Athletes are said to "burn" energy in the pursuit of their athletic goals; scientists refer to this process of internal bodily fuel consumption as oxidation. Both descriptions are accurate, the body requires both a fuel source (stored as one of the three sources: carbohydrates, fats, or proteins) combined with the delivery of oxygen to create energy.
Fats are a food group that includes a number of substances consumed in the typical diet. In the narrowest chemical sense, fats are a compound form of various kinds of fatty acids, found in various kinds of animal fats, vegetable fats, and oils. Once consumed, all types of fat have the potential to be employed as fuel within the body; the type and the quantity of fats consumed are a very important factor in both athletic performance and general human health.
Fats, along with carbohydrates and proteins, form the cornerstones of daily human nutritional needs. Although the ratio between each group may vary, depending upon individual circumstances, a general relationship of 60-65% carbohydrates, 12-15% proteins, and less than 30% fats is viewed as a healthy one. The intake of fat is the process of consumption; the oxidation of fat is the end use conversion into human energy.
While all three sources of energy may be consumed for this purpose, there exists a clear hierarchy as to when each is used for energy production. Proteins are the least desirable source of energy for all of the human energy systems, due to both the manner in which proteins are stored, as well as the significant amount of energy required to release the constituent parts of a protein into a useful form for energy conversion. Carbohydrates a useful energy form, and they are the only source of energy that the body will use to power the central nervous system and the process of erythrocyte (red blood cell) production. Fats have their uses as a fuel source because they are capable of virtually unlimited storage in the body, in the adipose cells created by the body for the storage of fat, and stored fats are utilized for a broad variety of functions.
Fat oxidation is not linked to the amount of fat consumed through diet. The body will have a positive balance of fat when there has been an excess consumption of energy sources (e.g., overeating), or a restriction in accustomed physical activity. When the amount of carbohydrates consumed is increased, the greater the percentage of carbohydrates consumed as energy will increase. This will necessarily decrease the amount of fats oxidized, increasing the available amount of fat. When the food energy is decreased, or when exercise levels are increased, fat oxidation will typically increase. The body will not readily convert excess carbohydrates into fat stores; it will first seek to use them as energy. This process means that, while a high level of carbohydrate intake does not always lead to increased fat stores, high intakes of fat will generally be reflected in the fat stored in the adipose tissues.
The process by which stored fats become energy sources begins with how fat is released from the adipose tissue. Fats are digested through conversion into free fatty acids, which are stored in a form known as triglycerides in the adipose tissue. Various hormones will trigger the release of the triglycerides from adipose tissue. These triglycerides, through the process known as lipolysis (a breakdown of the stored fats), are reduced to two distinct components: glycerol, (which is processed by the liver for further use), and fatty acids (which are released into the bloodstream). The fatty acids are transported to the mitochondria, the portion of a cell that produces power within each cell. The transport of the fatty acids to the mitochondria is facilitated by the enzyme carnitine, a transport mechanism that is found in food sources such as red meats and poultry. The mitochondria are also a storehouse for deoxyribonucleic acid (DNA), as well as the enzymes necessary to permit ongoing cellular construction. In the mitochondria, the fatty acids are oxidized in the process that creates adenosine triphosphate (ATP), the energy-producing fuel.
Various research studies have considered whether greater amounts of fatty acids could be oxidized if carnitine levels were elevated through supplements. Such increases were determined to have no discernible effect of fatty acid oxidization.
The oxidation of a molecule of fatty acid released from a fat cell stored in the body is a complete process. No portion of the fatty acid transported to the mitochondria is left over from the chemical process producing ATP.