1. Food fuels & the three energy systems
Chapter 5

2. Carbohydrates
Fats
Proteins
Summary of Fuels
ATP-PC energy system
Anaerobic Glycolysis energy system
Aerobic energy system
Predominant energy systems
Energy System interplay

3. Carbohydrates (CHO)
Sugars and starches such as fruit, cereal, bread, pasta and vegetables are the body’s preferred source of fuel, particularly during exercise.
Carbohydrates have a much lower oxygen cost to use aerobically than fats, so more energy can be produced with a given amount of oxygen using carbohydrates rather than fats. This is one of the reasons why carbohydrates are our preferred exercise fuel.
Carbohydrates are the only source of energy during maximal intensity exercise.
Energy for muscular contraction stems first from muscle glycogen and then liver glycogen. The level of CHO intake varies according to the nature of the activity.
For prolonged endurance activities, carbohydrate loading is required, a regime where as much as 80 per cent of the diet is carbohydrate. It is the practice of increasing CHO stores within the muscles and body by increasing CHO intake and tapering training in the time (up to 10 days) leading up to major competition.

4. It is important to have a carbohydrate-rich diet in order to increase glycogen stores. These glycogen stores facilitate high-intensity efforts via anaerobic glycolysis as well as aiding endurance performance via the aerobic energy system.
The diet of athletes and sedentary individuals should be essentially be the same, except that an athlete requires a greater energy intake.
CHO are the primary energy source during anaerobic (high intensity, short duration) exercise once PC has been depleted.
During aerobic exercise, (moderate intensity, longer duration) CHO and then fats are preferred as the energy source.
Carbohydrates are preferred over fats as a source of energy during exercise because they require less oxygen to produce the same amount of energy.

5. fats
Butter, margarine, cheese, oil, nuts, pork and fatty meats act as concentrated fuel storage in muscles and the body’s adipose tissue. It is the body’s main source of fuel at rest and during prolonged sub-maximal exercise.
Fats play an important role in the diet of athlete and non-athlete alike. Fats act as a large energy store, and provide the source and transport medium for the fat- soluble vitamins A, D, E and K.
Fats in the form of triglycerides are stored throughout the body in fat cells (adipose tissue) and in skeletal muscle.
Triglycerides are broken down into free fatty acids, which in turn are broken down aerobically to provide energy for movement.
The transport of free fatty acids to muscle fibres is slow, as the breakdown requires a greater amount of oxygen. Therefore, glycogen is the preferred fuel during exercise.

6. Protein
Meat, fish, poultry, legumes, eggs and grains contribute to energy production during exercise. It is used mainly for growth and repair.
Protein is vitally important in the diet, as it forms the building blocks of tissue (growth and repair).
All enzymes (which speed up chemical reactions) are proteins. Protein is also important in the synthesis of hormones and antibodies (the body’s immune defence system).
The basic structural units of proteins are amino acids.
As protein is not normally used as an energy source, it should form only 12 to 15 per cent of the average diet, significantly less than the percentage of fats or CHO.
Large amounts of oxygen are required for breaking down protein, therefore, it is used for energy in extreme circumstances (extended-duration exercise).

7. Summary of fuels
Our digestive system breaks down carbohydrates to glucose (stored in the liver and muscles as glycogen), as well as fats / triglycerides to free fatty acids (stored as adipose tissue), and proteins to amino acids (stored in muscles).
The body provides energy by breaking down fats, carbohydrates and, under extreme circumstances, proteins. Carbohydrates have a much lower oxygen cost to use aerobically than fats, so more energy can be produced with a given amount of oxygen using CHO rather than fats.
This in one reason why CHO are the preferred exercise fuel.
Carbohydrates, fats and proteins are broken down and used immediately or stored as chemical energy to allow muscular contractions and movement. Carbohydrates are the body’s preferred source of fuel, while fat acts as a back- up fuel and protein is used for growth and repair.

