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

Abstract

ABSTRACTA 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.