Amniotic fluid amino acid concentrations are modified by maternal dietary glucose, gestational age, and fetal growth in rats.

Amniotic fluid (AF) contains free amino acids that enter via transplacental and transmembranous routes from maternal sources; subsequently, the developing fetus "ingests" these amino acids early in gestation through unkeratinized skin and later through continuous AF swallowing. Our objectives were as follows: 1) to determine whether a restriction of maternal dietary glucose modulates the free AF amino acid pool, and 2) to establish whether any diet-induced changes were predictive of fetal weight near term (d 21.5). To produce varying in utero growth rates, pregnant rat dams were fed varying levels of glucose (0, 12, 24, 60%) throughout pregnancy. AF samples, collected on gestational days 18-21, were precolumn derivatized by 9-fluorenylmethyloxychloroformate to produce stable primary and secondary amino acid derivatives required for HPLC detection at low amino acid concentrations. Eighteen amino acids were identified. A 2-way ANOVA with main effects of diet (< or =12% and > or =24% glucose) and gestational age (d 18/19 and 20/21) showed that 2 AF amino acids, methionine and phenylalanine, and 12 AF amino acids were independently modified by diet and gestational age, respectively. Of note were the 364% increase in AF methionine and the constant decline in AF taurine as both gestational age lengthened and fetal weight increased. Multiple regression demonstrated that in addition to methionine, 3 specific AF amino acids, cysteine, lysine, and tyrosine, predicted fetal weight. These results demonstrate that the AF amino acid pool can be modified by the glucose content of the maternal diet and that specific AF amino acids are associated with gestational age and fetal growth.