Modelling nutrient utilization in farm animals.

Chapter 11 (Page no: 145)

Multiple-entry urea kinetic model: effects of incomplete data collection.

The consequences of incomplete urine collection on whole-body urea kinetics have been investigated. Information on the fates of urea can be obtained from the addition of double-labelled urea to the body urea pool, either as a single dose or by continuous infusion. In both cases it is assumed that labelled urea can re-enter the gastrointestinal tract (GIT) on multiple occasions. Analyses of ¹⁵N in urine and faeces allow the amount of urea-N utilized for anabolism (UUA) to be determined. For such an approach to be valid, it is important that the collection period is sufficiently long (single-dose experiment) or that samples are taken at an isotopic steady state (continuous infusion). Otherwise, ¹⁵N collected from urine and faeces will be incomplete, and hence the urea-N available for UUA will be overestimated. A similar problem occurs with incomplete recovery of urine or faeces. We investigated the effects of incomplete or inappropriate urine or faeces collection (or analytical errors which simulate collection errors) on the estimated fates of urea-N, using the following error types: (i) incomplete collection of urine; (ii) incomplete collection of faeces; and (iii) inappropriate collection period. Mathematical derivations show that such errors result in overestimation of UUA. To investigate the practical importance of such errors, these error types were applied to a single-dose urea kinetic experiment in humans and a continuous infusion study in sheep. The human (non-ruminant) study had low proportions of both urea production that entered the GIT and a low proportion of this entry that returned to the ornithine cycle, when compared with the sheep (ruminant) study (0.23 and 0.28 vs. 0.61 and 0.51, respectively). The application showed that errors on urine collection have a bigger impact on UUA than poor collection of faeces. Also, the effect of urine collection errors on UUA was larger for the human compared with the sheep study. Return of ¹⁵N into the body urea pool from protein turnover probably contributes an error of 2% or less.

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