Loss of deuterium in faecal solids and by sequestration in reindeer: effect on doubly labelled water studies

Geir Gotaas,Nicholas J.C Tyler,Paul Haggarty,Eric Milne

doi:10.7557/2.20.1.1500

Abstract

An underlying assumption when estimating total energy expenditure (TEE) using doubly labelled water (DLW) is that the injected isotopes (lsO and 2H) leave the body only in the form of CO, and H20. However, both isotopes have additional routes of loss. We quantified the loss of 2H (i) attached to faecal solids and (ii) by sequestration into newly synthesised fat in reindeer (Rangifer tarandus tarandus). Estimates of the errors caused by these processes were applied to data from DLW studies with reindeer in summer and in winter. Given the net rate of faecal dry matter output and lipid synthesis in the present study, ignoring both sources of error caused the TEE of reindeer to be underestimated by approximately 5% in winter and approximately 9% in summer. The separate effect of each source of error was evaluated in summer. If ignored, loss of 2H through sequestration alone caused TEE to be underestimated by approximately 3.7%. Similarly, if ignored, loss of 2H attached to faecal solids alone caused TEE to be underestimated by approximately 5.9%.

Highlights

The doubly labelled water (DLW) technique (Lifson et al, 1955) for the measurement of carbon dioxide production rate (r ) CC)2 and the estimation of total energy expenditure (TEE) has been applied in a wide variety of species of birds and mammals (e.g. Nagy, 1987; 1994)
2 H atoms in particular can be lost from the body water pool by exchanging with ' H in labile positions on other molecules which are subsequently exported from the body and by sequestration into de novo synthesised molecules
Exchangeable loss of deuterium in faecal solids The exchangeable loss of 2 H attached to faecal solids was equivalent to an apparent water flux of 0.09 to 0.17 g H 2 0

Summary

Introduction

The doubly labelled water (DLW) technique (Lifson et al, 1955) for the measurement of carbon dioxide production rate (r ) CC) and the estimation of total energy expenditure (TEE) has been applied in a wide variety of species of birds and mammals (e.g. Nagy, 1987; 1994). The method is based on injection and subsequent measurement of the rate of disappearance of the oxygen isotope l s O and a hydrogen isotope (most commonly 2 H , 3 H is used) from the body water pool. The amount of C 0 2 produced by the animal during the experiment can be calculated from the difference in the rates of disappearance of the two isotopes. The successful application of the DLW method rests on the assumption that the isotopes leave the body only as H 2 0 and C 0 2. 2 H atoms in particular can be lost from the body water pool by exchanging with ' H in labile positions on other molecules which are subsequently exported from the body (e.g. milk solids, faecal solids) and by sequestration into de novo synthesised molecules

Methods

Results

Conclusion