The re-establishment of the normal blood lactate response to exercise in humans after prolonged acclimatization to altitude

1. One to five weeks of chronic exposure to hypoxia has been shown to red-a ce peak blood lactate concentration compared to acute exposure to hypoxia, during exercise, the high altitude 'lactate paradox'. However, we hypothesi ze that a sufficiently long exposure to hypoxia would result in a blood lac tate and net lactate release from the active leg to an extent similar to th at observed in acute hypoxia, independent of work intensity. 2. Six Danish lowlanders (25-26 years) were studied during graded increment al bicycle exercise under four conditions: at sea level breathing either am bient air (0 m normoxia) or a low-oxygen gas mixture (10 % O-2 in N-2, O in acute hypoxia) and after 9 weeks of acclimatization to 5260 m breathing ei ther ambient air (5260 m chronic hypoxia) or a normoxic. gas mixture (47 % O-2 in N-2, 5260 m acute normoxia). In addition, one-leg knee-extensor exer cise was performed during 5260 m chronic hypoxia and 5260 m acute normoxia. 3. During incremental bicycle exercise, the arterial lactate concentrations were similar at submaximal work at 0 m acute hypoxia and 5260 m chronic hy poxia but higher compared to both 0 in normoxia and 5260 in acute normoxia. However, peak lactate concentration was similar under all conditions (10.0 +/- 1.3, 10.7 +/- 2.0, 10.9 +/- 2.3 and 11.0 +/- 1.0 mmol l(-1)) at 0 in n ormoxia, 0 m acute hypoxia, 52.60 m chronic hypoxia and 5260 m acute normox ia, respectively. Despite a similar lactate concentration at sub-maximal an d maximal workload, the net lactate release from the leg was lower during 0 m acute hypoxia (peak 8.4 +/- 1.6 mmol min(-1)) than at 5260 in chronic hy poxia (peak 12.8 +/- 2.2 mmol min(-1)). The same was observed for 0 in norm oxia. (peak 8.9 +/- 2.0 mmol min(-1)) compared to 5260 m acute normoxia. (p eak 12.6 +/- 3.6 mmol min(-1)). Exercise after acclimatization with a small muscle mass (one-leg knee-extensor) elicited similar lactate concentration s (peak 4.4 +/- 0.2 vs. 3.9 +/- 0.3 mmol l(-1)) and net lactate release (pe ak 16.4 +/- 1.8 vs. 14.3 mmol l(-1)) from the active leg at 5260 m chronic hypoxia and 5260 in acute normoxia. 4. In conclusion, in lowlanders acclimatized for 9 weeks to an altitude of 5260 m, the arterial lactate concentration was similar at 0 m acute hypoxia and 5260 m chronic hypoxia. The net lactate release from the active leg wa s higher at 5260 m chronic hypoxia compared to 0 m acute hypoxia, implying an enhanced lactate utilization with prolonged acclimatization to altitude. The present study clearly shows the absence of a lactate paradox in lowlan ders sufficiently acclimatized to altitude.