Abstract
For estimating the oxidation rates (Rox) of glucose and other substrates by use of [sup.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 [sup.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 (V[O.sub.2 peak]) after continuous infusions of [1-[sup.13]C]acetate, [2-[sup.13]C]acetate, or Na[H.sup.13][CO.sub.3]. In parallel trials, [6,6-2H]glucose and [1-[sup.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.sub.2] production increased (P < 0.05) in an intensity-dependent manner. Significant differences were observed in the fractional recoveries of [sup.13]C label as [sup.13][CO.sub.2] at rest (Na[H.sup.13][CO.sub.3], 77.5 [+ or -] 2.8%; [1-[sup.13]C]acetate, 49.8 [+ or -] 2.4%; [2-[sup.13]C]acetate, 26.1 [+ or -] 1.4%). During exercise, fractional recoveries of [sup.13]C label from [1-[sup.13]C]acetate, [2-[sup.13]C]acetate, and Na[H.sup.13][CO.sub.3] were increased compared with rest. Magnitudes of label recoveries during both exercise intensities were tracer specific (Na[H.sup.13][CO.sub.3], 93%; [1-[sup.13]C]acetate, 80%; [2-[sup.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 [sup.13]C label as [sup.13][CO.sub.2] from [[sup.13]C]acetate is decreased compared with bicarbonate; 2) the position of [sup.13]C acetate label affects carbon dilution estimations; 3) recovery of [sup.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 V[O.sub.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