8. ATP – PC energy system
The ATP-PC energy system produces energy by breaking down phosphocreatine (PC) to resynthesise adenosine triphosphate (ATP). ATP is resynthesised through chemical reactions that do not require oxygen (anaerobic). It is worth noting that all activities that are Carried out above 100% VO2 max (maximal oxygen uptake) depend on an anaerobic energy supply, and if PC has not had time to replenish, this will be powered by the lactic acid system.
The ATP-PC system does not require oxygen to release energy (anaerobic).
It provides the fastest rate of ATP release for energy because it depends on simple and short chemical reactions and ready availability of PC in muscles.
Once PC has been depleted in the muscle, ATP must be resynthesised from another substance – typically glycogen.
Once PC has been depleted, it can only be replenished through the aerobic pathway or during recovery once the activity has stopped.

9. Anaerobic glycolysis energy system
Anaerobic glycolysis refers to energy produced by the incomplete breakdown of glucose when oxygen isn’t available. Technically, there is enough glycogen stored in muscles to maintain maximum effort for approximately 90 seconds.
All of the pyruvic acid produced during anaerobic glycolysis is converted into lactic acid. A by-product of this process is the production of hydrogen ions that cause the muscle pH to fall (become more acidic), thereby inhibiting glycolysis.
These hydrogen ions are therefore responsible for the inability of muscles to contract maximally.
Hydrogen ions combine with pyruvate to form lactate, which is then converted to glycogen and made available to release further energy.

10. Aerobic energy system
Aerobic glycolysis refers to energy provided by the complete breakdown of glucose when plenty of oxygen is still available.
The aerobic energy system produces energy by breaking down glycogen (preferentially during exercise), or free fatty acids (as a last resort energy source) to resynthesise ATP.
The Aerobic system has the slowest rate of ATP resynthesis due to the complex nature of its chemical reactions.
It is capable of producing times more energy (high yield) in comparison to the other two energy systems.
It releases no toxic / fatiguing by-products and can be used indefinitely.

11. Predominant energy system
Eg 3km continuous running Predominant energy system: Aerobic energy System Explanation: Sub-max intensity for much of the duration. Long duration, typically minutes. Maximum push-up test: Predominant energy system: Anaerobic Glycolysis system Explanation: Maximal intensity effort required, duration takes approximately 30 seconds – 2 minutes. Phosphate Recovery Test: Predominant energy system: ATP-PC system Explanation: Maximal or 100% intensity of each repetition 7 seconds duration

12. Energy System Interplay
400m Sprint
All three energy systems will contribute to resynthesis of ATP throughout the entire 400m race, however, the anaerobic glycolysis system will be the predominant energy system over the duration of the activity.
At the beginning of the race as the athlete pushes off the blocks and begins to sprint, although all three energy systems are resynthesising ATP for energy, the PC system is the predominant system due to the maximal intensity of the activity and need for the fast rate of energy supply. It is likely that this will be predominant for the first 5-10 seconds.
As PC stores start to deplete, a continual need for a fast rate of supply of energy to the working muscles still exists. As a result, the anaerobic glycolysis system becomes the predominant energy system resynthesising ATP for energy from this point onwards until the later stages of the race.

13. As the anaerobic glycolysis energy system supplies energy at a slower rate, the athlete’s times slow as the race progresses.
Towards the closing stages of the race, and due to acute physiological changes in the athlete’s body (such as increase heart rate and respiratory rate), the aerobic system begins to assume greater significance in terms of energy supply as the body is now able to supply sufficient oxygen to meet the needs of the working muscles. The aerobic energy system becomes a significant supplier of energy for ATP resynthesis towards the closing stages of the race. As the aerobic energy system supplies energy at a slower rate, the athlete’s times in the final part of the race are slower again.
Due to the anaerobic glycolysis system being the major supplier of ATP resynthesis for the majority of the race, there is an accumulation of metabolic by-products such as hydrogen ions (H+) and Pi that cause fatigue resulting in a decrease in performance or slowing in the athlete’s times in the final stages of the race.