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[[File:1035px-Glycogen structure.svg.png|right|thumb|200px|A schematic of glycogen, showing a core protein surrounded by strands of glucose.]]
Our bodies store carbohydrate as glycogen, the critical fuel supply for endurance running. =Introduction=While the human body has sufficient stores of fat to run vast distances, the supply of carbohydrate is quite limited. This carbohydrate store is in the form of Glycogen, a branching chain of glucose molecules. * Burning glycogen for energy requires less oxygen than fat, making it more efficient. However, the store of glycogen is limited, and when the supply runs low we "hit the wall". * Glycogen is stored primarily in the muscles, but that glycogen can only be used by the muscle it’s stored in and cannot flow through the blood to other places. * Some glycogen is stored in the liver where it flows through the blood to all tissues.* The human liver typically stores between 90 and 160 grams of Glycogen, or 350 to 650 Calories.
* Blood typically contains less than 20 calories of glucose. (This assumes 5 liters of blood and 100mg/dL of blood glucose, which is 5g of glucose.)
* Glycogen can also be created from protein via a process called [http://en.wikipedia.org/wiki/Gluconeogenesis gluconeogenesis], but not from fat.
* Eccentric exercise, such as [[Downhill Running]], can cause [[[[Delayed Onset Muscle Soreness||DOMS]] and impair glycogen replenishment<ref name="O'Reilly-1987"/>.* Glycogen stores may not be replenished between daily hard runs, such as 10 miles at 80% of [[VO2max|V̇O<sub>2</sub>max]]<ref name="Costill-1971"/>.
=Glycogen Usage=
{| class="wikitable"
<ref name="Warhol-1985"> {{Cite journal | last1 = Warhol | first1 = MJ. | last2 = Siegel | first2 = AJ. | last3 = Evans | first3 = WJ. | last4 = Silverman | first4 = LM. | title = Skeletal muscle injury and repair in marathon runners after competition. | journal = Am J Pathol | volume = 118 | issue = 2 | pages = 331-9 | month = Feb | year = 1985 | doi = | PMID = 3970143 }}</ref>
<ref name="O'Reilly-1987"> {{Cite journal | last1 = O'Reilly | first1 = KP. | last2 = Warhol | first2 = MJ. | last3 = Fielding | first3 = RA. | last4 = Frontera | first4 = WR. | last5 = Meredith | first5 = CN. | last6 = Evans | first6 = WJ. | title = Eccentric exercise-induced muscle damage impairs muscle glycogen repletion. | journal = J Appl Physiol | volume = 63 | issue = 1 | pages = 252-6 | month = Jul | year = 1987 | doi = | PMID = 3624128 }}</ref>
<ref name="Costill-1971"> {{Cite journal | last1 = Costill | first1 = DL. | last2 = Bowers | first2 = R. | last3 = Branam | first3 = G. | last4 = Sparks | first4 = K. | title = Muscle glycogen utilization during prolonged exercise on successive days. | journal = J Appl Physiol | volume = 31 | issue = 6 | pages = 834-8 | month = Dec | year = 1971 | doi = | PMID = 5123660 }}</ref>
</references>