J Physiol. 2002 Nov 1;544(Pt 3):963-75.
Department of Integrative Biology, University of California, Berkeley 94720, USA.
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 approximately 4 mM by exogenous lactate infusion.
Subjects performed 90 min exercise trials at 65 % of the peak rate of oxygen
consumption (V(O(2))(,peak); 65 %), 55 % V(O(2))(,peak) (55 %) and 55 % V(O(2))(,peak)
with lactate clamped to the blood [lactate] that was measured at 65 % V(O(2))(,peak) (55 %-LC).
Lactate and glucose rates of appearance (R(a)), disappearance (R(d)) and oxidation (R(ox))
were measured with a combination of [3-(13)C]lactate, H(13)CO(3)(-), and [6,6-(2)H(2)]glucose
tracers. During rest and exercise, lactate R(a) and R(d) were increased at 55 %-LC
compared to 55 %. Glucose R(a) and R(d) were decreased during 55 %-LC compared to 55 %.
Lactate R(ox) 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 (13)C 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.