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

Abstract

Background: 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.

Objectives: 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.

Methods: 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 for 13C 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 in 13CO2 production (the requirement) in response to phenylalanine intakes.

Results: 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.

Conclusions: Our results suggest that the current EAR of 36 mg·kg-1·d-1 for 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.

Keywords: amino acid requirements; indicator amino acid oxidation; phenylalanine; pregnancy; stable isotopes; tyrosine.

FIGURE 1

Estimated average requirement of TAA in early gestation using the indicator amino acid oxidation method in healthy pregnant women. Biphase linear regression crossover analysis of L-[1-¹³C]leucine tracer oxidation (F¹³CO₂, μmol·kg⁻¹·h⁻¹) was used to determine the TAA requirement using the mixed and regression procedure in SAS (SAS/STAT Ver 9.4). TAA requirements were determined to be 44 mg·kg⁻¹·d⁻¹ (R² = 0.56, 95% CI: 28.3, 58.8 mg·kg⁻¹·d⁻¹; n = 10 women, individual study days = 24). Dashed line indicates the mean requirement. TAA, total aromatic amino acids.

FIGURE 2

Estimated average requirement of TAA in late gestation using the indicator amino acid oxidation method in healthy pregnant women. Biphase linear regression crossover analysis of L-[1-¹³C]leucine tracer oxidation (F¹³CO₂, μmol·kg⁻¹·h⁻¹) was used to determine the phenylalanine requirement using the mixed and regression procedure in SAS (SAS/STAT Ver 9.4). Phenylalanine requirements were determined to be 50 mg·kg⁻¹·d⁻¹ (R² = 0.67, 95% CI: 36.1, 63.1 mg·kg⁻¹·d⁻¹; n = 10 women, individual study days = 27). Dashed line indicates the mean requirement. TAA, total aromatic amino acids.

FIGURE 3

Plasma concentrations of phenylalanine and tyrosine in early pregnancy in response to graded phenylalanine intakes in healthy pregnant women. Linear regression analysis of phenylalanine concentrations (R² = 0.81, panel A) and tyrosine concentrations (R² = 0.71, panel B) (n = 10 women, individual study days = 24). TAA, total aromatic amino acids.

FIGURE 4

Plasma concentrations of phenylalanine and tyrosine in late pregnancy in response to graded phenylalanine intakes in healthy pregnant women. Linear regression analysis of phenylalanine concentrations (R² = 0.71, panel A) and tyrosine concentrations (R² = 0.31, panel B) (n = 10 women, individual study days = 27). TAA, total aromatic amino acids.