Aim: The aim of this study was to determine the effect of reduced muscle glycogen on maximal fat oxidation (MFO) and the exercise intensity at which maximal fat oxidation was occure (Fatmax) in untrained men.
Method: Eight healthy untrained males (age: 22.2±1; body mass index: 21.9±1.5 kg/m2; percent body fat (%): 15.1±1; VO2Peak: 2.82±0.41 L.min-1) participated in two studies carried out on separate days at least one week apart. Each subject in the fasting state (7:00 AM) performed graded exercise test with 3 min stages on a cycle ergometer to exhaustion, on two separate occasions randomly, in a normal glycogen state and under conditions of reduced muscle glycogen content. On one occasion, the day before the graded exercise test, subjects came to the laboratory at 18:00 pm to perform an exhaustive muscle glycogen lowering exercise. Indirect calorimetry was performed to determine oxygen consumption (VO2) and carbon dioxide production (VCO2) at during gradedexercise. Substrate oxidation, MFO and Fatmax was determined during graded exercise test, by use of the stoichiometric equations. The student’s t-test was used to analyze the variables.
Results: MFO in the lowering muscle glycogen (0.54±0.08 g.min-1) was significantly higher than in the normal muscle glycogen status (0.36±0.04 g. min-1) (P<0.01). In addition, Significantly difference in Fatmax occurred at an exercise intensity of around 46.12±3.13% and 65.01±3.42% VO2peak in the normal and reduces glycogen condition respectively (P<0.01).
Conclusion: In conclusion, reduced muscle glycogen before exercise can be causes a displacement of MFO and Fatmax to higher level of exercise intensity. This means that the onset of decrease of fat oxidation occurs at a higher intensity and individuals start to rely on carbohydrate sources later.

Key words: MFO, Fatmax, Lowering muscle glycogen, Graded exercise test