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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 from the muscles through the blood to other places. The glycogen is stored within a muscle fiber, not the overall muscle, so when a fiber gets glycogen depleted, it can't use glycogen from surrounding fibers.
* 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.)
* 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"/>.
* Each gram of Glycogen is stored with between 3-4g of water<ref name="OlssonSaltin1970"/>. This means you can lose a lot of weight rapidly after long runs or brief diets, but this is mostly water weight. This water can provide hydration, offset sweating or increasing urine volumes. Consequently, you may need less fluid earlier in a run compared with later.
* [[High Intensity Interval Training]] may deplete glycogen reserves rapidly. This is anerobic exercise only produces 1/15 the energy from glycogen, so the typical 2,000 calories energy reserve would only give 133 calories!
=Glycogen Usage=
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Similar things have been seen with [[High Intensity Interval Training| HIIT]]. One meta-analysis found that HIIT is the most effective way of depleting glycogen<ref name="Maclin-2019"/>, thought the analysis did not cover many different protocols.
==Estimating Glycogen Depletion During HIIT==
Assuming that you have 500g of muscle [[Glycogen]], that would normally provide about 2,000 Kcal (Calories) aerobically. However, anaerobically that Glycogen will only provide 1/15 of the energy, which is 133 Kcal. It's estimated that humans use 10.8 ml/min of O2 per watt<ref name="Swain2000"/>, and a liter of O2 is 5Kcal<ref name=" Janot2005"/>, so that works out to 10.8/1000*5=0.054 Kcal/w/min. If you do intervals at 100w above [[Critical Power]], that would burn through the 133 Kcal in (133Kcal/(0.054*100w), or about 25 minutes. At 150w above Critical Power, that drops to about 16 minutes. This is probably best performed as part of [[High Intensity Interval Training| HIIT]].
==Glycogen Restoration after High Intensity Exercise==
The glycogen depletion during HIIT seems largely due to the incomplete metabolism of glucose, resulting in lactate accumulation while producing far fewer calories than complete metabolism. The implications of this depend on how much glycogen is replenished from the conversion of lactate back to Glycogen.
<ref name="Maclin-2019">Macklin, Ian; Wyatt, Frank; Ramos, Malaeni; and Ralston, Grant (2019) "Muscle Glycogen Depletion and Replenishment: A Meta-Analytic Review," ''International Journal of Exercise Science: Conference Proceedings'': Vol. 2 : Iss. 11 , Article 10.
Available at: https://digitalcommons.wku.edu/ijesab/vol2/iss11/10</ref>
<ref name=" Janot2005">Janot, Jeffrey M. "Calculating caloric expenditure." ''IDEA Fitness Journal'' 2.6 (2005): 32-33.</ref>
<ref name="Swain2000">David P. Swain, Energy Cost Calculations for Exercise Prescription, Sports Medicine, volume 30, issue 1, 2000, pages 17–22, ISSN [http://www.worldcat.org/issn/0112-1642 0112-1642], doi [http://dx.doi.org/10.2165/00007256-200030010-00002 10.2165/00007256-200030010-00002]</ref>
</references>
[[Category:Advanced]]