Influence of a 24 h fast on high intensity cycle exercise performance in man

The influence of a 24 h fast on endurance performance and the metabolic response to maximal cycle exercise was investigated in 6 healthy men (mean±SD: age = 21±7 years; weight = 73±10 kg; $\dot V_{O_{2\max } }$ = 46±10 ml·kg⁻¹·min⁻¹). Subjects performed in randomised order two exercise bouts to exhaustion separated by one week. Test rides were performed in fasted (F) and post-absorptive (normal-diet, ND) conditions on an electrically braked cycle ergometer at a workload equivalent to 100% of $\dot V_{O_{2\max } }$ . Acid-base status and selected metabolites were measured on arterialised venous blood at rest prior to exercise and at intervals for 15 mins following exercise. Exercise time to exhaustion was shorter after F compared with ND (p<0.01). Pre-exercise blood bicarbonate (HCO₃ ⁻) concentration, $P_{CO_2 }$ and base excess (BE) were lower after F compared with ND (p<0.05). Prior to exercise, circulating concentrations of free fatty acids (FFA), gb-hydroxybutyrate (B-HB) and glycerol were higher after F compared with ND (p<0.01) but blood glucose and lactate concentration were not different. On the F treatment, after exercise, blood pH, HCO₃ ⁻, and BE were all significantly higher (p<0.01) than on ND; blood lactate concentration was significantly lower for the whole of the post-exercise period after F compared with ND (p<0.01). Circulating levels of FFA and B-HB after exercise on the F treatment fell but levels of these substrates were not altered by exercise after ND. Blood glucose and glycerol concentrations increased following exercise on both treatments. The present study provides evidence that a 24 h fast is detrimental to high-intensity exercise performance and possibly influences the metabolic response following maximal cycle exercise. These changes may be related to the altered pre-exercise acid-base status and/or a change in the pattern of substrate utilisation.