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We examined the influence of various carbohydrates of fuel homeostasis and glycogen utilization during prolonged exercise. Seventy-five grams of glucose, fructose, or placebo were given orally to eight healthy males 45 min before ergometer exercise performed for 2 h at 55% of maximal aerobic power (VO2max). After glucose ingestion, the rises in plasma glucose (P less than 0.01) and insulin (P less than 0.001) were 2.4- and 5.8-fold greater than when fructose was consumed. After 30 min of exercise following glucose ingestion, the plasma glucose concentration had declined to a nadir of 3.9 +/- 0.3 mmol/l, and plasma insulin had returned to basal levels. The fall in plasma glucose was closely related to the preexercise glucose (r = 0.98, P less than 0.001) and insulin (r = 0.66, P less than 0.05) levels. The rate of endogenous glucose production and utilization rose similarly by 2.8-fold during exercise in fructose group and were 10-15% higher than in placebo group (P less than 0.05). Serum free fatty acid levels were 1.5- to 2-fold higher (P less than 0.01) after placebo than carbohydrate ingestion. Muscle glycogen concentration in the quadriceps femoris fell in all three groups by 60-65% (P less than 0.001) during exercise. These data indicate that fructose ingestion, though causing smaller perturbations in plasma glucose, insulin, and gastrointestinal polypeptide (GIP) levels than glucose ingestion, was no more effective than glucose or placebo in sparing glycogen during a long-term exercise.

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Resting ECG was recorded in 59 endurance trained athletic and 81 non-athletic boys aged 10-17 years and the findings were correlated with heart volume and cardiorespiratory fitness. The two groups were physically similar, but the athletes had significantly higher maximal oxygen uptake and the 15-17-year-old athletes had larger heart volumes. ECG findings were rather similar in both groups, the major differences being a lower heart rate in the athletes than in the controls (71 vs 82 beats/min) and a longer PQ interval (0.151 vs. 0.140 s) and a greater sum of SV2 and RV4 (59 vs. 50 mm) in the 15-17-year-old athletes. In the controls no correlation existed between precordial voltage criteria for ventricular hypertrophy and heart volume or between heart volume and cardiorespiratory fitness. Contrary to this, in the athletes both SV2 + RV5 and SV2 + RV4 correlated significantly (r about 0.40) with relative heart volume, and relative heart volume with both maximal oxygen uptake per kg (r = 0.41) and calculated work at heart rate 170 beats/min expressed per kg (r = 0.61). Our findings seem to indicate that as a consequence of endurance training the high level of cardiorespiratory fitness becomes related to a large heart volume. It is obvious that ECG changes due to relative vagal dominance develop earlier in the adolescent athletes than those attributable to anatomical changes.