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  Last updated:11/10/2012


Energy to power muscle contractions is released when oxygen combines with carbohydrates, fats, and proteins in the cell to produce Adenosine Triphosphate or ATP. These chemical reactions are called oxidation. The amount of energy produced is limited by either the amount of oxygen available to the cell or limited fuel (carbohydrates, fats, and protein) to be oxidized as an energy source.

The foods we eat provide these three energy containing compounds: carbohydrates, fats, and protein. Carbohydrates are the primary energy source for short, maximum performance events (sprints at or near 100% VO2 max). Both fats and carbohydrates can serve as an energy source for the cell in endurance events (generally performed at less than 50% VO2 max.) Proteins are used to maintain and repair body tissues, and are only rarely used as an energy source to power muscle activity (almost always when severe Calorie restrictions are in effect such as with starvation or malnutrition).

The cardiovascular system delivers the oxygen necessary for oxidation. Oxygen is extracted by the lungs from the air we breathe and then transported via the circulatory system (bound to hemoglobin) to the cells where it will be utilized. One of the end products of energy production, carbon dioxide, is transported from the cell back to the lungs by the circulating blood and then leaves the body in expired air.

The two limiting factors in cellular energy production are:


Food energy is released via oxidation - a chemical reaction with oxygen. When oxidation occurs outside the body - for example in the burning of oil (a fat) in a lamp, or the use of a flaming sugar cube (a carbohydrate) as a decoration in a dessert - the energy is released as heat and light. In the cell, this same energy from carbohydrates and fats in food is released more slowly and in a form that can be harnessed to support basic cell functions or transformed into mechanical movement by muscle cells.

Controlled oxidation in the cell is accomplished by "refining" these three basic compounds (carbohydrates, fats, and proteins), to a single common chemical compound adenosine triphosphate (ATP). It is ATP, the intermediate synthesized as the cell metabolizes (or breaks down) these three basic compounds that actually transfers the energy content of all foods to muscle action.

ATP is composed of a base (adenosine), a sugar (ribose) and three phosphate groups. The chemical bonds between the phosphate groups contain the energy stored in this molecule, and it is the breaking of these bonds (as ATP is converted into ADP or adenosine diphosphate) that provides the energy to power muscle contractions and other cellular functions.


There is a limited storage capacity for ATP in the cell, and at maximum work levels the ATP stored in the muscle cells will be depleted in a few seconds at most. Thus, to sustain physical activity, the cells need to continually replenish (resynthesize) their ATP. There are three pathways for ATP synthesis, and the one used by the cell depends on both the degree and duration of the physical activity.

The first pathway involves the metabolism of phosphocreatine - another high energy molecule found in all muscle cells - to directly resynthesize and resupply cellular ATP. But phosphocreatine is also in limited supply and provides at most another 5 to 10 seconds of energy which limits its usefulness to sprint activities. Once phosphocreatine supplies have been depleted, the cells must switch to one of the other two pathways that regenerate ATP - one requiring oxygen (an aerobic pathway) and another that does not (anaerobic).

Aerobic metabolism, which requires oxygen (is oxygen dependent) refers to several different chemical processes in the cell, which produce ATP from all three food elements - carbohydrates, fats, and protein. Aerobic metabolism supplies the ATP needed for endurance activities.

Glycolysis, also known as anaerobic metabolism, occurs in the absence of oxygen and is limited to carbohydrates (glucose, glycogen) as a fuel source. Anaerobic metabolism is limited by the buildup of excess protons (H+ ions, acidic compounds) in the cell within minutes. This acidic environment then impairs muscle cell contraction and producing actual physical discomfort or pain resulting in a degradation of athletic performance. Anaerobic is generally the source of energy only for short bursts of high level activity lasting several minutes at most (sprints). At one time it was thought that lactic acid was THE acidic compound responsible for muscle pain and impaired muscle cell function (and physical performance). However recent work has shown that to be untrue. Although lactic acid is produced in the cell during energy production, it appears to be an intermediary in glucose metabolism serving an additional protective function to prevent the development of excessive acidity, while other acidic compounds are actually responsible for the impairment of muscle cell function.


As one begins to exercise, the anaerobic pathways provide for the initial ATP needs while the body shifts into gear to increase adequate oxygen to the cell - increasing both breathing and heart rates. As more oxygen is transported to the exercising muscle cell, the aerobic pathways (metabolizing both fats and carbohydrates) pick up the slack and anaerobic metabolism tapers off. However, anaerobic pathways continue to provide a small amount of ATP energy, and small amounts of lactic acid are still being produced. At this level of production, lactic acid is actually a source of cell energy as it is metabolized by liver and muscle cells. It is only as cell energy needs rise (as in a sprint, for example) and a significant shift occurs towards anaerobic metabolism as the more dominant metabolic pathway for energy production that lactic acid levels begin to rise (paralleling the rise in other intracellular acidic compounds).

