Urea kinetics were measured in normal women aged 22-34 years at weeks
16, 24 and 32 on either their habitual protein intake (HABIT) or a con
trolled intake of 60 g protein/d (CONTROL), using primed-intermittent
oral doses of [(NN)-N-15-N-15]urea and measurement of plateau enrichme
nt in urinary urea over 18 h (ID) or a single oral dose of [(NN)-N-15-
N-15]urea and measurement of enrichment of urea in urine over the foll
owing 48 h (SD). The intake of protein during HABIT-ID (80 g/d) was gr
eater than that on HABIT-SD (71 g/d); urea production as a percentage
of intake was significantly greater at week 16 for HABIT-ID than HABIT
-SD, whereas urea hydrolysis at week 16 was greater for HABIT-SD than
HABIT-ID and urea excretion at week 32 was greater for HABIT-ID than H
ABIT-SD. The combined results for HABIT-ID and HABIT-SD showed a signi
ficant reduction in urea production at week 32 compared with week 24.
Urea excretion decreased significantly from week 16 to week 24 with no
further decrease to week 32 and urea hydrolysis was significantly gre
ater at week 24 than either week 16 or week 32. Compared with HABIT, o
n CONTROL there was a decrease in urea production at week 16, and urea
excretion was significantly reduced at week 16. For all time periods
urea production was closely related to the sum of intake plus hydrolys
is. Hydrolysis was greatest at week 24 and closely related to urea pro
duction. There was a significant inverse linear relationship overall f
or hydrolysis as a proportion of production and excretion as a proport
ion of intake, The results show that on HABIT N is more effectively co
nserved in mid-pregnancy through an increase in urea hydrolysis and sa
lvage, and during late pregnancy through a reduction in urea formation
. Lowering protein intake at any stage of pregnancy increased the hydr
olysis and salvage of urea. The staging of these changes was later tha
n that in pregnancy in Jamaica.