear regression crossover analysis that identified breakpoints in13CO2production (the requirement) in response to phenylalanine intakes. TAA requirement during early pregnancy was 44 mg·kg–1·d–1(95% CI: 28.3, 58.8) and during late pregnancy was 50 mg·kg–1·d–1(95% CI: 36.1, 63.1). In early and late pregnancy, plasma phenylalanine and tyrosine concentrations rose linearly in response to graded phenylalanine intakes. Our results suggest that the current EAR of 36 mg·kg–1·d–1for TAAs is underestimated. When compared with results previously determined in nonpregnant adults, early pregnancy requirements were similar (43 compared with 44 mg·kg–1·d–1, respectively). During late pregnancy, a 14% higher TAA requirement was observed when compared with early pregnancy. The results from this study have potential implications for creating gestation stage-specific TAA recommendations.