Pre-Exercise Glucose Ingestion at Different Time Periodsand Blood Glucose Concentration During Exercise

The purpose of this study was to investigate the effects of glucose ingestion (GI) at different time periods prior to exercise on blood glucose (BG) levels during prolonged treadmill running. Eight subjects (X ± SD), age 20 ± 0.5 yr, bodymass 70.7 ±4.1 kg, height 177 ± 4 cm, V˙O₂max 52.8 ± 7.8 ml × kg^-1 × min^-1 who underwent different experimental conditions ingested a glucose solution (1 g/kg at 350 ml) 30 min (gl-30), 60 min (gl-60), 90 min (gl-90), and a placebo one 60 min (pl-60) prior to exercise in a counterbalanced design. Afterwards they ran at 65 % of V˙O₂max for 1 hour and then at 75 % of V˙O₂max till exhaustion. Fingertip blood samples (10 µl) were drawn every 15 min before and during exercise for the determination of BG levels. Oxygen uptake (V˙O₂), heart rate (HR), and blood lactate (La) were also measured every 15 min during exercise. Peak BG values were reached within 30 min after GI but were different (p < 0.01) at the onset of exercise (gl-30: 147 ± 22, gl-60: 118 ± 25, gl-90: 109 ± 22, pl-60: 79 ± 5 mg/dl). The two-way ANOVA repeated measures and the Tukey post-hoc test revealed a higher BG concentration (p < 0.05) for the gl-30 and the pl-60 as compared to the gl-60 and gl-90 during running (e. g. 15 min run: 82 ± 11, 68 ± 5, 64 ± 3, 78 ± 7, and 60 min run: 98 ± 12, 85 ± 12, 83 ± 11, 94 ± 11 mg/dl for gl-30, gl-60, gl-90, and pl-60, respectively). However, this did not significantly affect the duration of treadmill running. The La levels were higher (p < 0.05) after GI as compared to placebo throughout exercise (values at exhaustion: 4.6 ± 0.2, 5.0 ± 1.5, 4.8 ± 1.7 mmol/l for gl-30, gl-60, gl-90, and 3.5 ± 0.8 mmol/l for placebo). The gl-30 and the placebo fluctuated closer to normoglycaemic levels. The glucose ingestion (60 to 90 min) prior to exercise lowered the blood glucose levels without affecting the duration of running performance at 75 % V˙O₂max. Thus, in order to maintain normoglycaemic levels, pre-exercise glucose supplementation should be given 30 min before the onset of exercise.

Key words:

Glucose ingestion, blood glucose concentration, running performance.

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