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Metabolic responses to exercise after fasting.

G L Dohm, R T Beeker, R G Israel

    Journal of Applied Physiology (Bethesda, Md. : 1985)
    |October 1, 1986
    PubMed
    Summary
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    Fasting before exercise enhances fat burning and maintains blood glucose levels. This study shows that the body effectively manages glucose homeostasis during exercise after a 23-hour fast.

    Area of Science:

    • Exercise Physiology
    • Human Metabolism

    Background:

    • Fasting increases fat utilization and spares muscle glycogen during exercise.
    • A 24-hour fast significantly depletes liver glycogen stores.

    Purpose of the Study:

    • To investigate the impact of fasting on blood glucose homeostasis during exercise.
    • To analyze blood metabolite concentrations during exercise following a prolonged fast.

    Main Methods:

    • Nine male subjects completed two counterbalanced trials: one fed and one after a 23-hour fast.
    • Subjects performed exercise at 70% of maximum oxygen consumption.
    • Plasma glucose, lactate, free fatty acids, glycerol, and beta-hydroxybutyrate were measured.

    Main Results:

    • Blood lactate levels were significantly higher in the fasted trial throughout exercise.

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  • Fat mobilization and utilization increased in the fasted state, indicated by elevated free fatty acids, glycerol, and beta-hydroxybutyrate.
  • Despite liver glycogen depletion, plasma glucose levels were maintained during exercise in the fasted trial.

    • Conclusions:

    • Human blood glucose concentration is maintained during exercise after fasting, despite depleted liver glycogen.
    • Glucose homeostasis is likely achieved through increased gluconeogenesis and reduced muscle glucose utilization due to lower pyruvate dehydrogenase activity.