The digestion of starch in carbohydrates begins in the mouth and then continues in the small intestine. The main product of carbohydrate metabolism is glucose, or blood-sugar. In this form it enters our blood stream and first supplies the energy needs of our central nervous system. Any glucose not used immediately is stored in the liver or muscles as glycogen. The excess is converted to fat and stored throughout the body. Glycogen reserves are important because this is the primary fuel of hard working muscles, and supply of it is limited.
The body can store only a limited supply of glycogen: approximately 350 grams when the supply is at its peak. One-third of the amount is stored in the liver and the remainder in the muscles. Liver glycogen is available for immediate use. It is quickly converted into glucose when needed by the body. Muscles glycogen, however, does not have the necessary enzymes for this direct secretion into body fuel. It furnishes glucose indirectly. When the muscle contracts, glycogen is converted into Lactic Acid. The Lactic Acid is carried in the bloodstream to the liver and then converted into glycogen or glucose as needed by the body. For this reason, it does not reach the brain and nervous system as directly as liver glycogen.
The reserve of glycogen lasts 2-15 hours, depending on activity levels. Someone playing checkers can have enough to last most of the day. Body-builders in heavy training can use their entire supply of glycogen within 2-3 hours. The body will then switch to alternative, but, less efficient energy fuels. Muscle protein, for instance, can be converted by the liver into glucose in order to keep the brain and nerves supplied with fuel. However, this puts unnecessary stress on the liver. It also drains the supply of amino acids needed for building muscle and repairing the body.
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