Tracer methods underestimate short-term variations in urea
production in humans.
Hamadeh, Mazen J., and L. John Hoffer.
School of Dietetics and Human Nutrition, and McGill Nutrition and
Food Science Centre, McGill University, Montreal, Quebec H3A 1A1
Canada
APStracts 4:0278E, 1997.
Urea production rate (Ra) is commonly measured using a primed
continuous tracer urea infusion, but the accuracy of this method has
not been clearly established in humans. We used intravenous infusions
of unlabelled urea to assess the accuracy of this technique in
normal, postabsorptive men under four different conditions: (1)
tracer 13C-urea was infused under basal conditions for 12 h
(control); (2) tracer 13C-urea was infused for 12 h, and unlabelled
urea was infused from hours 4-12 at a rate 2-fold greater than the
endogenous production rate ("step" infusion); (3) tracer 13C
-urea was infused for 12 h, and unlabelled urea was infused from hours
4-8 ("pulse" infusion); and (4) tracer 13C-urea was infused
for 9 h, and unlabelled alanine was infused at a rate of 120 mg/kg.h
(1.35 mmol/kg.h) from hours 4-9. Urea Ra was calculated using (a) the
isotopic steady state equation (i/TTR), (b) Steele's nonsteady state
equation, and (c) urinary urea excretion corrected for changes in
total body urea. For each subject, endogenous urea Ra was measured at
hour 4 of the basal condition, and the sum of this rate plus
exogenous urea input was considered as "true urea input".
Under control conditions, urea Ra at hour 4 was similar to that
measured at hour 12. After 8-hour step and 4-hour pulse unlabelled
urea infusions, Steele's nonsteady state equation underestimated true
urea input by 22% (step) and 33% (pulse); whereas the nonisotopic
method underestimated it by 28% (step) and 10% (pulse). Similar
conclusions derived from the alanine infusion. These results indicate
that while Steele's nonsteady state equation and the nontracer method
more accurately predict total urea Ra than the steady state equation,
they nevertheless seriously underestimate it for as long as 8 h after
a change in true urea Ra.
Received 14 July 1997; accepted in final form 9 December 1997.
APS Manuscript Number E326-7.
Article publication pending Am. J. Physiol. (Endocrinol. Metab.).
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 January 1998