The 3 energy systems are ATP (IMMEDIATE), Lactic Acid system (SHORT TERM) and Aerobic system (LONG TERM).The energy for the synthesis of ATP comes from the breakdown of foods and phosphocreatine (PC). Phosphocreatine is also known as creatine phosphate and it is stored inside muscle cells. The fact that it is stored in muscle cells means that phosphocreatine is readily available to produce ATP quickly. However it is only stored in limited quantities and therefore like our ATP stores it also runs out very quickly.
It is estimated that there is only about 100g of ATP and about 120g of phosphocreatine stored in the body, mostly within the muscle cells.Together ATP and phosphocreatine are called ‘high-energy’ phosphates as large amounts of energy are released quickly during their breakdown. When the stores of PC run out quickly other substrates that are stored in larger quantities in the body are also used to synthesise ATP. These include the sources gained from everyday foods that provide the following macronutrients:1. Carbohydrates 2. Proteins. 3. Fats
Carbohydrates are the bodies preferred source of food energy for the synthesis of ATP, with one gram of CHO providing four calories of energy. When digested carbohydrates are broken down into glucose and chemical reactions involving glucose then produce ATP. Glucose is always present within the blood as it circulates and provides a readily available source of energy. Huge levels of glucose in the blood are not healthy as it becomes thick and sticky, making it harder to flow through small blood vessels. So to ensure the blood glucose levels are healthy excess glucose that is not needed immediately to produce energy for the body is converted into a substance called glycogen and this is stored in the muscles and liver. When needed, glycogen can then be converted back to glucose for energy.
Fats are broken down into free fatty acids (FFA) and triglycerides which can produce ATP through chemical reactions.Fatty acids either circulate in the blood or are stored as triglycerides in adipose tissue and muscle.Fat is a very energy dense nutrient, one gram of it provides nine calories of energy.
Despite the large quantity of available energy that fat has it provides this energy at a much slower rate than carbohydrate. This is because the chemical reactions required for its breakdown are much more complex and time consuming.
Protein contains four calories per gram and provides energy at a much slower rate than carbohydrates. It only makes a small contribution to energy production. However it can become a more significant energy source under periods of prolonged starvation or in ultra endurance events where other energy sources become severely depleted. Protein is converted into amino acids. Amino acids are normally responsible for the growth and repair of body tissue but they can also be converted into glucose or into other substances used by the aerobic energy system to synthesise ATP.Excess consumption of any or all of these food sources (carbohydrates, fats or proteins) does not result in more energy being produced, rather it results in the consumed excess being converted to and stored as adipose (fat) tissue.
There are three separate energy systems through which ATP can be synthesised, these are the:
- ATP-PC system (also known as the phosphagen system)
- Anaerobic glycolytic system (also known as the lactate system).
- Aerobic system (also known as slow glycolysis or oxidative phosphorylation).
The ATP-PC system and anaerobic glycolytic system are both anaerobic systems, meaning that oxygen is not used by these systems to synthesise ATP. The aerobic system on the other hand relies heavily on oxygen to synthesise ATP.
Most exercise involves ATP being synthesised through a mix of all three systems. The factors that determine which system is most dominant at any time are the intensity and duration of exercise.
When exercise starts energy will come from the anaerobic energy systems, the initial 10 seconds or so are almost exclusively through the ATP-PC system.
As exercise continues the anaerobic systems become depleted and the aerobic system becomes increasingly dominant as it can break down more complex fuels for energy such as fats and proteins as well as glycogen.The higher the intensity of the exercise the quicker the anaerobic systems will be depleted. For exercise to continue once the anaerobic systems have become significantly depleted the intensity of exercise needs to drop to a level that allows the aerobic system to provide enough energy.
When it comes to nutrition the mixture of fuel we need for different types of exercise depends on the energy production system being utilised for the purpose.
During exercise the body always burns a mix of carbs and fats. At lower intensity fat is the primary fuel source for the aerobic energy system, but as exercise intensity increases it shifts towards anaerobic glycolysis, which requires an adequate supply of glucose. The specific intensity and ratio of carbs to fat can vary significantly both within and between individuals. Our day to day diet affects our ability to utilise different fuel sources. If we constantly supply the body with carbohydrates and glucose it impairs our ability to burn fat for fuel and we Become ’sugar burners’ . The modern western diet promotes sugar burning.
Sugar burners must rely mainly on the glycolytic system, which depends on limited stores of glycogen. So during endurance exercise we might need to consume carbohydrates like sports drinks, gels etc. this is necessary to keep supply of sufficient fuel.
Being metabolically flexible, or fat adapted spares glycogen stores for when they are needed later in an event. It also reduces the need to consume carbohydrates during training or competition. In order to improve fat utilisation we can 1. Get fitter and increase the mitochondria in muscle cells. 2. Reduce refined carbohydrates in the diet and eat more whole foods. 3. Periodise carbohydrates based on training needs such as lowering carbs on rest days.
A low carb diet for athletes can have some potential benefits such as boosting the ability to burn fat, reduction in body fat and mass and it may also decrease the post muscle damage that can occur.
There are however a number of potential drawbacks such as the decrease in ability to perform high intensity work through down-regulation of enzymes needed for glucose metabolism, increase the perceived effort of exercise, increased cortisol with effect on immune function and thyroid function.
Research has shown that performing some training sessions in a glycogen depleted state can increase the training adaptations. This is known as ‘training low’. It should only be used for low intensity training as it is hard on the body. Eating immediately after these types of sessions is highly recommended for recovery.
Before training any of the energy systems it is vital never to ignore Hydration. Dehydration can be devastating for anyone during exercise especially of high intensity. Dehydration can lead to thicker blood. This causes the heart to pump much harder and be very uncomfortable during training or competition.
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