Pulse injection, 13C tracer studies of lactate metabolism in humans during rest and two levels of exercise.

In order to determine the role of lactic acid as a metabolic substrate during exercise, the extent of its oxidation was studied using (13C) lactate under three different metabolic conditions in two subjects. During rest, easy exercise (work below the lactate inflection point), and hard exercise (work above the lactate inflection point), 100 mg of Na+-D,L(+)-2,3-(13C) lactate was injected via an indwelling catheter inserted in an antecubital vein. Blood as well as expired gas samples were collected up to 2 h post-injection. Subjects worked at average intensities of 53% and 74% VO2max during easy and hard exercise, respectively. During rest and easy exercise, blood lactate concentrations remained stable at 1-2 nM. During hard exercise, blood lactate increased to 3-4 times those observed at rest. Excretion of 13CO2 peaked much sooner and enrichment of 13C in CO2 was greater during both exercise intensities than during rest. Cumulative recovery of injected 13C as 13CO2 averaged 13.2 and 86.2% through 120 min during rest and easy exercise. Through 45 min of hard exercise, recovery of tracer as CO2 was the same as during a similar time point of easy exercise, 51.8%. The results support the contention that oxidation is the major fate of lactate during exercise.