Initial Release from Tissues
During exercise, fat stored in adipose tissues and muscle fibers is broken down into fatty acids and glycerol, the basic compounds of fatty acids. Then they are released into the bloodstream to be taken up by muscle cells where the fatty acids are transported into the cell's mitochondria to make ATP (adenosine triphosphate), a high-energy compound that all cells use to make energy. Glycerol is transported to the cell's cytosol and combines with glucose in the process of glycolysis (breakdown of glucose) to generate a few ATPs and pyruvate. The latter compound is a catalyst of starting the citric acid cycle, one of the major steps in the fat-burning process in the cell. The net amount of energy generated via glycolysis and the citric acid cycle from one molecule of glycerol is 19 ATP.
Inside the mitochondria, fatty acids are metabolized via aerobic metabolism, in which oxygen and acetyl-CoA (a byproduct of glucose and fat metabolism) are combined and used to start the citric acid cycle to generate more ATPs. First, fatty acids turn into acetyl-CoA via beta-oxidation, which involves splitting off two carbon acyl fragments from the fatty acid chain. During the degrading process, hydrogen is released from the fatty acid chain, which is combined with oxygen to form water. Once acetyl-CoA is formed, it combines with oxaloacetate to trigger a chain reaction of chemical reactions to form ATP, carbon dioxide and hydrogen exchanges. There are a total of 11 steps in the citric acid cycle, and each irreversible step is dependent on the previous step to occur. Otherwise, the process would slow down, which causes a decrease of energy production. The net amount of energy generated from this process from three molecules of fatty acids is 441 ATP.
The Carbohydrate Flame
The fat-burning process occurs only with the presence of carbohydrates. Pyruvate is the byproduct of glycolysis and a catalyst of the citric acid cycle. It is converted to acetyl-CoA, which is combined with oxaloacetate (a byproduct from pyruvate) to start the citric acid cycle. Low carbohydrate levels reduce the amount of pyruvate generated, which reduces the amount of oxaloacetate and other intermediates of the citric acid cycle. This would slow the whole process down and reduce ATP production. Therefore, fat metabolism occurs in a "carbohydrate flame."