To test the hypothesis that lactate plays a central role in the distribution of carbohydrate (CHO) potential energy for oxidation and glucose production (GP), we performed a lactate clamp (LC) procedure during rest and moderate intensity exercise. Blood [lactate] was clamped at ∼4 mM by exogenous lactate infusion. Subjects performed 90 min exercise trials at 65% of the peak rate of oxygen consumption (̇VO2,peak; 65%), 55% VO2,peak (55%) and 55% ̇VO2,peak with lactate clamped to the blood [lactate] that was measured at 65% VO2,peak (55%-LC). Lactate and glucose rates of appearance (Ra), disappearance (Rd) and oxidation (ROX) were measured with a combination of [3-13C]lactate, H13CO3-, and [6,6-2H2]glucose tracers. During rest and exercise, lactate Ra and Rd were increased at 55%-LC compared to 55%. Glucose Ra and Rd were decreased during 55%-LC compared to 55%. Lactate ROX was increased by LC during exercise (55%: 6.52 ± 0.65 and 55%-LC: 10.01 ± 0.68 mg kg-1 min-1) which was concurrent with a decrease in glucose oxidation (55%: 7.64 ± 0.4 and 55%-LC: 4.35 ± 0.31 mg kg-1 min-1). With LC, incorporation of 13C from tracer lactate into blood glucose (L → GNG) increased while both GP and calculated hepatic glycogenolysis (GLY) decreased. Therefore, increased blood [lactate] during moderate intensity exercise increased lactate oxidation, spared blood glucose and decreased glucose production. Further, exogenous lactate infusion did not affect rating of perceived exertion (RPE) during exercise. These results demonstrate that lactate is a useful carbohydrate in times of increased energy demand.
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