Energy Metabolism
Energy is stored in our bodies in combination with the following chemical compounds: adenosine triphosphate (ATP), creatine phosphate (CP), carbohydrates fats and proteins.
ATP
ATP is the only source of energy that our bodies can use for muscular contraction. All the other energy containing chemicals are used to recycle ATP after its energy has been used for muscular work. The energy from ATP becomes available for muscular contraction in the following manner: When muscle fibers contract, they activate an enzyme, (ATPase), that causes one of the phosphate molecules to split away from the ATP molecule and in the process release the energy that bound it to that molecule. What is left is adenosine diphosphate (ADP). ATP cannot be transported to working muscle fibres from other parts of the body. Therefore, when the amount in a particular muscle fibre loses some of its energy and phosphate, other sources of energy within the same fibre must replace it almost immediately or the fibre will not be able to release enough energy to continue contracting. This recycling of ADP back to ATP requires that another phosphate molecule and energy be made available. The donors are the remaining 4 chemicals in muscles: CP, carbohydrates, fats and proteins.
Creatine Phosphate
Good stuff this creatine phosphate! It is what provides the most rapid source of energy and phosphate for ATP recycling. It is composed of one molecule of creatine and one molecule of phosphate. Energy binds the two molecules together. The enzyme creatine kinase (CK) catalyzes the splitting of the phosphate molecule from creatine, which also releases the energy that bound these two molecules together. That energy and the phosphate then combine with ADP to reform ATP. The process of replacing ATP with phosphate and energy from CP requires only two steps: the splitting of CP and the combination of is phosphate and energy with ADP. These two steps occur so rapidly that no delay will occur in the process of releasing energy from ATP. The upshot of this is that athletes can maintain a maximum rate of muscular contraction as long as sufficient CP is available to restore the energy provided by ATP. Fast-twitch muscle fibres have a higher concentration of CP than do slow-twitch muscle fibres.
Unfortunately, the amount of CP that can be stored in either type of muscle fibre is quite small, small enough in fact that humans can only use CP to recycle ATP for approximately 4 to 5 seconds of an all-out effort. This means that humans can only maintain a maximum rate of muscular contraction for 4 to 6 seconds. Now athletes must rely on the metabolism of carbohydrates, fats, and proteins for the energy and phosphate they need to recycle ATP. This circumstance will slow the rate of muscular contraction because several additional steps are needed to release energy from these foods. In the absence of sufficient CP, the next most rapidly available source of energy and phosphate is carbohydrates in the form of glycogen stored in the muscles.
Carbohydrates
Carbohydrates are made up of simple sugars and starches, which supply the energy for all body functions. Glucose is the simple sugar used for ATP recycling. Glucose, after being taken in as food, can be used immediately or stored for later use. The storage form of glucose is termed glycogen. Glycogen is stored in the muscles and in the liver. Glucose therefore provides 3 forms of energy: blood glucose (used immediately), muscle glycogen and liver glycogen.
Muscle glycogen is the primary source of energy from carbohydrate. When combined with phosphate, it results in the rapid recycling of ATP in all but the shortest of events. When exercise begins, the glycogen stored in muscles is converted back to glucose by glycolysis. Energy and phosphate for ATP recycling is released at several points during the glycolysis process. The energy and phosphate released most quickly come from anaerobic glycolysis, which does not require oxygen for release to take place. The longer, slower process, aerobic glycolysis, requires oxygen.
The liver and blood also contain supplies of glucose that can be mobilized and transported to the muscles when they are needed for energy. The glucose that previously entered the liver was stored there as glycogen. It must be converted back to glucose before it can be transported to the muscles and used to supplement their glycogen supply. The reconversion process occurs whenever the blood glucose supply drops below normal. So when muscles are contracting and glucose is diffusing into them from the blood, liver glycogen will be converted to glucose and poured into the bloodstream to replenish the supply of glucose in the blood.
Blood glucose is more commonly known as blood sugar. When an athlete is training, glucose that was circulating in the blood can diffuse into the muscle cells and enter the metabolic process without first being converted to glycogen. So glucose from the blood helps athletes maintain a high level of glucose in the muscles during exercise. Blood glucose may supply 30 to 40% of the total amount of energy used during training. But both liver glycogen and blood glucose can supply only small amounts of energy during racing. The process of converting liver glycogen to blood glucose is too slow to provide energy for recycling ATP. Similarly, the diffusion of blood glucose into muscle cells requires too much time to sustain the high intensity of racing. Blood glucose and liver glycogen play important roles in replacing the glycogen supplies of muscles during the recovery period following exercise.
