Glycogen Depletion During Athletic Excercise

[CREAO]

I just read this and thought it would be good to share with the board...enjoy

Glycogen Depletion During Athletic Exercise​

by Maria I. Martos
Glucose, the primary source of fuel for all body cells, is derived primarily from carbohydrates, although, if needed, glucose can also be metabolized from protein. After a meal, some of the glucose not used immediately for fuel travels to the liver or skeletal muscles, where it is converted to a compound called Glycogen--through a process called glycogenesis--and stored for energy. Any excess glucose is stored in adipose tissue as fat. The liver has a greater capacity for glycogen storage than muscle: Liver cells can typically store up to 8% of their weight as glycogen, while muscle cells can typically store up to only 3%. The liver is responsible for maintaining adequate levels of glucose in the body. As the body’s glucose level drops, the liver converts some of the glycogen back into glucose--through a process called glycogenolysis--and releases it back into the bloodstream. Muscle cells, on the other hand, are unable to reconvert glycogen to glucose. Instead, they convert glycogen directly to fuel through a process called glycolysis.
Glycolysis is a cellular anaerobic process which, through a complex series of steps, breaks down muscle glycogen into pyruvic acid during high-intensity exercise. This process rapidly produces a small amount of adenosine triphosphate (ATP), the necessary fuel for body cells. However, if too much pyruvic acid accumulates in the muscle during glycolysis, it can substantially slow down or even stop the process of ATP formation. Therefore, after one or two minutes of high-intensity exercise, a subsequent process of energy formation begins--oxidation
Oxidation, an oxygen-requiring process of energy formation, produces over 95% of the energy used by muscles during moderate and prolonged exercise. Oxidation immediately converts much of the pyruvic acid formed through glycolysis to ATP. However, during prolonged exercise, if an athlete is unable to breathe in oxygen quickly enough to oxidize pyruvic acid into ATP, some pyruvic acid is converted to lactic acid and diffused out of the cell. It then circulates throughout the body until it can be reconverted to pyruvic acid once oxygen again becomes available. If excess accumulation of lactic acid occurs, extreme fatigue can set in, which can greatly impair the athlete’s performance.
Glucose is needed by the central nervous system to keep the body functioning. Therefore, during periods of moderate exercise lasting longer than 20 minutes, the body works to conserve stored muscle and liver glycogen. It does so by reducing the percentage of fuel derived from glycogen to only 40% or 50%, with the remainder supplied by fat. During exercise periods lasting longer than 4 or five hours, as much as 60% to 85% of fuel produced by oxidation may be derived from fat.
Fats need carbohydrates in order to burn efficiently. The breakdown of carbohydrates generates oxaloacetic acid, which is needed for the breakdown of fats into fuel. If insufficient carbohydrate levels exist, the levels of oxaloacetic acid may also drop, making it difficult for the body to continue producing a high level of fuel from fat. Although the body can break down fats in the absence of carbohydrates, it does so at a much slower rate. When the glycogen stores in the muscles and liver are depleted, and the blood glucose level begins to fall, athletes begin to experience fatigue, lack of coordination, light-headedness and lack of concentration. This experience is commonly known as "hitting the wall" or "bonking".
Following exhaustive exercise, the body needs to replenish the depleted glycogen reserves. Increasing the intake of carbohydrates promotes the storage of glycogen in the liver and muscles. Therefore, according to Hickson and Wolinsky in their book Nutrition in Exercise and Sport, a diet consisting of approximately 60% or more of complex (starch) carbohydrates is recommended after strenuous exercise in order to promote glycogen replenishment. With adequate consumption of complex carbohydrates, coupled with extra rest, most of the glycogen replenishment occurs within 24 hours. If a diet high in protein and fat is consumed, glycogen replenishment may take longer than one week.
While proper diet is important after an endurance event, it is probably of even greater importance prior to an event. The larger the stores of glycogen in the liver and muscles, the longer and more effectively an athlete can perform during prolonged strenuous exercise. Although many schools of thought exist regarding appropriate nutrition for athletes, most seem to agree that the most important nutrient for endurance athletes is carbohydrates. As much as 60% to 70% of the diet should consist of carbohydrates.

