Validation of the [1,2‐¹³C]acetate recovery factor for correction of [U‐¹³C]palmitate oxidation rates in humans

Abstract

The validity of estimations of plasma fatty acid oxidation using tracers has often been questioned. The appearance of isotopic markers in breath CO2 is delayed and incomplete. Recently suggestions have been made that substantial amounts of tracer are incorporated into products of the tricarboxylic acid cycle (e.g. glucose, glutamine and glutamate) and that an acetate correction factor can be used to correct for tracer fixation. In the present study we investigated whether the appearance of 13CO2 during a separate infusion of [1,2‐13C]acetate could be used for correction of [U‐13C]palmitate oxidation rates in studies lasting <2 h and we quantified the appearance of tracer in the glutamine, glutamate and glucose pools of the body. An infusion of either [1,2‐13C]acetate (0.104 μmol min−1 kg−1) or [U‐13C]palmitate (0.013 μmol min−1 kg−1) was given to eight male subjects and continued for 2 h at rest. In six subjects the infusion of [1,2‐13C]acetate was repeated to determine reproducibility of the acetate recovery. Fractional recovery in breath from [1,2‐13C]acetate gradually increased during the infusion period at rest from 14.1 ± 0.6 % at 60 min to 26.5 ± 0.5 % at 120 min after the start of the infusion. Intersubject coefficient of variance was 8.3 ± 0.6 % and intrasubject coefficient of variance of the acetate recovery tests was 4.0 ± 1.5 %. After 2 h of [1,2‐13C]acetate infusion, 12.4 ± 0.8 and 10.3 ± 0.9 % of infused 13C was incorporated in the glutamine and glutamate pools, respectively. In conclusion, the [1,2‐13C]acetate recovery factor can be used for correcting the rate of [U‐13C]palmitate oxidation in infusing studies of 2 h in resting conditions. Failure to use this recovery factor leads to a substantial underestimation of the rate of plasma free fatty acid oxidation. The extent of label fixation could largely be explained by accumulation of tracer carbon in glutamine and glutamate, and the accumulation in glucose is negligible.

Substrate source utilisation in long-term diagnosed type 2 diabetes patients at rest, and during exercise and subsequent recovery

“…where TTRCO 2 is the breath 13 C/ 12 C ratio at a given time point, VCO 2 is carbon dioxide production (l/min), k is the volume of 1 mole of CO 2 (22.4 l/mol), and Ar is the fractional 13 C label recovery in breath CO 2 observed after the infusion of labelled acetate [27,31,32], calculated as:…”

The acetate recovery factor to correct tracer-derived dietary fat oxidation in humans

“…Indeed, labeled carbons were found in glutamate and glutamine and in lower quantity in glucose and lactate and pyruvate (15). On the basis of the observation that acetate is converted into acetyl-CoA and directly enters the TCA cycle, Sidossis et al (14) were the first to propose that infused [ 14 C]acetate can be used to correct for the retention of a 13 C-fatty acid ( 13 C-FA) in the organism.…”

“…Together with the low intraindividual coefficient of variation (CV; 4%), this suggested that although the iARF is reproducible it should be measured in every subject, except when only the group mean data are of interest. In many countries the use of radioactive isotopes is restricted for ethical reasons, and the conclusions of Schrauwen and colleagues (12,13) imply that two separate tests, performed under identical conditions, are needed to determine the iARF required to correct the tracer oxidation data. Although this might be feasible for infusion studies given the relatively short duration required to obtain a steady state (Ϸ2 h) (13), it presents operational constraints for studies investigating dietary fat oxidation that require Ն10-h tests due to digestion and trafficking processes.…”

“…exogenous fatty acid oxidation; stable isotopes; mass spectrometry STABLE ISOTOPES ARE EMPLOYED in clinical nutrition studies to measure fatty acid oxidation. The rate of excretion of labeled CO 2 following a constant infusion or ingestion of 13 C-labeled fatty acids is used to measure the tracer oxidized (2,5,6,11). However, early studies (3) showed that the appearance of 13 CO 2 in breath is very low in the first hours of infusion, especially at rest.…”

“…The rate of excretion of labeled CO 2 following a constant infusion or ingestion of 13 C-labeled fatty acids is used to measure the tracer oxidized (2,5,6,11). However, early studies (3) showed that the appearance of 13 CO 2 in breath is very low in the first hours of infusion, especially at rest. This finding suggested that part of the labeled carbon is temporarily sequestrated in the organism due to label dilution and/or redistribution, resulting in an underestimation of the tracer fat oxidation rates.…”