Abstract
Exercise Physiology Laboratory, Department of Integrative
Biology, University of California, Berkeley, Berkeley, California
94720-3140
For estimating the
oxidation rates (Rox) of glucose and other substrates by use of
13 C-labeled tracers, we obtained correction factors to
account for label dilution in endogenous bicarbonate pools and TCA
cycle exchange reactions. Fractional recoveries of 13 C
label in respiratory gases were determined during 225 min of rest and
90 min of leg cycle ergometry at 45 and 65% peak oxygen uptake
( O 2 peak ) after continuous infusions of
[1- 13 C]acetate, [2- 13 C]acetate, or
NaH 13 CO 3 . In parallel trials,
[6,6- 2 H]glucose and [1- 13 C]glucose were
given. Experiments were conducted after an overnight fast with exercise
commencing 12 h after the last meal. During the transition from
rest to exercise, CO 2 production increased ( P < 0.05) in an intensity-dependent manner.
Significant differences were observed in the fractional recoveries of
13 C label as 13 CO 2 at rest
(NaH 13 CO 3 , 77.5 ± 2.8%;
[1- 13 C]acetate, 49.8 ± 2.4%;
[2- 13 C]acetate, 26.1 ± 1.4%). During exercise,
fractional recoveries of 13 C label from
[1- 13 C]acetate, [2- 13 C]acetate, and
NaH 13 CO 3 were increased compared with rest.
Magnitudes of label recoveries during both exercise intensities were
tracer specific (NaH 13 CO 3 , 93%;
[1- 13 C]acetate, 80%; [2- 13 C]acetate,
65%). Use of an acetate-derived correction factor for estimating
glucose oxidation resulted in Rox values in excess ( P < 0.05) of glucose rate of disappearance during hard exercise. We
conclude that, after an overnight fast: 1 ) recovery of
13 C label as 13 CO 2 from
[ 13 C]acetate is decreased compared with bicarbonate;
2 ) the position of 13 C acetate label affects
carbon dilution estimations; 3 ) recovery of 13 C
label increases in the transition from rest to exercise in an isotope-dependent manner; and 4 ) application of an acetate
correction factor in glucose oxidation measurements results in
oxidation rates in excess of glucose disappearance during exercise at
65% of O 2 peak . Therefore,
bicarbonate, not acetate, correction factors are advocated for
estimating glucose oxidation from carbon tracers in exercising men.
stable isotopes; free fatty acid oxidation; glucose oxidation; carbon dioxide; exertion