The Importance of Glycogen for ATP Resynthesis Before, During, and After Physical Activity

Sam Spaiser


Introduction:

Everyone knows that exercise is good for them, but it is often only understood on the outside, the “seen;” how it can shape our bodies into slim runway models or Mr. Universe body builders. What is most commonly misunderstood or rarely contemplated is the “unseen”, or what happens inside the body during and following exercise. This paper will address the link between glycogen stores and the rate of ATP resynthesis, and in turn, how this affects the power of athletic output. In doing so, the follow will be explained: how muscles contract, the different mechanisms for ATP resynthesis according to exercise intensity, and the various metabolic pathways for generating it. In discussion of metabolic pathways for ATP generation, it is explained how the body uses carbohydrate, fat, and protein during athletic activity, and how this affects the athlete’s choice for post-workout refueling. Glycogen is explained as the limiting factor on the maximum intensity of exercise that may be performed. Likewise, eating for recovery is important to replenish the exercise-induced depleted stores of glycogen, and post-workout food choices become paramount in determining the efficiency of recovery, glycogen repletion, and resolutely the intensity and performance of the next training session. In short, consuming fruit immediately after exercise increases recovery rate by replenishing muscle glycogen stores, and increased stores of glycogen result in an ability to increase the maximum intensity of athletic performance.

Concept 1: ATP and Muscle Contractions

To understand what food to eat for optimum athletic performance, it is imperative to understand how the muscles are functioning during the athletic activity. During exercise, muscles shorten, or contract, to generate force, and when they are applied to joints, the limb moves, resulting in a concentric contraction. The sliding filament mechanism is used to explain the muscle contraction. The sarcomere is the smallest functional contractile unit within a muscle that produces force, and it is composed of actin (thin filament) and myosin (thick filament). When calcium ions are released onto the sarcomere from a nerve impulse, myosin, with energy provided by ATP, bind to and moves actin, generating a muscle contraction. It is during this “power stroke” that the split ATP (ADP + Pi) falls off of myosin so that a new ATP can bind to it and repeat the power stroke, if more calcium ions are present.

Why is all of this important? Firstly, without calcium ions, contraction cannot occur. Secondly, and more important, without ATP, the myosin heads will not have the energy to bind to actin and cause the “power stroke,” or the sarcomere contraction, which essentially is the muscles firing to carry out the desired athletic endeavor. Furthermore, without additional ATP, after a single power stroke, actin and myosin are stuck linked together until a new ATP is introduced. Thus, without another ATP, the muscles cannot contract any further, which is a problem, whether you’re running a race or just lifting weights. Conversely, if the actin/myosin cross bridge is not broken the muscles cannot relax, and this will impair the muscle’s recovery ability. So not only is ATP vital for muscle contraction, but also for post workout muscle relaxation and recovery.

Concept 2: ATP Resynthesis – An Athletic Perspective

The main obstacle that the athlete must overcome is the fact that ATP stores within muscle are extremely small, and when used, they must be resynthesized immediatedly. ATP can be generated from the breakdown of glycogen in the muscles themselves via glycogenolysis. ATP is required for breaking the cross-bridges, but there is only enough on hand ATP available for a few contractions, so to continue to contract muscle, and to continue ATP resynthesis, creatine phosphate’s phosphate group can be used to convert the ADP released by myosin back into ATP, although this is generally not enough to keep up with the rigorous demands from muscle contraction. Thus, glycolysis occurs in order to aid and increase the rate of ATP resynthesis.

The rate of resynthesis of ATP is so important not just for athletics, but because if it does not meet the rate that it is being used, the reduction in ATP availability on the cellular level would result in a decreased ability, and eventually a failure, to maintain life. ATP can be generated in many ways, depending on the type and intensity of the exercise. At rest, ATP can be resynthesized by the oxidation of fat, or fat catabolism, where very little glucose is needed. In contrast, during maximal exercise, ATP use increases over 1000 times, and muscle stores of creatine phosphate (CP), a chemical compound stored in muscle, aids ATP resynthesis by breaking down and combining with ADP to form new ATP. This process is sufficient to maintain energy needs for about 15 seconds. As exercise continues, glycolysis is needed to increase the supply of pyruvate for oxidation, and the increased glucose-phosphate intermediates generated lowers the activity of hexokinase (an enzyme that controls the muscles’ uptake of glucose) and consequently inhibits glucose uptake into the muscles from the blood at the beginning of exercise in order to reserve it for when ATP generation from fat catabolism will be forced to decrease.