 

[jonny21]

It is important for a Sports Massage Therapist to understand the process of glycogen depletion during athletic exercise. While working with athletes during training or competitive events, the Therapist can provide information to them about the process. Additionally, the Therapist can encourage the athlete to eat a proper diet during training, as well as prior to and after an endurance event in order to provide the body with necessary nutrients for optimum performance. Additionally, and possibly more importantly, the Therapist can detect signs of glycogen depletion and be able to assist the athlete during an event.

^^That part kinda threw me for a bit of a loop. Just seemed out of place. I have a hard time picturing an athlete needing their massage therapist to tell them that are not adequately nourished especially during the event.


Thanks for the posting though.

 

[CREAO]

oops, forgot to take that out...I just thought it was a good article explaining glycogen storage..it cleared up some of my questions

 

[jminis]

good post bro.

 

[Ant.]

Good post. I have a test in Exercise Physiology on that topic on Wednesday.

 

[ssor1005]

Good read, so if one is carb cycling, they should most definitely not have low carb days on workout/cardio days?

 

[CREAO]

Good read, so if one is carb cycling, they should most definitely not have low carb days on workout/cardio days?

I'm not entirely sure, but if was doing a low carb diet..I would still have some kind of carb post workout just to keep the muscles full

 

[scoher]

Good read, so if one is carb cycling, they should most definitely not have low carb days on workout/cardio days?

Depends. UD2 uses high rep workouts in conjunction with low carbs to ensure the body is fully glycogen depleted by time the carb load rolls around.

 

[sublimejeh]

nice post man! good research

 

[UnicronSpawn]

Nice article man, very concise and easy to understand. Pieced together a few of the steps I was unsure of. I have the text book "Advanced nutrition and the human metabolism." and picked up a lot of that sort of thing from it, but it's incredibly detailed and some sections it literally takes me weeks to get a hold on cuz I have to either skip ahead and come back, or keep re-reading parts over and over again for like days at a time before it registers in my mortal brain. Even though the goal of the article was to explain the energy production pathways for the purpose of prescribing an endurance athletes reccomendations, the glycogenesis, glycolosis, glycogenolysis, and oxiditave pathways are valuable for an anaerobic athlete to understand.

 

[greekgeorge]

60-70% carbs, does this study apply to endomorphs, most endos agree on less carbs, less calories in general, did the study go into the body types of the individuals. Good post and study but there are always research flaws

 

[russy_russ]

Glucose, the primary source of fuel for all body cells, is derived primarily from carbohydrates, although, if needed, glucose can also be metabolized from protein. After a meal, some of the glucose not used immediately for fuel travels to the liver or skeletal muscles, where it is converted to a compound called Glycogen--through a process called glycogenesis--and stored for energy.

There are many cells which can use glucose for ATP synthesis. Neurons and red blood cells can only use glucose. However, Type I muscle fibers primarily rely on stored intramuscular triacylglycerols for ATP synthesis and lack high concentrations of ATP synthase. At rest, triacylglycerols are utilized at a greater extent for ATP synthesis than carbohydrates. Alanine is the only amino acid that can directly be converted to glucose in the liver. Other BCAAs can serve as metabolic intermediates.

Any excess glucose is stored in adipose tissue as fat. The liver has a greater capacity for glycogen storage than muscle: Liver cells can typically store up to 8% of their weight as glycogen, while muscle cells can typically store up to only 3%. The liver is responsible for maintaining adequate levels of glucose in the body

Excess glucose promotes its own metabolism and in turn slows down (in some cases can inhibit) oxidation of lipids and protein through neuroendocrine mechanisms. Skeletal muscles can store more glycogen than the liver. It's irrelevant to compare concentration to organ weight, it provides useless meaning to define ATP synthesis. The pancreas is responsible for maintaining serum glucose levels by hormonal regulation.

