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Abstract

Maximal fat oxidation (MFO), assessed via a graded exercise test (GXT), also known as the FATMAX test, demonstrates considerable inter-individual variability due to factors such as metabolic efficiency, body composition, and physiological responses to exercise. PURPOSE: The purpose of this study was to examine the factors driving this variability, including fasting blood glucose (FBG), hydration status, anthropometric measures, and cardiorespiratory fitness. METHODS: Male participants (N = 41; BMI = 25.0 ± 4.2 kg/m2) underwent a graded exercise test (GXT) to exhaustion using an electromagnetically braked cycle ergometer. The GXT protocol began with a 10-minute warm-up at 50 watts, with power output increasing by 35 watts every 3 minutes. The test continued until the respiratory quotient (RQ) exceeded 1.0 for the last minute of a stage, at which point the power output was increased every minute. A metabolic cart system measured gas exchange and ventilation in breath-by-breath mode, with average values for oxygen uptake and carbon dioxide production calculated for the final minute of each stage. Fat oxidation rates were derived using stoichiometric equations, assuming negligible protein oxidation. MFO was determined using a third-order polynomial function for each participant. RESULTS: Linear regressions showed that among the factors measured, 16% of the variability in MFO could be explained by baseline weight (p = 0.168), height (p = 0.920), FBG (p = 0.891), urinary specific gravity (USG; p = 0.832), and VO2 max (p = 0.918). Stronger correlations with MFO were observed for weight (p = 0.004, r = 0.44) and height (p = 0.042, r = 0.323). CONCLUSION: The results suggest that anthropometric factors, particularly weight and height, are more strongly correlated with MFO than physiological variables such as fasting blood glucose, hydration status, or cardiorespiratory fitness. These findings support the notion that individual variability in fat oxidation rates during exercise is driven primarily by body size and composition. Further research is needed to explore additional physiological and metabolic variables that might account for the remaining variability. These findings align with previous research showing a moderate association between anthropometry and fat metabolism during exercise.

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