This process occurs relatively slowly as compared with the mobilization of carbohydrate for fuel. These fatty acids are transported through the blood to muscles for fuel. During exercise, stored fat in the body (in the form of triglycerides in adipose or fat tissue) is broken down into fatty acids. Foods that you eat or drink during exercise that supply carbohydrate can help delay the depletion of muscle glycogen and prevent hypoglycemia.įat is the body's most concentrated source of energy, providing more than twice as much potential energy as carbohydrate or protein (9 calories per gram versus 4 calories each per gram). When the liver is out of glycogen, you'll “bonk” as your blood glucose level dips too low, and the resulting hypoglycemia (low blood sugar) will further slow you down. To keep up with this greatly elevated demand for glucose, liver glycogen stores become rapidly depleted. As we exercise, our muscle glycogen reserves continually decease, and blood glucose plays an increasingly greater role in meeting the body's energy demands. If you've ever hit the wall while exercising, you know what muscle glycogen depletion feels like. The capacity of your body to store muscle and liver glycogen, however, is limited to approximately 1,800 to 2,000 calories worth of energy, or enough fuel for 90 to 120 minutes of continuous, vigorous activity. The carbohydrate content of your diet and the type and amount of training that you undertake influence the size of your glycogen stores. The body constantly uses and replenishes its glycogen stores. Blood glucose also serves as the most significant source of energy for the brain, both at rest and during exercise. During exercise, your muscles pick up some of this glucose and use it in addition to their own private glycogen stores. The liver converts its glycogen back into glucose, too however, it's released directly into the bloodstream to maintain your blood sugar (blood glucose) level. During exercise, muscle glycogen is converted back into glucose, which only the muscle fibers can use as fuel. Glucose can be used immediately as fuel, or can be sent to the liver and muscles and stored as glycogen. Carbohydrates, such as sugar and starch, for example, are readily broken down into glucose, the body's principal energy source. (See table 2.1, Estimated Energy Stores in Humans.) The body can store some of these fuels in a form that offers muscles an immediate source of energy. This energy takes three forms: carbohydrate, fat, and protein. Our daily food choices resupply the potential energy, or fuel, that the body requires to continue to function normally. To sustain physical activity, however, cells must constantly replenish both CP and ATP. It's another high-energy compound that can be rapidly mobilized to help fuel short, explosive efforts. Creatine phosphate (CP), like ATP, is also stored in small amounts within cells. ATP, in fact, is the only molecule able to provide energy to muscle fibers to power muscle contractions. Anytime you need energy-to breathe, to tie your shoes, or to cycle 100 miles (160 km)-your body uses ATP molecules. Think of ATP molecules as high-energy compounds or batteries that store energy. As potential fuel sources, the carbohydrate, fat, and protein in the foods that you eat follow different metabolic paths in the body, but they all ultimately yield water, carbon dioxide, and a chemical energy called adenosine triphosphate (ATP). Our ability to run, bicycle, ski, swim, and row hinges on the capacity of the body to extract energy from ingested food. This is an excerpt from Endurance Sports Nutrition-3rd Edition by Suzanne Girard Eberle.
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