Carbohydrate metabolism refers to the chemical turnover and breakdown of sugars in order to meet the increased energetic needs of the body's tissues, particularly muscle, during physical exercise. This includes both the mobilization of stored sources of sugar (glycogen metabolism) in muscle and liver, and the chemical breakdown of monosaccharides (simple sugars such as glucose, fructose) into pyruvate via a metabolic pathway known as glycolysis.
Exercise requires the mechanical contraction of muscle tissue. These contractions enable individuals to move their limbs or modulate their heart rate to meet the physical demands of exercise. Muscle contractions require energy, and muscle tissue generates this energy by releasing it from a chemical fuel called adenosine
triphosphate (ATP). However, the amount of ATP in each gram of muscle is only enough to support about ten cycles of contraction (a few seconds worth) before it is exhausted. Thus, muscles must constantly regenerate ATP to support rapid contractions during exercise.
Muscles can replenish their supply of ATP by two principal mechanisms. First, they can transfer energy from another chemical fuel, creatinine phosphate, to regenerate ATP. Stores of creatinine phosphate can support an additional 50 contraction events. Beyond this, muscles must harness energy for ATP production from the breakdown of nutrients such as simple sugars (carbohydrates) or fats in a metabolic process known as catabolism.
Upon completion of exercise, the consumption rate of oxygen in muscle no longer outstrips the supply available from the bloodstream. At this point, oxygen levels in muscle rise and allow the resumption of normal aerobic metabolism. The lactic acid produced during anaerobic metabolism is transported by the bloodstream to the liver, where it is converted back to glucose and/or glycogen. Restoration of the stores of glycogen in muscle is dependent on diet and can take hours to days, depending on the intensity of physical exertion. The dependence of muscle stores of glycogen on diet is the basis for “carbohydrate loading” in endurance athletes. These diets are designed to maximize the levels of glycogen storage in muscles immediately preceding a period of intense exercise, such as a marathon.
The muscles of trained athletes can have oxygen uptakes levels up to twice those of average individuals, allowing them to perform much greater levels of activity without triggering anaerobic metabolism and lactic acid accumulation.
The ability of exercise to increase the rate of carbohydrate metabolism in muscle is an important factor in how exercise can promote the consumption of excess calories and facilitate weight loss. It is nearly universally beneficial in both the average individual and the trained athlete. The increased level of carbohydrate metabolism during exercise does not normally disrupt blood glucose homeostasis (maintaining a resting supply of glucose in the blood that is absolutely required by the brain and red blood cells) because the liver can regenerate glucose from lactic acid (Cori cycle) or certain amino acids, such as alanine.
Combinations of poor diet, high levels of physical exertion, and hormone imbalances can cause exercise-induced hypoglycemia (too little sugar in the bloodstream). Individuals dependent on insulin injections or otherwise prone to hypoglycemia should be careful to keep dietary sources of glucose available during periods of exercise. Even in healthy individuals, the body cannot sustain elevated carbohydrate metabolism by muscles for more than two hours of moderate or intense exercise. At this point, athletes can “hit a wall,” at which point their muscles require either a dietary source of carbohydrate, or a switch to aerobic metabolism and fatty acid fuels, and consequently a lower rate of muscle activity.
See also Fat ; Calories ; Carbohydrates ; Metabolism and energy ; Weight loss .
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Silbernagl, Stefan, and Agamemmnon Despopoulos. Color Atlas of Physiology, 7th ed. New York: Thieme, 2015.
Quinn, Elizabeth. “Energy for Exercise—How What You Eat Helps You Move.” Verywell. April 5, 2016. https://www.verywell.com/how-carbs-fat-and-protein-fuelexercise-3120663 (accessed January 15, 2017).
Daniel M. Cohen, PhD