Higher weight at birth is related to decreased maternal amino acid oxidation during pregnancy

Small size at birth is associated with cardiovascular disease in adult life. Decreased fetal growth may result from a limitation in the nutrient supply to the fetus. Net tissue deposition in the mother and fetus increases the demand for nitrogen, but because maternal consumption of protein does not increase, there must be a change in the partitioning of amino acids, away from oxidation and toward deposition.

Our objective was to characterize amino acid oxidation in pregnancy and to investigate whether the relative partitioning of amino acids was related to fetal growth.

We determined amino acid oxidation as urea production in 25 women during mid (17-19 wk) and late (26-29 wk) gestation. Urea production was measured from urinary [(15)N-(15)N]urea excretion over 48 h after a single oral dose of [(15)N-(15)N]urea. We measured the infant's size at birth.

For the group as a whole, urea excretion decreased and amino acid oxidation remained similar between mid and late pregnancy, but there was wide variation between the women. Heavier infants were born to the mothers in whom amino acid oxidation decreased the most during pregnancy (slope of regression line: -80 g x g N(-1) x d(-1); 95% CI: -129, -31; P = 0.003). After adjustment for length of gestation and the infant's sex, the change in maternal amino acid oxidation explained 34% of the variation in birth weight.

Amino acid oxidation varied widely between the women during pregnancy. Understanding the ability of a pregnant woman to adapt metabolically may have implications for establishing dietary recommendations in pregnancy.