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Glycogen

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=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~~ 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|V̇O2max]]<ref name="Costill-1971"/>.

* Each gram of Glycogen is stored with between 3-4g of water<ref name="OlssonSaltin1970"/>.

=Glycogen Usage=

{| class="wikitable"

At low exercise intensity the majority of the energy comes from free fatty acids in the blood, with a little bit of blood glucose and a little bit of muscle triglyceride. As the exercise intensity increases the contribution of free fatty acids drops. The contribution of blood glucose increases with exercise intensity, but not as dramatically as the contribution of muscle glycogen. At higher intensity muscle glycogen is the major energy source and is critical for performance.

At 65% [[VO2max|V̇O2max]], the usage of different substrates changes over time. The reduced usage of muscle glycogen may be due to a reduction in the availability of the glycogen. Over the two hour period shown, the fat:carbohydrate ratio changes from around 55:45 to 65:35. This change would reduce power output (running speed) at the fixed percentage of [[VO2max|V̇O2max]] (see ~~‘Glycogen~~ 'Glycogen Depletion and [[Breathing]]’ ' below).

=Glycogen Depletion=

The chart<ref name="selective"/> below shows that muscles do not become glycogen depleted at the same time. At all intensities shown, slow twitch fibers become depleted before fast twitch. The depletion within a fiber type is also not equivalent, with some fibers becoming depleted while others are fully loaded. This pattern implies a pattern of [[Muscle|Muscle Recruitment]], where a subset of muscle fibers are recruited until they become exhausted, at which point other fibers are then used. As the slow twitch fibers become exhausted, fast twitch fibers are used in turn.

[[File:Glycogen depletion ST FT.jpg|none|thumb|800px|Glycogen depletion in human muscle fibers. The bars are colored with black indicating high glycogen content through to white indicating glycogen depletion. Three different intensities are shown; high (84% [[VO2max|V̇O2max]]) medium (64 %[[VO2max|V̇O2max]]) and low (31 %[[VO2max|V̇O2max]]) for each of Slow Twitch and Fast Twitch muscle fibers.]]

=Glycogen Depletion and Breathing Rate=

It requires more oxygen to produce energy from fat than carbohydrate<ref name="vent"/>. This may be why higher intensity exercise harder shifts to burning more carbohydrate. When our muscles become depleted of glycogen, muscles are forced to burn more fat. At any given exercise intensity , we will use more oxygen when we are glycogen depleted. This means our [[Heart Rate]] will be higher and out [[Breathing]] will be deeper and faster. It also means our perceived exertion is much higher for a given pace when glycogen depleted. This effect is most noticeable at the end of a long run or a marathon race, and it becomes much harder to stay on target pace. In fact, it can become up to 207% harder and this can be the difference between relaxed easy [[Breathing]] and panting for breath. This [[Heart Rate Drift| increased demand for oxygen]] can often be seen in the [[Running Efficiency Calculator|calculated running efficiency]]. In addition, the amount of O2 that is extracted from the air is lower with glycogen depletion, probably because breathing rate is driven by CO2 concentrations<ref name="KyrPullinen2000"/>.

[[File:Ventilatory response and glycogen depletion.jpg|none|thumb|400px|This graph <ref name="vent"/> shows the relationship between a cyclist's power output and their breathing rate in normal and glycogen depleted states.]]

=Glycogen Depletion and Muscle Damage=

Muscle biopsies taken after a marathon show damage to muscle fibers, but this damage appears focused on a subset of the fibers<ref name="Warhol-1985"/>. Some fibers show no damage, but adjacent fibers are badly affected. The damaged fibers are depleted of Glycogen and lipids (fat). It seems reasonable to me that this pattern of selective damage is due to the pattern of fibers recruitment, with the fibers that are recruited first becoming both glycogen depleted and damaged. Similar damage can be seen with [[Delayed Onset Muscle Soreness]]. The images below are taken from the gastrocnemius (calf), 24-48 hours after a marathon race,

|[[File:MarathonFiberDamage2.jpg|thumb|400px|Here you can see extreme damage, with only the Z band of the fiber remaining (marked Z). Adjacent fibers show far less damage.]]

|}

=Glycogen and other fuels=

Glycogen is one of several types of fuel the human body can metabolize. Below is a table describing the characteristics of the primary types of fuel<ref name="plowman-2007"/>. Some important notes:

* Carbohydrate requires less oxygen than fat to produce a calorie of energy.

* Muscles generally burn BCAA<ref name="plowman-2007"/>.

* The ratio of O2 to CO2 is called the respiratory quotient or RQ.

* The figures for fat assume full metabolism rather than a ketogenic state. A ketogenic metabolism of fat can result in an RQ below 0.7<ref name="Schutz-1980"/>.

