Glycine becomes conditionally indispensable during late gestation

Protein requirements are elevated during pregnancy to support the many adaptations and high rates of protein synthesis to support fetal and maternal needs.  Recent work using the indicator amino acid oxidation method has resulted in estimated average requirements higher than those currently recommended during early and late gestation.  Although emphasis is placed on the indispensable amino acids, studies conducted using animals suggest requirements may be elevated for dispensable amino acids during gestation as well.  In order to determine if dispensable amino acid requirements are elevated during gestation in humans, Rasmussen and colleagues conducted a study in healthy women late gestation.

Subjects in this study were pregnant women who consumed isocaloric diets containing 0.88 g.kg-1.d-1 of protein and a range of glycine (5-100 mg.kg-1.d-1) during midgestation (17 observations in 9 women) and late gestation (19 observations in 8 women).  Oxidation of labeled phenylalanine was determined at baseline and at isotopic steady state, and plasma was collected to determine 5-oxoproline (an indicator of glycine availability).

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During midgestation, there were no significant responses observed.  During late gestation, lower levels of glycine intake led to elevated oxidation of phenylalanine and higher levels of plasma 5-oxoproline.  The breakpoint in the oxidation of phenylalanine was above 37 mg glycine.kg-1.d-1 and for 5-oxoproline it was above 27 mg glycine.kg-1.d-1.  The authors concluded that glycine becomes conditionally indispensable during late gestation if protein intake is maintained at the current estimated average requirement of 0.88 g.kg-1.d-1.

In a commentary on this article, Matthews states this innovative study provides sufficient information to warrant future studies to validate the conclusion that glycine becomes conditionally indispensable in late gestation, and thus determine the correct the EAR and RDA of this amino acid.

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