Influence of glucose and fructose ingestion on the capacity for long-term exercise in well-trained men

Summary. The aim of the present study was to examine the influence of glucose and fructose ingestion on the capacity to perform prolonged heavy exercise. Eight well-trained healthy volunteers exercised on a bicycle ergometer at 68±3% of their VO₂ max until exhaustion, on three occasions, with 8-day intervals. During the exercise they ingested either glucose (250 ml, 7%), fructose (250 ml, 7%) or water (250 ml) every 20 min in a double-blind randomized study design. Arterial blood samples were collected at rest and during exercise for the determination of substrates and hormones. Muscle glycogen content (m. quadriceps femoris) was measured before and after exercise. The duration of exercise lengthened with repeated exercise (3rd test: 136±13 min v. 1st test: 110±12 min, P<0·01). Corrected for the sequence effect, total work time until exhaustion was significantly longer with glucose (137±13 min) than with either fructose (114±12 min) or water (116±13 min) (both P<0·01). When glucose or fructose was ingested, the arterial plasma glucose concentration was maintained at the normoglycaemic level; with water ingestion, plasma glucose values fell during exercise in seven subjects and remained at the resting level in the eighth subject. The muscle glycogen concentration was 467±29 mmol kg d.w.^-1 at rest and fell to approximately half the initial value at exhaustion. In the subgroup of seven subjects in whom glucose values decreased with water intake, the mean rate of glycogen degradation was significantly lower (P<0·05) with the ingestion of glucose (1·3±0·4 mmol kg d.w.^-1 min^-1) as compared to fructose (2·1±0·5 mmol kg d.w.^-1 min^-1) or water (2·3±0·5 mmol kg d.w.^-1 min^-1). Intermittent glucose ingestion (3×17·5 g h^-1) during prolonged, heavy bicycle exercise postpones exhaustion and exerts a glycogen-conserving effect in the working muscles. In contrast, fructose ingestion during exercise maintains the glucose concentration at the basal level but fails to influence either muscle glycogen degradation or endurance performance.