As the intensity of the exercise increases, more oxygen is delivered to the muscles and the mitochondria becomes more efficient. This allows more pyruvate to enter the mitochondria from the intermediates and take the aerobic pathway to generate ATP. Also as intensity increases, even though oxygen is pumped to the muscles more quickly, some muscle cells will receive insufficient oxygen to meet energy demands and thus, some pyruvate is converted to lactic acid as a pyruvate storage point for when sufficient oxygen is present. The combination of the reverse lactic acid pathway and the aerobic pathway reduces the rate at which glycogen is being broken down. As muscle glycogen becomes depleted, glucose is released from the liver into the blood and this provides energy for ATP resynthesis. As exercise continues and glycogen stores become severely depleted, ATP resynthesis shifts back toward fat oxidation, resulting in fatigue, muscle failure, and a decreased level of physical exertion.

Fat provides the bulk of ATP synthesis because it releases a tremendous amount of energy. However, the rate at which fat oxidation occurs is very slow. About 50% of max intensity can be supported by fat oxidation alone, anything greater will require ATP resynthesis from carbohydrate catabolism, and this applies to most athletics. When the exercise intensity is at or just below maximum, ATP resynthesis shifts to predominantly carbohydrate oxidation and lactate formation, because the fat oxidation pathway is too slow. At this intensity, if allowed, the body would deplete its entire glycogen stores in a minute, but fatigue prevents this from occurring. Thus, the greater the exercise intensity, the greater the rate that glycogen stores are used. In other words, the most rapid and greatest glycogen depletion occurs with short-term, intense exercise.

Concept 3: ATP Resynthesis – A Metabolic Perspective

In order to understand the body’s choice of mechanism for ATP resynthesis for the muscles during exercise, it is necessary to understand how the different mechanisms for generating ATP work. The follow paragraphs provide an overview of carbohydrate, fat, and protein metabolism, as well as the aerobic and anaerobic metabolic pathways.

In carbohydrate catabolism, both anaerobic and aerobic metabolism work together to form ATP. First, glucose, formed from glycogenolysis, undergoes glycolysis and forms two pyruvate, 1 ATP, and 2 NADH (anaerobic metabolism). If oxygen is present, the pyruvate enters the mitochondria where it is converted into acetyl CoA (aerobic catabolism of pyruvate). Acetyl CoA is then broken down via the tricarboxylic acid cycle (TCA), also known as the Krebs cycle, which requires two turns and produces 2 ATP, 2 FADH, 6NADH. However, it is the electron transport chain of oxidative phosphorylation that produces a lot of ATP, and for which oxygen is needed. Oxidative phosphorylation uses the NADH and FADH previously generated to produce about 34 ATP, which is the major contributor of energy. In total, carbohydrate catabolism generates 38 ATP.

Fat molecules may also generate ATP, and to do so are first broken down into glycerol and fatty acids via lipolysis. Fatty acids can then be converted to Acetyl CoA and go through the TCA cycle and oxidative phosphorylation to generate ATP. Glycerol can be converted into pyruvate, which then goes through the same mechanism to generate ATP. This is known as fat catabolism. Depending on the length of the fat molecule, over 300 ATP can be generated per molecule of fat. However, even though fat catabolism provides a great deal more ATP than carbohydrate catabolism does, glycolysis of carbohydrate catabolism is a much faster and easier process than the burning of glycerol or fatty acid chains of fat catabolism, which is why during intense athletic activity the body primarily chooses glucose from glycogen and the glycolysis pathway for generating ATP in order to keep up with the increased rate of demand.

The third option for resynthesizing ATP is via protein catabolism. However, protein catabolism is never used for generating ATP, except in times of starvation, after all glycogen and fat stores have been consumed, and it is for this reason that a high protein diet is not recommended for athletes. Consequently, protein catabolism is inefficient and not much ATP is generated, less than 38. Furthermore, protein is not synthesized by the body in order to be metabolized; it is synthesized to build bodily structures, of which muscle is a part of. Thus, instead of a high protein diet, a high carbohydrate diet results in increased glycogen stores and enhanced athletic training, which results in the body’s adaptive response in synthesizing protein to build more muscle to meet the new and increased athletic demands placed on the body.

