Food Components to Enhance Performance An Evaluation of Potential Performance-Enhancing Food Components for Operational Rations (1994) / Chapter Skim
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11. The Metabolic Responses to Stress and Physical Activity
11. The Metabolic Responses to Stress and Physical Activity
Pages 209-220
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Beisel2 INTRODUCTION Metabolic responses to stress and physical activity are extremely complex, involving many interacting variables. These multiple factors include endocrinological, physiological (cardiovascular and neuromuscular)
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A focus on body fuel metabolism is important, because humans must have Mel available for both the CNS (primarily glucose) and for skeletal muscle (a mixture of glucose and fatty acids)
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At rest, skeletal muscle derives about 85 percent of its energy from fatty acid oxidation, only about 10 percent from glucose oxidation, and maybe 1-2 percent from oxidation of branched chain amino acids. The blood glucose concentration stays very constant in these circumstances, because glucose is being taken up by all of these tissues, and at the same time an equivalent amount is being produced, primarily by the liver.
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The fourth level of integration is the regulation of blood flow, which controls the delivery rate of substrates to exercising muscle. Fifth, there are local cellular factors that regulate glucose uptake and its intracellular metabolism.
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Then, as exercise persists, free fatty acid concentrations in blood increase, and the muscle gradually shifts over to burning more fatty acids and less glucose. The effects of physical training tend to cause a more effective adaptation of skeletal muscle, allowing them to oxidize fatty acids more effectively, and thus to spare both muscle and liver glycogen.
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During an 8-12 min maximum stress test, blood glucose values of a cyclist will overshoot baseline values for a brief time. This is the result of intense sympathetic nervous system stimulation, with an activation of hepatic glucose production that transiently exceeds peripheral glucose utilization.
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As noted, glucagon stimulates hepatic glucose production, while insulin suppresses it. The declining insulin glucagon ratio during exercise is a contributing factor in stimulating hepatic glucose output.
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The chronic norepinephrine infusion had apparently changed the glucose transport system by increasing blood flow to skeletal muscle as well as to brown fat (Lupien et al., 1990~. Some of the mechanisms by which exercise stress increases glucose turnover thus include changes in hepatic metabolism, changes in blood flow to skeletal muscle, and changes in the glucose transport system itself.
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The surprising increase in peripheral glucose uptake, in the absence of an increase in plasma insulin, may have resulted from an increased blood flow to muscles and/or possibly to an activation of the glucose transport system in muscle cells. To determine if the increase in glucose clearance was insulin-dependent or not, Vranic's group did other studies before and after the production of partial alloxan diabetes in the dogs (Miles et al., 1991; Yamatani et al., 1992~.
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There must be, then, a mechanism for increasing peripheral glucose clearance in stress situations that is independent of the increase in peripheral glucose uptake stimulated by physical exercise. The skeletal muscle's uptake of glucose is an important area for study, because glucose transport in skeletal muscle is the major rate-limiting step in glucose utilization.
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Horton 1992 The glucose transport system in skeletal muscle: Effects of exercise and insulin.
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