SUBSTRATE USAGE DURING PROLONGED EXERCISE ON MORNING AND EVENING

Abstract

Substrate utilization during 60 min of endurance exercise (50-60%VO₂max) performed in the morning and evening was compared in 11 subjects. After rising at 0600-0630 with a 10-h fast, they exercised without a meal (fasting) or after intake of 40g of carbohydrate feeding (snack) . In addition, subjects consumed curry and rice for lunch 4-h before exercise and exercised at about 1630 (evening) .
In the fasting trial, there was no significant change in plasma glucose (J 100.8 vs 93.0; R 101.0 vs 105.6 mg/dl) before and after exercise between the joggers (J) and the runners (R) . However, plasma glucose in only one subject, who had no night meal, decreased to less than 70 mg/dl. Also, although the plasma free fatty acid level was elevated by more than 50% in both groups, the highest value observed was 0.86 mmol/l. Therefore it was suggested that no subjects developed hypoglycemia and weakness that plasma free fatty acid was not markedly elevated during 60 min of endurance exercise after the fasting trial, whereas there was a tendency for plasma β-hydroxybutyrate to be increased in the joggers compared with the runners.
The availability of blood-borne substrates in the trial evening was characterized by a two-fold elevation (p<0.01) of the insulin level prior to exercise, a 10% decline in plasma glucose and suppression of the normal increase in plasma glycerol and FFA turnover during exercise compared with the other two trials, whereas the results for the fasting and snack trials were similar.
The total energy expenditure for 60 min of exercise in the snack trial was significantly 4-5 % higher than in the fasting and evening trial (mean ± SE= 654.4 ± 26.7 kcal, 619.2 ± 21.2 kcal and 627.5 ± 27.5 kcal, respectively) . Percentage of energy obtained from lipid was determined based on the respiratory quotient, which was similar (48.6 vs 49.7%) between the fasting and snack trials. This, however, differed significantly (p<0.01) from the percentage of lipid metabolized (32.6%) during the evening trial. These data indicate that fasting and snack trials in the morning result in preferential oxidation of fat during endurance exercise.