All three metabolic cycles began with the generation of pyruvate to form Acteyl CoA and then entered the TCA cycle, followed by oxidative phosphorylation, to acquire the necessary ATP. However, when the rate of pyruvate formation exceeds the mitochondria’s capacity to accept and oxidize it, it is converted into lactate. This is paired with the fact that during high intensity athletic activities, the athlete goes into a state of oxygen debt, and the body is forced to anaerobically catabolize pyruvate into lactic acid, which acts as a temporary storage point until oxygen is present again. When oxygen is again present, the reaction reverses causing the lactic acid to reform pyruvate, and ATP can once again be generated. If too much lactic acid builds up in the athlete’s muscles, and thus the oxygen debt results in a significantly reduced capacity to generate more ATP, the athlete will fatigue. The point at which the athlete starts to fatigue depends on the muscle adaptations gained from previous exercise, of whose intensity was dependent on the availability of glycogen to the muscles, which is increased from a carbohydrate rich diet.

Concept 4: Glycogen, The Limiting Factor

Glycogen is the limiting reagent in the exercise reaction. This is because an athlete’s muscles can only work as hard as ATP can be resynthesized fast. The harder the athlete works, the more glycogen that is used. Every time an athlete trains, the glycogen reserves in the muscles are used and decrease. The limited stores must be adequately repleted in order to train at the same or greater intensity, or the athlete will train with below normal reserves, and this will result in an impaired ability to carry out the athletic activity because of an inability to resynthesize ATP at the previously faster rates. In this scenario, the point at which glycogen becomes limiting occurs more rapidly, and this reduces both the quality and quantity of training accomplished. Complete repletion of glycogen reserves for the average individual may take up to 48 hours or more, and intense muscle use from heavy training, including speedwork, gymnastics, and body building can result in prolonged glycogen repletion delays. Through endurance training, the ability to increase fat metabolism is obtained so that glycogen can be spared and the intensity of activity can be maintained for a longer period of time, or a greater intensity can be achieved for the same amount of time. However, during maximal training, carbohydrate metabolism prevails. While training may not affect the capacity of glycogen stores, diet does. The amount of carbohydrates consumed overall in the diet preceding exercise will determine the amount of carbohydrate stored as glycogen in the liver and muscles. In this sense, diet is the limiting factor of the limiting factor. The less carbohydrates comprise the diet, the less energy that will be available to the muscles during exercise.

Even though there is a shift toward fat metabolism as the main energy source during endurance exercise, the intensity at which glycogen limits the desired rate of ATP resynthesis is reached before fat is metabolized, and the ability to perform the exercise would already be impaired. At maximal training, like sprinting (the maximum intensity of running which is limited by CP stores), the performance is immediately and dramatically affected by glycogen stores, in which case a frugal store of glycogen would severely impair performance. Eating more carbohydrates will create greater stores of glycogen within the liver and muscle and allow a greater proportion of the energy supply to come from carbohydrate instead of fat. This will allow one to perform optimally for a greater duration, or at a greater intensity for the same duration. If the diet is dominated by fats and proteins, then the body is forced to rely on gluconeogenesis (where the liver converts fat molecules, lactate, and pyruvate to glucose) to create glycogen and glucose, and the decreased reserves of glycogen will limit performance at high intensities. Because of the limited performance at high intensities, the lack of carbohydrate in the diet results in a limited ability to train, resulting in minimal gains. At maximal intensity, the energy requirement is so great and needed so quickly that only carbohydrates can produce energy fast enough, and as a result very little fat is used. Thus, the more intense the exercise, the greater the rate at which you use up your glycogen stores. The harder the exercise, the greater the body’s adaptive response will be.

Because the amount of glycogen stores set the upper limit of training intensity, greater glycogen stores mean that the athlete can train harder. It is important to understand that eating protein does not build muscle and aid adaptive gains; only training leads to the body’s adaptive response of building muscle. No matter what exercise is being performed, some glycogen is always used, and becomes the limiting factor in performance. If the supply of glycogen is not maintained, then the rate at which ATP may be resynthesized, and exercise performed, decreases. Thus, an increase in glycogen stores increases fatigue resistance and enhances performance.

