Assessment of the amount of body water in the Red Knot (Calidris canutus): an evaluation of the principle of isotope dilution with ²H, ¹⁷O, and ¹⁸O as measured with laser spectrometry and isotope ratio mass spectrometry

Abstract

We have used the isotope dilution technique to study changes in the body composition of a migratory shorebird species (Red Knot, Calidris canutus) through an assessment of the amount of body water in it. Birds were quantitatively injected with a dose of water with elevated concentrations of ²H, ¹⁷O, and ¹⁸O. Thereafter, blood samples were taken and distilled. The resulting water samples were analysed using an isotope ratio mass spectrometry (for ²H and ¹⁸O only) and a stable isotope ratio infrared laser spectrometry (²H, ¹⁷O, and ¹⁸O) to yield estimates of the amount of body water in the birds, which in turn could be correlated to the amount of body fat. Here, we validate laser spectrometry against mass spectrometry and show that all three isotopes may be used for body water determinations. This opens the way to the extension of the doubly labelled water method, used for the determination of energy expenditure, to a triply labelled water method, incorporating an evaporative water loss correction on a subject-by-subject basis or, alternatively, the reduction of the analytical errors by statistically combining the ¹⁷O and ¹⁸O measurements.

Keywords:

• Birds
• Body water
• Hydrogen-2
• isotope dilution
• IR spectroscopy
• IRMS
• Oxygen-17
• Oxygen-18

Acknowledgement

We thank Mrs. B. M. A. A. Verstappen-Dumoulin, Mrs. J. Spriensma, and Mr. H. G. Jansen for excellent analytical support.