FUEL METABOLISM DURING ULTRA-ENDURANCE EXERCISE

Cyclists either ingested 300 ml 100 g/l U-[C-14] glucose solution ever y 30 min during 6 h rides at 55% of VO2max (n = 6) or they consumed un labelled glucose and were infused with U-[C-14] lactate (n = 5). Maint enance of euglycaemia limited rises in circulating free fatty acids, n oradrenaline and adrenaline concentrations to 0.9+/-0.1 mM, 27+/-4 nM and 2.0+/-0.5 nM, respectively, and sustained the oxidation of glucose and lactate. As muscle glycogen oxidation declined from 100+/-13 to 7 1+/-9 mu mol/min/kg in the last 3 h of exercise, glucose and lactate o xidation and interconversion rates remained at approximately 60 and 50 and at about 4 and 5 mu mol/min/kg, respectively. Continued high rate s of carbohydrate oxidation led to a total oxidation of around 270 g g lucose, 130 g plasma lactate and 530 g muscle glycogen. Oxidation of s ome 530 g of muscle glycogen far exceeded the predicted (about 250 g) initial glycogen content of the active muscles and suggested that ther e must have been a considerable diffusion of unlabelled lactate from g lycogen breakdown in inactive muscle fibres to adjacent active muscle fibres via the interstitial fluid that did not equilibrate with C-14 l actate in the circulation.