• maximal fat oxidation
• training status
• diet
• low CHO
• high CHO
• FATmax
• MFO

Carbohydrates and lipids are the most important fuels of energy metabolism. During exercise the contribution of fat oxidation is parabolic: as exercise intensity increases (more than 65 % VO2max), the contribution of carbohydrate oxidation increases in parallel to the decrease in lipid oxidation. The point when lipid oxidation reaches maximum is “maximal fat oxidation” (MFO). MFO and the exercise intensity eliciting MFO (FATmax) are considered biological markers of metabolic health and performance. Variability in MFO is quite large and is explained by many factors such as gender, body composition, genetic factors, nutritional status, exercise duration and intensity ecc. The aim of this study is to understand how external factors, in particular diet and training status, affect MFO and FATmax. Twenty healthy active men and women were asked to follow a high CHO and a low CHO diet for about 36 or 41 hours before performing a graded test on the cycle ergometer, the FATmax protocol. Breath-by-breath gaseous exchange measurements were made continuously throughout exercise by using a face mask and measuring VCO2, VO2 and RER, to calculate substrate oxidation and maximal oxygen uptake. Each participant was tested twice (the order of the diet was randomly assigned), with more or less one week in between the two tests. During the days preceding the dietary manipulation, subjects were asked to maintain regular training, but to refrain on the day preceding the exercise trials. Differences between the two diets were detected in MFO (0,69 ± 0,22 g/min vs 0,41 ± 0,18 g/min; p=0,006) and FATmax (59,1 ± 6,3 % VO2max vs. 57,0 ± 4,6 % VO2max; p = 0,002), with lower values in the high CHO one. Differences between the two diets can be explained by an acute effect of insulinemia due to the pre-exercise meal and differences in pre-exercise glycogen levels. Moreover diet appeared to affect the ability to oxidize the primary macronutrient consumed, resulting in higher values of RER in the high CHO diet. RER values indeed reflected the results seen with MFO and FATmax: short-term dietary intake have an impact on substrate utilization. Athletes were then split up into high and low trained according to their VO2max/FFM but no differences were found in MFO, FATmax and RER at 65 % VO2max in the high CHO condition, while in the low CHO one only FATmax resulted higher in the high trained participants (56,76 ± 6,49 % VO2max vs 62,54 ± 4,15 % VO2max, p = 0,026).