Concept 5: Eating For Glycogen Repletion

Fatigue is the enemy of athletic performance, and results when the rate of ATP synthesis falls below the demands of energy usage. The body has only limited ability to make carbohydrate from fats and proteins, and is dependent on sugars and/or starches to recover normal glycogen levels. Thus, glycogen stores, once depleted, will remain low unless carbohydrate is consumed, or else there will be nothing to replete them. Proceeding exercise-induced glycogen depletion, a high carbohydrate meal will dramatically increase the rate of refueling. In general, the higher the amount of carbohydrate in the diet (as a percentage of), the faster the muscle glycogen stores are replenished. If glycogen is consistently left unreplenished, and successive training continues, exercise that was once an easy warm up can become extremely difficult. Dietary supplements will have no effect on glycogen levels. On a high carbohydrate diet, time allocated to rest and recovery becomes reduced. To further reduce recovery time, it is important to eat immediately following the exercise. This is because insulin sensitivity is increased for a short period following exercise and therefore the ability of muscle to replete glycogen is greatest during this period (for about one hour).

Recovering normal muscle glycogen levels after training is most effectively done by eating simple sugars. This is because glycogen is formed from converted glucose, a simple sugar. Glycogen stores are replenished through the process called glucogenesis, which is simply the formation of glycogen from glucose. All animals, including humans, store glucose as glycogen, but all plants store glucose as starches. This created the belief that eating a diet high in starchy plant foods such as grains, legumes, roots, and tubers, would result in the greatest human glycogen reserves possible. However, the problem with this concept is that starches are large molecules that require extensive breakdown, are often difficult to digest, and are necessary to be cooked in order to render them edible, resulting in the loss of vitamins and minerals, such as calcium, necessary to not only carry out major bodily functions but also to trigger ATP synthesis and muscular contractions. The post workout goal is to replenish the depleted glycogen reserves as quickly and efficiently as possible.

Glucose is unique in that it can be absorbed through the lining of the mouth. Glucose itself is a monosaccharide, and is one of two sugars in every disaccharide, which requires less of a breakdown than obtaining it from a starch polysaccharide. The simple sugars glucose and galactose are absorbed through the cell lining in the small intestines by active transport. Fructose is absorbed by facilitated diffusion. The simple sugar monosaccharides are absorbed immediately, while branched, disaccharides, such as sucrose and maltose, are absorbed next most quickly. Unbranched, starch chains are absorbed and digested much slower than the simple sugars because, not only are they larger to begin with, but also because the lack of branching results in enzymes having less places to attack and rapidly release the glucose. Thus, to absorb glucose the fastest, replete glycogen stores, and optimize recovery, it is best to consume a source of simple sugars with all of the vitamins and minerals intact. The easy answer to this is ripe fruit. Fruit is abundant in the simple sugars sucrose, fructose, glucose, and occasionally maltose and galactose, providing both an instantaneous source of food and glucose energy. As a side benefit, packaging a mix of mono and disaccharide simple sugars in a single food, as fruit has, increases the sugars’ absorption rate at the same osmolarity as compared with isolated glucose because of the body’s ability to carry out multiple transport mechanisms simultaneously. Fruit is also easy to bring to the training session and consume immediately after, and providing a source of hydration as well. Starches need to be cooked and can be a hassle, and the destruction of key vitamins and minerals through cooking, many of which starches lack to begin with, is a heavy price to pay for the inconvenience.

Conclusion:

Fruit is the best source of glucose to replenish glycogen stores after exercise. Of course, that doesn’t mean athletic abilities will improve by spending more time in the kitchen; training is still paramount. However, the greater the athlete’s glycogen stores, the more muscular energy that can be physically exerted because of an increased ability to resynthesize ATP at a maintained and faster rate. Likewise, the consumption of the simple sugars in fruit will not only replenish glycogen stores after they’ve been depleted, but also provide the glucose to resynthesize more ATP to break any remaining actin/myosin cross bridges that left some muscles tensed and in a contracted state. As a result, the muscles can relax sooner after exercise and are given a greater opportunity to recover. What records could be broken, and what impossible feats might become possible, if athletes began to eat more fruit?


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