Carbohydrate intake, muscle metabolism, and enduring running performance in man
The purpose of this thesis was to study the effects of a pre-exercise carbohydrate meal on metabolism, endurance capacity and performance during prolonged running when carbohydrate was, or was not consumed during exercise. The first study (Chapter 4) examined the effects on endurance running capacity of ingesting a carbohydrate-electrolyte solution during treadmill exercise to fatigue at 70% V02 max after subjects (10 males) had undergone an overnight fast (P+C), or when fed with a 2.5 g. kg-1 BW carbohydrate meal 3 hours before exercise (M+C). Exercise time to exhaustion was longer in the M+C (147.4 ± 9.6 min) and P+C (125.1 ± 7.0 min) trials compared with the control condition (P+P: 115.1 ± 17.6 min) (p< 0.01 and p< 0.05 respectively). Also, exercise time was longer in the M+C compared with the P+C trial (p< 0.01). The improvement in endurance capacity in the M+C trial occurred despite a higher carbohydrate oxidation rate during the first hour of exercise. The second study (Chapter 5) examined whether a pre-exercise carbohydrate meal (M+W) can improve endurance capacity, and further examined if the combination of a pre-exercise meal together with the ingestion of a carbohydrate-electrolyte solution during exercise (M+C) would be superior to the carbohydrate meal (M+W) alone. Ten males volunteered in this study. Although the consumption of the meal increased carbohydrate oxidation during the first hour of exercise, exercise time to fatigue at 70% V02 max was longer in the M+C (125.1 + 5.3 min) and M+W (111.9 + 5.6 min) trials compared with the control trial (P+W : 102.9 ± 7.9 min) (p< 0.01 and p< 0.05 respectively). Also, exercise time was longer (p< 0.05) in the M+C compared with the M+W trial. The third study (Chapter 6) investigated whether the high carbohydrate meal can influence muscle glycogen levels. Eight male subjects participated in the study. Three hours after the ingestion of the 2.5 g. kg-1 BW carbohydrate meal, muscle glycogen concentration was 10.6% higher (p< 0.05) in the vastus lateralis muscle (347.3 + 31.3 mmol. kg dw-1) compared with the muscle glycogen concentration before feeding (314 ± 33.9 mmol. kg dw-1). The fourth study (Chapter 7) examined the influence of ingesting a carbohydrate-electrolyte drink (M+C) on the muscle glycogen utilisation during 60 min running at 70% V02 max in subjects (8 males) who had consumed a carbohydrate meal 3 hours before exercise (M+W). Muscle glycogen concentrations were not different before (M+C : 321.9 ± 27.2 vs M+W : 338.8 ± 32.8 mmol. kg dw-1), as well as after exercise (M+C : 225.8 ± 26.7 vs M+W: 261 + 40.5 mmol. kg dw-1) between the two experimental trials. Neither was there any difference in the rate of muscle glycogen utilisation (M+C : 96.1 ± 22.1 vs M+W: 77.9 ± 11.7 mmol. kg dwl. h-1). The aim of the last study (Chapter 8) was to investigate whether, after an overnight fast, the ingestion of a carbohydrate-electrolyte solution during a 30 km self-paced treadmill run (C) would be as effective as the consumption of a carbohydrate meal (M) (2.0 g. kg-1 BW carbohydrate) 4 hours before exercise. Ten males volunteered for this study. The overall performance times in the M and C trials were identical (M: 121.8 ± 3.6 min vs C: 121.7 ± 4.1 min). No differences were found between the two trials in running speeds over each successive 5 km, or even when running speed was analysed every kilometre. Also, no reduction in the self-selected speeds of subjects was observed towards the end of the 30 km run in both conditions. The ingestion of a carbohydrate meal, providing 2.5 g. kg-1 BW carbohydrate, 3 hours before exercise increases muscle glycogen concentration and improves endurance running capacity, despite an elevated carbohydrate oxidation rate during the first hour of exercise. It seems that the amount of carbohydrate given before exercise compensates for the greater carbohydrate used. Furthermore, the combination of both a pre-exercise carbohydrate meal and a carbohydrate-electrolyte solution ingested during exercise further improves endurance capacity.