Glucose-alanine relationship in normal human pregnancy

Abstract

In order to quantify the glucose-alanine relationship in normal human pregnancy, the turnover rates of alanine and the incorporation of alanine carbon into glucose were quantified in 15 pregnant women during the last 4 weeks of gestation following a ten-hour fast. Eight nonpregnant women of similar age group were studied as controls. l-[2,3- 13C 2]Alanine and d[6,6- 2H 2]glucose were infused as tracers. The 13C enrichment of alanine, lactate, and glucose and the deuterium enrichment of glucose were measured by gas chromatography-mass spectrometry. In five pregnant and five nonpregnant women, the contribution of alanine carbon to expired CO 2 directly and via glucose was estimated by combining indirect respiratory calorimetry with the tracer infusions. The alanine turnover rates in the pregnant and nonpregnant women were similar (pregnant, 4.43 ± 0.82 μmol/kg × min; nonpregnant, 4.11 ± 1.08 μmol/kg × min, mean ± SD). However, the fraction of alanine incorporated into glucose was significantly lower during pregnancy (23.5 ± 8.3% v 30.8 ± 8.2%, P < .04). In pregnant women, 20% of lactate pool was derived from alanine as compared with 28% in nonpregnant subjects ( P < .02). Twenty-four percent of alanine turnover was converted to CO 2 in both pregnant and nonpregnant women. The plasma insulin concentration was increased significantly during pregnancy ( P < .05). These data suggest that gluconeogenesis from alanine is attenuated during pregnancy. This decrease in gluconeogenesis is not the result of decreased alanine flux, but due to intrinsic intrahepatic mechanism such as decreased deamination of alanine mediated by the predominant insulin effect or a decreased hepatic uptake of alanine. These unique adaptive responses in human pregnancy result in conservation of maternal nitrogen for new maternal and fetal protein synthesis.