Aerobic pathways are used by the muscle cells for energy production (metabolizing both fats and carbohydrates as an energy source) up to exercise levels of approximately 50% VO2 max. At that point (called the "crossover point" ), although metabolism is still oxygen based (aerobic), it shifts towards glycogen as an energy source (and fat metabolism decreases). Then, as 100% VO2max is approached, and the cardiovascular system can no longer provide adequate oxygen to the muscle cell to continue aerobic ATP production, either the phosphocreatine system, or anaerobic metabolism pathways once again cover the cells energy needs. When the level of activity drops back to less than 100% VO2max, and oxygen is once again available to the cells, metabolism once again shifts away from anaerobic pathways, and excess oxygen is available to regenerate phosphocreatine and metabolize (clear) the excess acids produced during the anaerobic sprint type activity. With training, changes occur in the cardiovascular system and muscle cells that support higher levels and longer duration of physical activity before anaerobic pathways are needed, and also clear lactic acid more quickly leading to faster recovery from anaerobic sprints.


Could lactic acid be considered as good for us - not the message of the 70's? The answer is a definite yes. Not only is lactate One reason trained athletes can perform at a high level is their training leads to muscle adaptation to perform at a higher level before anaerobic metabolism occurs and lactic acid buildup develops.


The energy contained in equal weights of carbohydrate, fat, and protein is not the same. Energy content is measured in Calories (note the capital C). Carbohydrates and protein both contain 4.1 Calories per gram (120 Calories per ounce) while the energy "density" of fat is more than double at 9 Calories per gram. The disadvantage of fat as a fuel for exercise is that it is metabolized through pathways that differ from carbohydrates and can only support an exercise level equivalent to 50% VO2 max. It is an ideal fuel for endurance events, but unacceptable for high level aerobic (or sprint) type activities.

Carbohydrate metabolism is much more efficient than fat metabolism assuming adequate oxygen is available (ie aerobic metabolism). But once VO2max has been reached, and anaerobic metabolism takes over, the efficiency of carbohydrate metabolism drops off dramatically. Carbohydrate will produce 19 times as many units of ATP per gram when metabolized in the presence of adequate cell oxygen supplies (aerobic) as opposed to its metabolism in an oxygen deficient (anaerobic) environment.

In the well fed and rested state, the human body contains approximately 1500 carbohydrate Calories (stored as glycogen) in the liver and muscle tissue, and over 100,000 Calories of energy stored as fat. The muscles are the main glycogen repository containing anywhere from 300 - 600 grams of glycogen (which equates to 1200 to 2400 Calories) while the liver contains 80 to 110 grams (300 to 400 Calories). The exact numbers are less important than the concept that internal carbohydrate stores are only adequate for several hours of brisk cycling (80 to 100 % VO2max). On the other hand, there is enough stored fat to continue to cycle at a reduced speed (50 - 60% VO2@max) for days.

If one is not supplementing their internal carbohydrates with oral supplements, the muscles first use their internal glycogen reserves for fuel, then call upon liver glycogen which has been maintaining a constant blood sugar level, and finally, when there is no more glucose as an energy source, the muscles switch to fat metabolism and you " bonk". This term describes the fatigue resulting from muscle glycogen depletion. Without adequate carbohydrate to fuel continues high level muscle activity, it is impossible to maintain a high level of energy output and one has to slow to speeds of 50% VO2max where fat metabolism can provide the needed Calories. Any oral supplements delay the onset of this mandatory switch in energy source and associated inability to maintain a high level of performance. The bonk can be delayed by using oral sugars to supplement muscle glycogen stores. Thus, on a long ride, a rider that snacks will have more glucose remaining in the body to fuel that final sprint than one who cuts corners on their nutrition.

Two other strategies are to 1) minimize extremely energy inefficient anaerobic sprints earlier in the ride (remember they are very inefficient in terms of ATP production) and 2) whenever possible, ride closer to 50% VO2max to take advantage of supplemental Calories available from fat metabolism. In addition to eating while riding, these two strategies will help to save a few more grams of muscle glycogen for that final sprint to the line.


Insulin is made by special cells in the pancreas, is released when they sense a rise in the blood glucose level, and works by stimulating the cells of the body to extract that extra glucose from the blood stream. Insulin is essential to the normal functioning of exercising muscle cells - and the replenishment of glycogen post exercise.

The disease of diabetes, at its most basic, is the result of a lack of glucose inside the cells to power them. The blood glucose level is usually elevated but it can't get into the cells where it is needed to supply the energy they need.

Exercise moves carbohydrates into the cell, where it is needed to produce the energy to power the cell, in 2 ways.

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