A novel doubly labelled ¹³C, ¹⁵N amino acid method for measuring energy and protein metabolism in man

ABSTRACT

A novel doubly [1-¹³C, α-¹⁵NH₂]-labelled amino acid method (DLAAM) is presented for the determination of the CO₂ production (RCO₂) and energy expenditure in humans. This method is based on the simultaneous administration of [1-¹³C]glycine and [¹⁵N]glycine followed by the measurement of excretion kinetics of breath ¹³CO₂ and urinary ¹⁵N. The basic idea of the DLAAM is that the unknown ¹³C recovery RF(¹³C) of the 1-¹³C amino acid, essential for the calculation of the net CO₂ production, can be approximated by the easily measureable ¹⁵N recovery RF(¹⁵N) of the α-¹⁵NH₂ labelled amino acid. In four healthy adult men (76–97 kg) the DLAAM was tested parallel to the IC and in one man (74 kg) parallel to the DLWM. Using the approximation RF(¹³C) ≈ RF(¹⁵N) the RCO₂ (in l CO₂ d^–1) was calculated to 387.0 ± 30.3 (DLAAM) vs. 382.8 ± 22.6. (IC). The Bland–Altman plot shows that the difference between the DLAAM and IC of individual RCO₂ is within the 95 % confidence interval (mean ± 2 SD): +4.3 ± 37.5 l CO₂ d^–1. We conclude that the DLAAM and IC may be used interchangeably. The physical activity level (PAL) was calculated based on the DLAAM vs. DLWM to about 1.5.

KEYWORDS:

• Carbon-13
• doubly labelled water
• energy expenditure
• glycine
• human
• hydrogen-2
• nitrogen-15
• mass spectrometry
• oxygen-18
• protein metabolism
• stable isotope tracer technique

Acknowledgements

We thank Ingrid Brüning, Kirsten Karpati, Axel Fischer and Helmut Scholze for analytical and statistical assistance; Heike Pröhl and Kerstin Pilz are gratefully acknowledged for management and for help with safety care. Finally, we would like to thank Anne-Helene Tauson for helpful comments and the final preparation of the text.

Disclosure statement

No potential conflict of interest was reported by the authors.