By Nenad Rodic,
posted 04/05

The process of energy supply, exchange and conversion within the body is referred to as metabolism. It is the understanding of metabolism that is largely responsible for the advances in training methodology.
There are several sources of energy our bodies can use, but it is important to know that the only energy any cell can use directly, comes from the molecules of ATP (Adenosine Triphosphate) and the energy is stored in the phosphate bonds between phosphate groups, and conversely, the energy is released when phosphate bonds are cleaved.


This simple equation shows ATP splitting into ADP (adenosine diphosphate) and phosphate group with a release of energy, although the reverse is also possible. This is accomplished with the help of enzymes just like all the other chemical reactions in the body. Once ATP in a muscle cell is used it cannot be replaced with ATP from another muscle or another store. It needs to be recycled or synthesized from other sources. Muscles have enough ATP stored for about one second of work (maximum lift). After that, ATP gets restored/recycled from the ADP molecules and the phosphate groups with the input of energy from other sources.
The most readily available energy source is Creatine phosphate (CP).


This is a very fast reaction in which ATP is replenished very quickly. There is only enough CP available to supply energy for about six seconds, hence the maximum speed is usually achieved around the sixtieth meter in a 100 m running race. I doubt that the attempts to increase creatine stores with supplements (creatine-monohydrate) have any effect. At least it’s never been proven that orally taken creatine-monohydrate improves sprint performance.
It is obvious that these metabolic pathways are of little concern for someone racing eight hours, but it is important to be aware of them since they are used during FT fibers conditioning (sprinting).
Both ATP and CP are stored in the muscle cells. When they are depleted, other sources of energy must be used, some of which can be found within the cell, while others require transport from the blood. Whatever the source is, its function is to provide energy.

Glucose is the most readily available energy substrate, stored in the muscles and the liver. Depending on the availability of oxygen, one molecule of glucose can re-synthesize 36 molecules of ATP relatively slowly (aerobic glycolysis) or two very quickly (anaerobic glycolysis). If energy demands are very high, anaerobic glycolysis will become the primary source of energy for ATP recycling. The glycolysis metabolic pathway transforms glycogen into pyruvate that will combine with the hydrogen ions, released continuously throughout the process, and become lactic acid (lactate), if there isn’t sufficient oxygen available. As a result, two molecules of ATP are synthesized. 1
Lactic acid, which is the unwanted side product of this process, lowers the pH of the muscle (increases the acidity, leading to condition known as acidosis), which is believed to be the primary cause of fatigue in all exertion events lasting over 30 seconds. Most well-trained sprinters can tolerate lactic acid up to 40 seconds and maintain energy supply by ways of anaerobic glycolysis. After 40 seconds, aerobic capacities will engage regardless of your ability to tolerate lactate. Naturally, the anaerobic glycolysis energy system hardly has any impact on triathletes. However, athletes who never train in this energy system will have a difficult time recovering form accidental anaerobic efforts during the race.

Glycolysis, first 10 steps

It is wise to do a few pick-ups during a warm-up that will elevate lactate level to the point where lactate removal process will engage, so if this happens in the race later, your systems are good to go. Swimming, as the most advanced endurance sport, places huge emphasis on warm-ups and mid-distance swimmers are known to do race pace sets (VO2max-SP1 zone) in duration of 4 to 5 minutes during the warm-up sessions. If this is not done correctly, the body will always respond the same way; it gets tight and heavy, while the muscles end up in a serious acidosis very quickly. Make sure this does not happen in a triathlon by doing a simple set in a descending manner (increase speed or power output with each repetition), making sure that the last few repeats are over the anaerobic threshold effort.



If there is enough oxygen available, pyruvate will undergo a different conversion path by creating the Acetyl-coenzyme A and entering The Krebs cycle. Suffice it to say that oxygen is the principal regulator of the energy release since it is the final acceptor of hydrogen in the electron transport chain. Aerobic process takes place in mitochondria, a cell organelle where over 90 % of ATP is re-synthesized. Both pyruvate and hydrogen ions must enter mitochondria while oxygen is diffused through the cell membrane, where myoglobin picks it up and transports it to the mitochondria. Endurance training can increase the size and number of mitochondria to some extent as well as the amount of myoglobin within a cell. This metabolic pathway is the most important for endurance athletes, especially those competing in events up to two hours. This is how long glycogen stores can provide energy for medium intensity exercise (90-100 % of AT, 70-95 % of VO2max). Training can increase the amount of glycogen stored in muscles from 14 g per kg of muscle in untrained, to up to 40 g per kg of wet muscle in a trained athlete’s muscle. Continuous exercise at 70-95 % of VO2max causes the greatest degree of muscle glycogen depletion. Glucose taken by mouth does not reduce the degree of muscle glycogen utilization but slows down or replaces the use of liver glycogen. It is essential to keep the liver glycogen at safe levels since its depletion will cause hypoglycemia (fall of blood glucose levels), which will impair the brain function and lead to exhaustion. The optimum amount of carbs needed to fill the glycogen stores is 500 g a day, three days preceding the race. This should be done only in conjunction with a controlled depletion via energy demanding training 72 hours prior to a race. This process is also known as super compensation.

Krebs cycle shown below:


Once glycogen becomes unavailable, or after about 30 min of continuous exercise metabolism of fat will engage. In long events, or after about two hours of continuous effort, fat will become the main source of energy. While a molecule of glucose can restore only 36 molecules of ATP a molecule of fat can restore 457. Triglycerides (fat molecules) are stored throughout the body. Process called lypolysis converts triglycerides into glycerol and three molecules of free fatty acids (FFA). Glycerol is transported to liver where it is transformed into glucose and glycogen. FFA’s are transported via blood to the muscle cells (mitochondria specifically), where they produce Acetyl-coenzyme A via a process called beta-oxidation. Unfortunately, this process is very slow and entirely aerobic. If you could sustain a VO2max effort until you ran out of fuel you could keep going about 60 minutes on glycogen and about 60 hours on fat. Since most triathlons are very long, it is of great importance to improve everything related to the metabolism of fat (except reserves; we all have enough of those-even the 4% body fat guys). Training can increase the number and size of mitochondria, number of blood vessels that supply muscle fiber with oxygen and FFA, lower the insulin and glucagon secretion.* etc. Improving fat metabolism can be achieved only through long training. Effects are amplified if glycogen levels are low to begin with, due to multiple glycogen depleting sessions in succession, or a low calory diet.

Protein is not really an energy source per se, even though it can provide up to 15 % of energy for muscle work during its breakdown (catabolism). It is the last resort for your body and should be avoided by extra supply of energy as mentioned above. Research shows that there is a considerable loss of muscle tissue if energy is not replenished via carbohydrates within two hours after the end of exercise (glycogen window)**.

*See hormonal response during exercise.
**See recovery

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By Nenad Rodic, founder of