* It is possible for RQ to be above 1.0 if the carbohydrate is converted to fat rather than metabolized<ref name="Gottschlich-2001"/>. {| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"! style="background-color: #F2F2F2;" |'''Fuel Type'''! style="background-color: #F2F2F2;" colspan="2"| '''Carbohydrate'''! style="background-color: #F2F2F2;" colspan="2"| [['''Protein]]'''! style="background-color: #F2F2F2;" |'''Fat'''

|-

|-

| style="background-color: #F9F9F9;" | Oxygen needed per gram (l/g)| style="background-color: #F9F9F9;" | 0.75| style="background-color: #F9F9F9;" | 0.83| style="background-color: #F9F9F9;" | 0.965| style="background-color: #F9F9F9;" | 1.24| style="background-color: #F9F9F9;" | 2.02

|-

| style="background-color: #F9F9F9;" | Energy per gram (Kcal/g)| style="background-color: #F9F9F9;" | 3.75| style="background-color: #F9F9F9;" | 4.17| style="background-color: #F9F9F9;" | 4.3| style="background-color: #F9F9F9;" | 3.76| style="background-color: #F9F9F9;" | 9.3

|-

| style="background-color: #F9F9F9;" | Energy per Liter of Oxygen (Kcal/l O2)| style="background-color: #F9F9F9;" | 5.03| style="background-color: #F9F9F9;" | 5.03| style="background-color: #F9F9F9;" | 4.46| style="background-color: #F9F9F9;" | 3.03| style="background-color: #F9F9F9;" | 4.61

|-

| style="background-color: #F9F9F9;" | Carbon Dioxide produced per gram (l/g)| style="background-color: #F9F9F9;" | 0.75| style="background-color: #F9F9F9;" | 0.83| style="background-color: #F9F9F9;" | 0.781| style="background-color: #F9F9F9;" | 0.92| style="background-color: #F9F9F9;" | 1.43

|-

| style="background-color: #F9F9F9;" | RQ (O2:CO2)| style="background-color: #F9F9F9;" | 1.00| style="background-color: #F9F9F9;" | 1.00| style="background-color: #F9F9F9;" | 0.81| style="background-color: #F9F9F9;" | 0.74| style="background-color: #F9F9F9;" | 0.71

|}

=Glycogen Storage and Body Fat Scales (Bioimpedence)=One study<ref name="ShioseYamada2018"/> concluded that changes in [[Glycogen]] storage shouldn't change the way [[Body Fat Scales]] estimate fat using bioimpedence. This means that increased Glycogen will result in a lower body fat estimate, as fat mass stays constant and body weight increases<ref name="ShioseYamada2016"/>. Of course, different scales will use different models that may give different results.

=References=

<ref name="ShioseYamada2018">Keisuke Shiose, Yosuke Yamada, Keiko Motonaga, Hideyuki Takahashi, Muscle glycogen depletion does not alter segmental extracellular and intracellular water distribution measured using bioimpedance spectroscopy, Journal of Applied Physiology, volume 124, issue 6, 2018, pages 1420–1425, ISSN [http://www.worldcat.org/issn/8750-7587 8750-7587], doi [http://dx.doi.org/10.1152/japplphysiol.00666.2017 10.1152/japplphysiol.00666.2017]</ref>

<ref name="selective">Selective glycogen depletion in skeletal muscle fibers of man following sustained contractions http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1331072/</ref>

<ref name="romijn">Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration http://ajpendo.physiology.org/content/265/3/E380.short</ref>

<ref name="LoonGreenhaff2001">Luc J. C. van Loon, Paul L. Greenhaff, D. Constantin-Teodosiu, Wim H. M. Saris, Anton J. M. Wagenmakers, The effects of increasing exercise intensity on muscle fuel utilisation in humans, The Journal of Physiology, volume 536, issue 1, 2001, pages 295–304, ISSN [http://www.worldcat.org/issn/0022-3751 0022-3751], doi [http://dx.doi.org/10.1111/j.1469-7793.2001.00295.x 10.1111/j.1469-7793.2001.00295.x]</ref>

<ref name="KyrPullinen2000">H. Kyröläinen, T. Pullinen, R. Candau, J. Avela, P. Huttunen, P. V. Komi, Effects of marathon running on running economy and kinematics, European Journal of Applied Physiology, volume 82, issue 4, 2000, pages 297–304, ISSN [http://www.worldcat.org/issn/1439-6319 1439-6319], doi [http://dx.doi.org/10.1007/s004210000219 10.1007/s004210000219]</ref>

<ref name="OlssonSaltin1970">Karl-Erik Olsson, Bengt Saltin, Variation in Total Body Water with Muscle Glycogen Changes in Man, Acta Physiologica Scandinavica, volume 80, issue 1, 1970, pages 11–18, ISSN [http://www.worldcat.org/issn/00016772 00016772], doi [http://dx.doi.org/10.1111/j.1748-1716.1970.tb04764.x 10.1111/j.1748-1716.1970.tb04764.x]</ref>

<ref name="ShioseYamada2016">Keisuke Shiose, Yosuke Yamada, Keiko Motonaga, Hiroyuki Sagayama, Yasuki Higaki, Hiroaki Tanaka, Hideyuki Takahashi, Segmental extracellular and intracellular water distribution and muscle glycogen after 72-h carbohydrate loading using spectroscopic techniques, Journal of Applied Physiology, volume 121, issue 1, 2016, pages 205–211, ISSN [http://www.worldcat.org/issn/8750-7587 8750-7587], doi [http://dx.doi.org/10.1152/japplphysiol.00126.2016 10.1152/japplphysiol.00126.2016]</ref>

</references>

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