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Department of Pediatrics, Case Western Reserve University School of Medicine, Rainbow Babies and Children's Hospital, Cleveland, Ohio 44106
Protein and nitrogen
(N) accretion by the mother is a major adaptive response to pregnancy
in humans and animals to meet the demands of the growing conceptus.
Quantitative changes in whole body N metabolism were examined during
normal pregnancy by measuring the rates of leucine N
(QN) and carbon
(QC) kinetics
with the use of
[1-13C,15N]leucine.
Rate of synthesis of urea was measured by
[15N2]urea
tracer. Pregnancy-related change in total body water was quantified by
H2[18O]
dilution, and respiratory calorimetry was performed to quantify substrate oxidation. A significant decrease in the rate of urea synthesis was evident in the 1st trimester (nonpregnant 4.69 ± 1.14 vs. pregnant 3.44 ± 1.11 µmol · kg
1 · min1;
means ± SD, P < 0.05). The lower
rate of urea synthesis was sustained through the 2nd and 3rd
trimesters. QN
was also lower in the 1st trimester during fasting; however, it reached
a significant level only in the 3rd trimester (nonpregnant 166 ± 35 vs. 3rd trimester 135 ± 16 µmol · kg1 · h1;
P < 0.05). There was no significant
change in QC
during pregnancy. A significant decrease in the rate of transamination
of leucine was evident in the 3rd trimester both during fasting and in
response to nutrient administration (P < 0.05). The rate of deamination of leucine was correlated with the
rate of urea synthesis during fasting
(r = 0.59, P = 0.001) and during feeding
(r = 0.407, P = 0.01). These data show that
pregnancy-related adaptations in maternal N metabolism are evident
early in gestation before any significant increase in fetal N
accretion. It is speculated that the lower transamination of
branched-chain amino acids may be due to decreased availability of N
acceptors such as 
-ketoglutarate as a consequence of resistance to
insulin action evident in pregnancy.
leucine; stable isotopes; nitrogen accretion
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