Where a more extended form of physical activity demands more energy, glucose becomes the next most immediate source of energy or ‘fuel’. For example, after the first 50-100 metres have been run, the ATP and CP stores have been depleted, but the person is still running and needs more energy. The glucose is broken down in a chain of chemical reactions to form a substance called pyruvate (a process called glycolysis which generates 2 ATPs per glucose molecule along the way). It is then broken down aerobically to H2O and CO2, generating a further 36 ATPs. If the rate of energy demand is greater than the aerobic system’s capacity to handle pyruvate, the anaerobic part of glucose metabolism (glycolysis) supplies the energy needed, and the pyruvate is turned into lactic add. At high concentrations, lactic acid acts to stop muscles continuing to contract.
It should be noted that lactate salts produced from anaerobic glycolysis are not waste products. Lactates are transported out of the musde to the liver for conversion to glucose or glycogen and to the cardiac musde which can use them directly as fuel. Anaerobic glycolysis can provide energy for intense exercise up to 45-60 seconds, beyond which its necessary to use oxygen for further fuel breakdown. If oxygen is present for aerobic metabolism, there can be a continual breakdown of glucose until, theoretically at least, this is exhausted. In this case, pyruvate is involved in further energy breakdown through a process called the Krebs cyde, during which more ATP is produced (around 36 molecules per glycogen molecule compared to 2 under anaerobic conditions) providing more energy for longer effort. As we have seen in Chapter 3, however, the glucose supplies in the body of a 70kg male only amount to around 450g, or the equivalent of 2250 calories, about a day’s energy requirement at rest, and less than would be needed to run a marathon, if glucose was the only fuel source. Hence another source of fuel for physical activity is required.
This is where fat comes into the equation. Fats, in the form of free fatty acids, play little part in the energy equation while there is no oxygen available, because these are only broken down after entering the Krebs cyde where aerobic metabolism is initiated. When oxygen is available, fats take a major role in energy production because each molecule of fat oxidised during this process leads to a production of around 132 ATP molecules, far more than the 36 molecules supplied through slow-aerobic metabolism or the 2 ATP molecules through fast-anaerobic glycolysis.
After stored ATP is reduced to adenosine diphosphate (ADP) (over 4 seconds of maximal effort), creatine phosphate (CP) combines with ADP to reform ATP + creatine and another 4-6 seconds energy at maximal effort is made available. This process occurs in the cytoplasm of the muscle cell, no oxygen is used and the breakdown of neither fat nor glucose is required. Hence lifting a heavy weight, or an explosion of energy such as in a sprint, bums essentially no fat. When a slightly longer term (i.e. 1-2 minutes) of, still intense, muscular contraction is needed, glucose through anaerobic glycolysis is metabolised in the absence of oxygen in the cytoplasm of the cell. Again no fat is involved. Fat provides a better option for longer term performance, simply because it is twice as energy dense and there is relatively much more of it than glucose. The key, however, is that oxygen must be present In other words, the activity must be aerobic for fat to be utilised. The extent to which it becomes utilised as part of the energy mix is then dependent on a number of factors, including genetics, state of glycogen scores, the intensity and duration of exercise, the amount of glucose coming from the gut, and the level and type of obesity. Our specific interests here are the exercise parameters that influence fat utilisation and the practical implications of this for general exercise planning.
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