Leucine kinetics and fuel utilization during a brief fast in human pregnancy.

To examine proteolysis, protein and leucine oxidation, and fuel utilization during a brief fast (approximately 17 hours) in human pregnancy, we determined leucine kinetics, urea nitrogen excretion, and respiratory quotient (RQ) in 11 pregnant subjects during the second half of gestation, and in 11 normal nonpregnant controls. The total rate of appearance (Ra) of leucine was similar in the pregnant and control groups (pregnant 4.99 +/- 0.60 v control 5.25 +/- 1.60 mmol/h [mean +/- SD]). However, leucine Ra per kilogram was significantly lower in pregnant subjects (pregnant 68 +/- 7 v control 82 +/- 13 mumol/kg/h, P less than .01). In addition, urinary urea nitrogen excretion was also significantly less in pregnant subjects (pregnant 3.74 +/- 1.09 v control 5.58 +/- 1.6 mg/kg/h, P less than .01). The RQ measured in the pregnant group was significantly higher than controls (0.82 +/- 0.05 v 0.76 +/- 0.04, P = .01), resulting in higher calculated carbohydrate oxidation rates during fasting in pregnancy. These data suggest that total rates of proteolysis (reflected by leucine flux) are similar in pregnant and nonpregnant subjects after an overnight fast. When normalized to body weight, proteolysis is lower in pregnant subjects. Urea excretion rates are also lower in pregnancy. These findings support the hypothesis that there is a pregnancy-induced adaptation to conserve maternal protein stores during a brief fast. The higher rate of carbohydrate oxidation during fasting in pregnancy may be a reflection of the fetal-placental unit's use of glucose as its predominant oxidative substrate.