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Carbohydrate is typically the limiting energy substrate in exercise, meaning that it will run out before protein or fat runs out. Glycogen depletion is a term used to describe when carbohydrates are used up and no longer exist as a fuel source for working muscles.

In other words energy stores are depleted, which can result in cells and muscle tissue being damaged and the immune system being stressed if exercise continues.In endurance sports such as triathlon, or long-distance cycling and running, hitting the wall or “bonking” describes a condition caused by the depletion of glycogen stores in the liver and muscles, which manifests itself by sudden fatigue and loss of energy.

The condition can usually be avoided by ensuring that glycogen levels are high when the exercise begins, maintaining glucose levels during exercise by eating or drinking carbohydrate-rich substances, or by reducing exercise intensity. Athletes engaged in exercise over a long period of time produce energy via two mechanisms, both facilitated by oxygen: via fat metabolism and via breakdown of glycogen into glucose, followed by glycolysis.

How much energy comes from either source depends on the intensity of the exercise. During intense exercise that approaches one's VO2 max, most of the energy comes from glycogen.The carbohydrates that a person eats are converted by the liver and muscles into glycogen for storage. Glycogen burns rapidly to provide quick energy. Well-conditioned athletes can store about 8 MJ or 2,000 kcal worth of glycogen in their bodies without refueling. This is enough for about 18–20 miles of running. If marathon runners don’t replenish carbohydrate, running becomes a death march for the last 6-8 miles. Intense cycling can easily burn 600-800 or more kcal per hour. Unless the athlete learns to fuel while on the bike, glycogen stores will be depleted after less than 2 hours of continuous cycling.

Training and carbohydrate loading can raise these reserves as high as 880 g (3600 kcal), correspondingly raising the potential for uninterrupted exercise.When glycogen runs low, the body must then obtain energy by breaking down muscle or burning stored fat, which does not burn as readily. When this happens, the athlete will experience dramatic fatigue and is said to "hit the wall".

The aim of training for the marathon or Ironman is to maximize the limited glycogen available so that the fatigue of the "wall" is not as dramatic. This is accomplished in part by replenishing carbohydrates during the race, and in part by utilizing a higher percentage of energy from burned fat during the early phase of the race, thus conserving glycogen.


The amount of additional carbohydrate that is able to be stored in the body is dependent on diet and athlete conditioning level. For an untrained individual consuming a high carbohydrate (75%) diet, glycogen stores may increase up to 130g and 360g for liver and muscle respectively, for a total storage of 490g.

For an athlete training on a daily basis but consuming a normal/low-carb diet (45% carbohydrate), glycogen levels approximate 55g and 280g for liver and muscle respectively, yielding a total of only 330g.However, should this same well-conditioned athlete consume a high-carb diet (75% carbohydrate), their total carbohydrate reserves may soar up to 880g with approximately 160g stored in the liver and 720g in the muscle. Clearly the conditioned athlete’s muscles are much more efficient at storing carbohydrates than those of his or her unconditioned competitor

Carbohydrate-based "energy gels" are used by athletes to avoid or reduce the effect of "hitting the wall", as they provide easy to digest energy during the long-distance, high- intensity events. Energy gels usually contain a blend of high-glycemic carbohydrates, varying amounts of electrolytes and some also contain caffeine. Due to their high carbohydrate concentration, they need to be consumed with fluid. Alternatives to gels include various forms of concentrated sugars and other high-glycemic foods (high in simple carbohydrates) that can be digested easily. Many athletes experiment with consuming energy supplements during training runs to determine what works best for them, but trial and error by athletes untrained in the science of fueling can have disastrous results, not the least of which is race failure. For instance, consumption of food high in fat or protein while training or racing at high intensity sometimes makes the athlete sick due to the slow rate of digestion for fat and protein and the additional body fluid needed to process these foods.

In addition to having a well-crafted, individualized fuel plan, endurance athletes are advised not to ingest any food (or medicine) they have not previously used just prior to or during a race. Recommendations for how often to take an energy gel during the race range widely and should be spelled out in a fueling plan created by a qualified coach or dietician familiar with endurance events.

Consequences of unrecovered glycogen depletionFailure to appropriately replete glycogen may result in chronic glycogen depletion. There is evidence linking muscle glycogen depletion with both fatigue and injury. Symptoms are very similar to those of over-training. Muscles that are fatigued lose their strength, and thus their ability to protect joints, with the unfortunate risk of injury. The purpose of a good recovery nutrition strategy is to avoid chronic glycogen depletion that can take place over several days of training and avoid injury. Furthermore, consistency in glycogen repletion results in effective and recovery between workouts and competitions and thus overall improved performance during competition.

ReferencesKatz, Ilana MS, RD, LD, Costill, D.L., Miller, J.M. Nutrition for endurance sport: Carbohydrate and fluid balance. Int. J. Sports. Med. 1980;1:2-14.Hargreaves, M., Finn, J.P., Withers, R.T., Halbert, J.A., Scroop, G.C., Mackay, M., Snow, R.J., Carey, M.F. Effect on muscle glycogen availability on maximal exercise performance. Eur J Appl Physiol 1997;75:188-192.Rankin Walberg, J. Glycemic index and exercise metabolism. Sports Science Exchange 1997;10 (1)Robergs, RA., Perason, D.R., Costill, D.L., Fink, W.J., Pascoe, D.D. Benedict, M.A., Lambert, C.P., and Zachweija, J.J. Muscle glycogenolysis during differing intensities of weight-resistance exercise. J Appl Physiol 1991;70:1700-1706.

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