EXERCISE INTENSITY AT THE ONSET OF PLASMA LACTATE ACCUMULATION COINCIDES WITH MAXIMAL FAT OXIDATION

Abstract

Fat oxidation increases from low to moderate exercise intensities and decreases from moderate to high intensities. The decrease in fat oxidation at intensities higher than 63% VO2max is associated with increases in glycolytic flux. Recently a test was developed to determine the intensity which elicits maximal fat oxidation (Fatmax, Achten et al. MSSE Jan,2002). PURPOSE: In the present study this test was used to determine whether the onset of the decrease in fat oxidation correlates with the onset of plasma lactate accumulation. It was hypothesized that the intensity at the onset of plasma lactate accumulation (OPLA) would coincide with Fatmax. METHODS: 26 trained male cyclists (VO2max 60.9±1.3 ml/kg/min) performed a graded exercise test to exhaustion on a cycle ergometer, starting at 95W with 35W increments every 3 min until exhaustion. Indirect calorimetry was used to calculate fat oxidation rates and venous blood samles were taken for measurement of lactate concentration. Fat oxidation rates and lactate concentrations were plotted against exercise intensity. OPLA was defined as the intensity where lactate starts to increase above baseline and lactate threshold (LT) was determined with the D-max method (Cheng et al. Int J Sports Med 13,1992). RESULTS: Fatmax was found at 63.2 ± 1.7%VO2max where fat oxidation rate was 0.51 ± 0.03g/min. Fatmin (exercise intensity at RER = 1.0) occurred at 89.1 ± 1.4%VO2max. OPLA was located at 60.7 ± 1.1%VO2max and LT at 87.3 ± 1.0%VO2max. Significant correlations (p < 0.01) were found between Fatmax and OPLA (r = 0.65) and between Fatmin and LT (r = 0.80), when intensity was expressed as VO2. Similar finding were obtained when intensity was expressed as heart rate (r = 0.72 and 0.78 resp.). CONCLUSION: The exercise intensity at OPLA coincides with the exercise intensity that elicits maximal fat oxidation. Whether this is a causal relationship needs to be determined but these findings seem to support studies that suggest a direct relationship between lactic acid accummulation and fat oxidation. Supported by a research grant of Polar Electro Oy, Finland