Exogenous carbohydrate oxidation during ultraendurance exercise

The purposes of this study were: 1) to obtain a measure of exogenous carbohydrate (CHOExo) oxidation and plasma glucose kinetics during 5 h of exercise; and 2) to compare CHOExo following the ingestion of a glucose solution (Glu) or a glucose + fructose solution ( 2: 1 ratio, Glu + Fru) during ultraendurance exercise. Eight well-trained subjects exercised three times for 5 h at 58% maximum O-2 consumption while ingesting either Glu or Glu + Fru (both delivering 1.5 g/min CHO) or water. The CHO used had a naturally high C-13 enrichment, and five subjects received a primed continuous intravenous [6,6-H-2(2)] glucose infusion. CHOExo rates following the ingestion of Glu leveled off after 120 min and peaked at 1.24 +/- 0.04 g/min. The ingestion of Glu + Fru resulted in a significantly higher peak rate of CHOExo (1.40 +/- 0.08 g/min), a faster rate of increase in CHOExo, and an increase in the percentage of CHOExo oxidized (65-77%). However, the rate of appearance and disappearance of Glu continued to increase during exercise, with no differences between trials. These data suggest an important role for gluconeogenesis during the later stages of exercise. Following the ingestion of Glu + Fru, cadence (rpm) was maintained, and the perception of stomach fullness was reduced relative to Glu. The ingestion of Glu + Fru increases CHOExo compared with the ingestion of Glu alone, potentially through the oxidation of CHOExo in the liver or through the conversion to, and oxidation of, lactate.