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Most of the stored energy in our bodies is found in the fatty acids of triglycerides. Most of it exists in adipose tissue deposits. When those deposits of fat stores are broken down for the production of energy, triglyceride molecules yield fatty acids that through the blood stream travel to the muscles. Fatty acids from here move to mitochondria (energy factories) where they are broken down into carbon dioxide and water using the electron transport-chain, oxygen requiring electron carriers), for the production of energy, adenosine triphosphate (ATP). The rate at which muscles use fatty acids depends on several factors.
• The more fatty acids released from adipose tissue in the blood stream the more fat will be used by the muscles as energy. A practice that can increase the release of fatty acids in the blood stream from adipose tissue, employed by many athletes, is the consumption of caffeine containing beverages.

• The more conditioned a muscle is, regular exercise and/or physical activity , the greater its ability to utilize fat as a fuel for energy. Aerobic exercise and training after some period of time enables muscle cells to produce more ATP, from oxygen requiring pathways including the pathway used to burn fat for fuel. This becomes possible because muscle cells will develop more and larger mitochondria.

• When exercise increasingly becomes longer but remains at low to moderate aerobic rate the use of fat as a source of energy predominates. When there is demand for energy, for long duration physical activity or exercise, while stores of carbohydrates are always limited there is almost always plenty of fat supply to provide it. Fat actually supplies more than twice as much energy as carbohydrate does for a given weight of fuel.
The aerobically utilization of one glucosemolecule (6-carbon) produces 30 to 32 ATP while a fatty acid molecule (16-carbon) produces 108 ATP. However, carbohydrate is more efficient in one special way. It takes 23 O2 molecules to produce 108 ATP but it takes only 6 O2 molecules to produce 30 to32 ATP. This means athletes involved in competitive endurance exercise need that muscles cells also use carbohydrate as long as the supply lasts.

When engaging in lengthy physical activities or exercise such as ultra marathon, manual labor and even a desk job for 8 hours a day the energy requirements are supplied 70% to 90% from fat. It is only carbohydrate however that can support intense aerobic activity or exercise. Prolonged low intensity steady exercise or activity uses all available energy,
fat and carbohydrates.

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When physical activity or exercise is of low or moderate intensity and muscles are in an aerobic state (plenty of oxygen available), most of the pyruvate produced during glycolysis is further metabolized to turn more energy and finally carbon dioxide and water.

Almost 95% of the aerobically produced energy, adenosine triphosphate (ATP), from the complete metabolism of glucose, it takes place in the mitochondria. Several health supplements such as minerals and some of the B vitamins are important key factors in the production of energy of both anaerobic and aerobic pathways.

Aerobic cellular respiration includes those areas of the metabolic pathway that are oxygen dependent. The aerobic pathway produces ATP more slowly than the anaerobic pathway. However, aerobically released energy production is a lot higher and can be sustained for hours making it an important energy contribution to physical activity and/or sports. Its contribution lasts more than 2 minutes and up to 4 or 5 hours. The aerobic pathway uses carbohydrates and fatty acids as fuel so that it represents an enormous potential source of energy. For athletes, the aerobic system is the main provider of ATP in events lasting more than 3 minutes.

Glycogen is the ort term storage form of glucose in the liver and muscles about 100g and 300g respectively. Glycogen is the primary source of glucose for the production of ATP for muscles during intense activities that last about 2 hours or so. Glycogen is broken down into a form of glucose that becomes available to both aerobic and anaerobic pathways. Depletion of glycogen in the muscles contributes to muscle fatigue while depletion in the liver creates blood glucose fall.

When depletion of glycogen stores take place the maximum capacity that can be achieved by an athlete, while continue working, will be about 50% of maximum capacity and further exertion is hampered. When maximum volume of oxygen consumed per unit of time (VO2max) for longer than an hour consideration of increasing stored carbohydrates in the muscle is an appropriate decision. Exercise durations longer than 30 minutes will require maintenance of blood glucose levels and therefore intake of carbohydrates in the order of 40 to 60 g/hour during strenuous endurance exercise. Maintaining blood sugar levels can spare glycogen in the muscles in search of the edge over the opponents in the finish line

The intake of carbohydrates in short term events of 30 minutes exercise or so is not so important since the muscles during sort term exercise do not take up blood glucose due to the action of several hormones; Glycogen and epinephrine suppress the action of insulin to increase the uptake of glucose by the muscles. Carbohydrate intake during exercise is overall a very important source of energy before and during prolonged, strenuous and continuous exercise and/or physical activity.

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Utilization of carbohydrates as fuels during exercise depends primarily on the intensity and duration of the activity. Generally, carbohydrate use increases with increasing intensity and falls with increasing duration of an exercise activity . Glucose is an important carbohydrate in human biology.

The cell uses it as a source of energy. The breakdown of food molecules in glucose and other simple sugars is what fuels the reactions which take place inside of our bodies.

Glucose breaks down during glycolysis; production of ATP (adenosine triphosphate) through the degradation of glucose in the absence of oxygen. The process of glycolysis yields a small amount of ATP. During glycolysis the three carbon compound pyruvate is produced. When oxygen is present more ATP is produced from further metabolism of pyruvate. During intense exercise
, physical activity or when supply of oxygen in muscle is limited pyruvate produced from glycolysis accumulates in the muscle and is converted into lactate.

On glycolysis 1 glucose can yield 2 ATP and can re-supply depleted ATP in muscle activity. Most of the energy provided for exercise or physical activity, after it started, for about 30 seconds to 2 minutes derives from glycolysis. Fat utilization does not occur fast enough for ATP demands to be met. If the energy source for exercise or physical activity was only fat we wouldn’t be in a position to carry out activities more intense than a fast walk.

While the anaerobic pathway - glucose provide a major energy fuel for short term, high intensity exercise there are some major disadvantages: Only 5% of the available energy from glucose is released during glycolysis thus ATP production can not be sustained for long. Accumulation of lactate increases acidity of muscle cells that in turn inhibits the activity of key enzymes in glycolysis and production of ATP soon slows and muscle fatigue
sets in.

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