Fuel metabolism during ultra-endurance exercise

Abstract

 Cyclists either ingested 300 ml 100 g/l U-[¹⁴C] glucose solution every 30 min during 6 h rides at 55% of VO_2max (n=6) or they consumed unlabelled glucose and were infused with U-[¹⁴C] lactate (n=5). Maintenance of euglycaemia limited rises in circulating free fatty acids, noradrenaline 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 71±9 μmol/min/kg in the last 3 h of exercise, glucose and lactate oxidation and interconversion rates remained at approximately 60 and 50 and at about 4 and 5 μmol/min/kg, respectively. Continued high rates of carbohydrate oxidation led to a total oxidation of around 270 g glucose, 130 g plasma lactate and 530 g muscle glycogen. Oxidation of some 530 g of muscle glycogen far exceeded the predicted (about 250 g) initial glycogen content of the active muscles and suggested that there must have been a considerable diffusion of unlabelled lactate from glycogen breakdown in inactive muscle fibres to adjacent active muscle fibres via the interstitial fluid that did not equilibrate with ¹⁴C lactate in the circulation.