ATP (Adenosine triphosphate) is the high-energy compound for cells – it is the energy currency for the body. However, the body has a very limited ability to store it, therefore it must be generated quickly during exercise.
- The ATP stored in muscle cells can only give high-power output for a few seconds. It can be resynthesized anaerobically in a further few seconds (5-8 seconds) by using the phosphate from creatine phosphate. These short, intense bursts of muscular activity occur in sprints (100 m), track & field events and sports such as tennis, football, gymnastics and weightlifting.
- If maximum effort lasts for 30 seconds or longer, then breakdown of muscle glycogen can supply the energy anaerobically with a build-up of muscle lactic acid as a by-product. The lactic acid is also a form of stored energy, but it requires sufficient oxygen to re-enter the system.
- In steady-state, low intensity exercise, ATP can be adequately produced aerobically from the oxidation of carbohydrate (glycogen or stored glucose) and fat (lipids). Both duration and intensity of exercise determine the mix of fuel used.
The body stores 50,000 to 100,000 calories (or 5,000 to 10,000 grams) of lipids (fats) in adipose tissue – enough energy to walk or run 800 to 1,600 kms. Another 2,000 to 3000 calories of lipids are stored inside muscle tissue. Tissue lipids are available to be used as fuel for exercise under certain conditions. Maximal fat oxidation occurs between 60 to 65% of VO2Max. At higher intensities of exercise, there is not enough oxygen to derive the majority of energy from fat catabolism. As intensity increases, the proportion of energy from carbohydrate increases, as does total energy output. Fat is a highly concentrated fuel at 9 cals per gram, but fatty acid oxidation is a slow process, so as the intensity of exercises increases, the need for ATP speeds up and the body must produce more through the faster carbohydrate oxidation.
Glucose is the main source of energy muscular activity and the higher the intensity, the greater the reliance on glucose as a fuel. When it runs out, the body stops moving. Glycogen stored in muscle (300 – 400 grams or 1400 calories), in the liver (90 – 100 grams or 350 – 400 calories) and circulating in the blood (-5 grams or 20 calories) is a major fuel for physical performance. Glycogen stores usually can support just 1 to 3 hours of physical exertion, depending on the intensity of effort. Delivering more glucose to the system than can be stored results in the excess being stored results in the excess being stored as fat in muscle and fat cells. Liver glycogen primarily feeds blood glucose while muscle glycogen is primarily used for energy to exercising muscles, both aerobically and anaerobically. The rate of liver glucose delivered to the blood during exercise is a function of exercise intensity, with higher intensity exercise causing a faster rate of liver glucose release.
The body relies more on carbohydrate than fat for higher-intensity exercise, longer duration exercise, exercise in hot and cold temperature extremes and at high altitudes and more on fat than carbohydrate for less intense, shorter periods of exercise in less extreme conditions. Protein utilisation or catabolism during exercise is largely a function of total energy inadequacy. If you can supply enough carbohydrates to fuel your body’s energy needs, then protein will be spared from being burned.
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