Glycine, A Dispensable Amino Acid, Is Conditionally Indispensable In Late Stages Of Human Pregnancy

Background: Recently, we showed that there are higher protein, lysine, and phenylalanine requirements in late stages of pregnancy compared with early stages. Animal studies have suggested an increased dietary need for specific dispensable amino acids in pregnancy; whether such a need exists in human pregnancies is unknown.Objective: The objective of the current study was to examine whether healthy pregnant women at midgestation (20-29 wk) and late gestation (30-40 wk) have a dietary demand for glycine, a dispensable amino acid, using the indicator amino acid oxidation method and measurement of plasma 5-oxoproline concentrations.Methods: Seventeen healthy women (aged 26-36 y) randomly received different test glycine intakes (range: 5-100 mg.kg(-1).d(-1)) during each study day in midgestation (similar to 26 wk, n = 17 observations in 9 women) and late gestation (similar to 35 wk, n = 19 observations in 8 women). Diets were isocaloric with energy at 1.7 x resting energy expenditure. Protein was given as a crystalline amino acid mixture based on egg protein composition at current estimated average requirement (EAR; 0.88 g.kg(-1).d(-1)). Breath samples were collected at baseline and isotopic steady state to measure oxidation of L-[1-C-13]phenylalanine to (CO2)-C-13 ((FCO2)-C-13). Plasma was collected at the sixth hour of the study day. Linear regression crossover analysis and simple linear regression were used to assess responses in (FCO2)-C-13 and plasma 5-oxoproline concentrations to different glycine intakes.Results: No statistically significant responses were observed in midgestation. However, in late gestation, lower glycine intakes resulted in higher rates of (FCO2)-C-13 (suggesting low protein synthesis) with a breakpoint for phenylalanine oxidation at >37 mg glycine.kg(-1).d(-1) and higher plasma 5-oxoproline (suggesting low glycine availability) with a breakpoint >27 mg glycine.kg(-1).d(-1).Conclusions: The findings suggest that glycine should be considered a "conditionally" indispensable amino acid during late gestation, especially when protein intakes are at 0.88 g.kg(-1).d(-1), the current EAR.