Fats
Fats are also an important source of energy for ATP recycling during exercise. More ATP can be replaced with fat than with carbohydrate. However, the process of metabolizing fats is entirely aerobic, which means that energy can only be released slowly. Although a small amount of fat is stored in muscles, most is stored under the skin as adipose tissue.
Adipose tissue supplies about one-half of the fat that is metabolized for energy during exercise. The fat stored in the muscle cells supplies the other half. Slow-twitch fibres are better suited for fat metabolism than fast-twitch fibres because slow-twitch fibres have more fat stored within them, have a greater blood supply, and can transport additional fat from adipose tissue more rapidly. Slow-twitch muscle fibres also have more mitochondria, where fat from both the circulation and muscles can be metabolized. The rate of fat metabolism in slow-twitch fibres has been estimated to be 10 times greater than the same rate in their fast-twitch friends. Consequently, distance athletes, who generally have a higher percentage of slow-twitch fibres, burn more fat (and less muscle glycogen) for energy during training. Thus distance athletes should deplete their muscle glycogen supply more slowly. This may be one of the reasons why they seem to tolerate successive days and weeks of hard training better than sprinters do.
The major role that fat metabolism plays in replacing the ATP of athletes occurs during training. Fat metabolism can provide a significant amount of energy during long repeat sets at moderate speeds, thus reducing the rate of muscle glycogen use and delaying fatigue. Fat metabolism probably supplies between 30% and 50% of the total energy used during typical 2 hour training sessions that includes a significant amount of endurance training. The energy supply for sprinting and fast endurance repeats is another matter. The process of fat metabolism is simply too slow to supply any but a small amount of the energy needed to support fast speeds. Thus, the contribution of fats to ATP recycling falls dramatically when athletes train at speeds that approach and exceed their anaerobic thresholds. Therefore, most of the energy for these sets must come from glycogen and glucose.
Proteins
Often people will think of strength when they think of proteins because proteins are the basic structural elements of muscles and are intimately involved in the repair and rebuilding of those tissues. What is less known is the role they play in endurance. Many of the structural components of muscles (principally, mitochondria) that are involved in aerobic metabolism are built from protein. Proteins are made up of carbon, hydrogen, oxygen and nitrogen arranged in a way to form a large combination of amino acids.
Besides other functions, proteins can donate small amounts of energy for recycling ATP during exercise. This occurs when some of the nitrogen in certain amino acids is removed and passed to other proteins to form new amino acids. The carbon portions that remain from the old amino acids can then be converted to acetyl-CoA so that they can enter the Krebs cycle and be metabolized for energy in the same manner that glucose is.
Like fat, the recycling of ATP from protein is a slow, aerobic process that requires many steps. Protein metabolism is the slowest and least economical method for recycling ATP. Why use protein to recycle ATP if fat is available? The muscles must contain a certain amount of glucose to metabolize fat for energy. So if an athlete’s glucose supply is very low, he or she has no choice but to rely more heavily on the muscles’ own protein to recycle ATP.
Because it is an extremely slow process, protein metabolism does not contribute any substantial amount of energy during competition, but it does contribute to the energy supply for training, around 10 to 15% during a 2 hour session. Athletes should maintain adequate supplies of glycogen and glucose in their muscles during training so that they do not use excessive amounts of protein for energy. Doing so will cause muscles to lose some of their protein content and therefore some of their strength and endurance.
The small amount commonly used can generally be replaced overnight, so training adaptations should not be adversely affected. But when athletes train when muscle glycogen supplies are low, negative effects can become significant. For example, if an athlete’s muscle glycogen is low from days of previous training, the amount of energy derived from protein could increase from 15 to 45%. The energy for ATP recycling donated by protein will also increase significantly during long continuous training sessions if the glycogen supplies in muscles and the liver become depleted. When the amount of protein metabolized for energy becomes so great that the athletes cannot replace it on a regular basis, they may literally cannibalize the structural components of the contractile proteins, mitochondria, myoglobin, and metabolic enzymes within their muscles. Over time, the loss may become so severe that they lose strength and endurance.
