Effects of endurance training on lactate removal by oxidation and gluconeogenesis during exercise

This report describes the effects of 9 weeks of endurance-training on the relative rates of lactate removal via oxidation and gluconeogenesis in humans. Before and after training, eight subjects performed incremental (60 W plus 40 W every 6 min) exercise tests, while 14C-lactate was infused into one forearm vein and arterialized venous blood was sampled from the other forearm. During the trial, the volume of expired 14CO2 and circulating 14C-lactate and 14C-glucose specific radioactivities were measured. Such measurements revealed that training increased the estimated oxidation of equivalent venous blood lactate concentrations [VLa] of greater than 1.6 mmol/l. These increases in lactate oxidation were more than would be predicted from the approximately 40% higher O2 uptake values at any [VLa] after training. At a [VLa] of 6 mmol/l, rates of lactate oxidation were increased by some 100% following training, from 105 +/- 12 to 208 +/- 33 micromol/min/kg (P < 0.01). Improvements in lactate oxidation after training reduced the estimated rates of lactate-to-glucose conversion from 40 +/- 3 to 9 +/- 2 micromol/min/kg at a [VLa] of 2.5 mmol/l (P < 0.01). Thus, unlike in rats, human endurance-training does not increase gluconeogenesis. In the final stages of progressive exercise after training, more than 80% of lactate was oxidised and accounted for approximately 45% of overall carbohydrate oxidation.