Maximal fat oxidation during exercise is positively associated with 24-hour fat oxidation and insulin sensitivity in young, healthy men.

Disturbances in fat oxidation have been associated with an increased risk of obesity and metabolic disorders such as insulin resistance. There is large intersubject variability in the capacity to oxidize fat when a person is physically active, although the significance of this for metabolic health is unclear. We investigated whether the maximal capacity to oxidize fat during exercise is related to 24-h fat oxidation and insulin sensitivity. Maximal fat oxidation (MFO; indirect calorimetry during incremental exercise) and insulin sensitivity (Quantitative Insulin Sensitivity Check Index) were measured in 53 young, healthy men (age 24 ± 7 yr, V̇o2max 52 ± 6 ml·kg(-1)·min(-1)). Fat oxidation over 24 h (24-h FO; indirect calorimetry) was assessed in 16 young, healthy men (age 26 ± 8 yr, V̇o2max 52 ± 6 ml·kg(-1)·min(-1)) during a 36-h stay in a whole-room respiration chamber. MFO (g/min) was positively correlated with 24-h FO (g/day) (R = 0.65, P = 0.003; R = 0.46, P = 0.041 when controlled for V̇o2max [l/min]), 24-h percent energy from FO (R = 0.58, P = 0.009), and insulin sensitivity (R = 0.33, P = 0.007). MFO (g/min) was negatively correlated with 24-h fat balance (g/day) (R = -0.51, P = 0.021) but not significantly correlated with 24-h respiratory quotient (R = -0.29, P = 0.142). Although additional investigations are needed, our data showing positive associations between MFO and 24-h FO, and between MFO and insulin sensitivity in healthy young men suggests that a high capacity to oxidize fat while one is physically active could be advantageous for the maintenance of metabolic health.