Glycolysis is a cellular anaerobic process which, through a complex series of steps, breaks down muscle glycogen into pyruvic acid during high-intensity exercise. This process rapidly produces a small amount of adenosine triphosphate (ATP), the necessary fuel for body cells. However, if too much pyruvic acid accumulates in the muscle during glycolysis, it can substantially slow down or even stop the process of ATP formation. Therefore, after one or two minutes of high-intensity exercise, a subsequent process of energy formation begins--oxidation

Recent research suggests that the end result of glycolysis is lactate instead of pyruvate. This notion is supported that lactate is 10x the concentration of pyruvate at rest. During high-intensity exercise Type II myosin heavy chain isoforms are recruited which have a particular lactate dehydrogenase isozyme which promotes the conversion of pyruvate to lactate which relates to lactate accumulation during high intensity exercise. ATP synthesis only ceases when you are dead. High amounts of Acetyl-CoA produced from pyruvate inhibits beta-ketothiolase activity slowing down beta-oxidation. By oxidation you mean oxidative phosphorylation and this metabolic pathway is always active. The degree of intensity can have a direct impact on the percent of ATP synthesis utilized by this pathway.

Oxidation immediately converts much of the pyruvic acid formed through glycolysis to ATP. However, during prolonged exercise, if an athlete is unable to breathe in oxygen quickly enough to oxidize pyruvic acid into ATP, some pyruvic acid is converted to lactic acid and diffused out of the cell. It then circulates throughout the body until it can be reconverted to pyruvic acid once oxygen again becomes available. If excess accumulation of lactic acid occurs, extreme fatigue can set in, which can greatly impair the athlete’s performance.

Oxidative phosphorylation (TCA cycle, ETS) begins after pyruvate converted to acetyl-coa (from glycolysis). Acetyl-CoA is also a metabolic intermediate of beta-oxidation. Pyruvate is not a metabolic intermediate in this pathway. Oxyhemoglobin saturation rarely drops below ~70-80% during exercise. You never utilize 100% of the oxygen you breath in. Pyruvate is converted to lactate if either there isn't available electron acceptors (NAD, FAD) or it is catalyzed by LDH. Lactate or any monocarboxylic acid requires a transport protein (MCT) and depending on the cell determines which isoform is used for transport (MCT1, MCT2, etc). Lactate can only be converted to pyruvate when it is catalyzed by LDH.

Glucose is needed by the central nervous system to keep the body functioning. Therefore, during periods of moderate exercise lasting longer than 20 minutes, the body works to conserve stored muscle and liver glycogen. It does so by reducing the percentage of fuel derived from glycogen to only 40% or 50%, with the remainder supplied by fat. During exercise periods lasting longer than 4 or five hours, as much as 60% to 85% of fuel produced by oxidation may be derived from fat.

The reason why substrate utilization during moderate exercise lasting longer than 30 minutes is due to a greater recruitment of type I MHC isoform fibers which have high oxidative enzyme activity. Lactate and hydrogen ions are in lower concentrations during moderate and prolonged exercise. Increases in catecholamines increases lipid utilization.

One should be more concerned with caloric expenditure rather than substrate utilization. Evidence from several studies suggest that total substrate oxidation is relatively the same during recovery in the first 24 hours. One study sampled a group burning X kcalories in 30 minutes and another group burning the same kcalories in 60 minutes. Each group was monitored for 24 hours post exercise during recovery and found that the higher intensity group used carbohydrates as the major substrate during exercise. However, during the following 24 hours fatty acid oxidation was elevated, whereas the 60m group was subsequently lower. After the 24 hour monitor, total fatty acid oxidation was very close between both groups.