Abstract
As exercise intensity exceeds 65% of maximal oxygen uptake carbohydrate energy sources predominate. However, relative to the meager 4–5 g blood glucose pool size in a postabsorptive individual (0.9–1.0 g·L−1 × 5 L blood = 18–20 kcal), carbohydrate (CHO) oxidation rates of 20 kcal·min−1 can be sustained in a healthy and fit person for one hour, if not longer, all the while euglycemia is maintained. While glucose rate of appearance (i.e., production, Ra) from splanchnic sources in a postabsorptive person can rise 2–3 fold during exercise, working muscle and adipose tissue glucose uptake must be restricted while other energy substrates such as glycogen, lactate, and fatty acids are mobilized and utilized. If not for the use of alternative energy substrates hypoglycemia would occur in less than a minute during hard exercise because blood glucose disposal rate (Rd) could easily exceed glucose production (Ra) from hepatic glycogenolysis and gluconeogenesis. The goal of this paper is to present and discuss the integration of physiological, neuroendocrine, circulatory, and biochemical mechanisms necessary for maintenance of euglycemia during sustained hard physical exercise.
Highlights
Maintenance of blood glucose concentration in a narrow range is a major physiological priority [1]
Blood glucose concentration remains constant for exercise durations of one hour or more in untrained subjects, while in trained subjects blood may rise slightly during hard exercise such as at the lactate threshold (LT)
Not completely satisfied with the proposition that glucoregulatory hormone responses during exercise can explain observed phenomena, others have asked the question, paraphrased “How does the liver know what the muscles are doing?” In our experience we have studied the effects of nutrition, exercise intensity, and exercise training, gender, menstrual cycle phase, oral contraceptives, aging, and high altitude exposure; we and many others have observed that euglycemia is well maintained over a broad range of challenges
Summary
Maintenance of blood glucose concentration in a narrow range is a major physiological priority [1]. The problem of maintaining blood glucose homeostasis (euglycemia) is always a major challenge, after high carbohydrate containing meals and during physical exercise when the demand for carbohydrate (CHO) energy use is huge compared to blood glucose content. Noteworthy is that while the glucose rate of appearance (i.e., production, Ra) from splanchnic sources in a postabsorptive person can rise 2–3 fold during exercise, working and non-working muscle and adipose tissue glucose use must be restricted. Without those restrictions hypoglycemia would result in less than one minute during hard exercise because blood glucose disposal rate (Rd) could exceed glucose production (Ra) from hepatic glycogenolysis and gluconeogenesis (GNG)
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