[Sources for the following are Exercise Physiology by David H. Clarke (1975) and Principles of Physiology by Roy Hartenstein (1972)]
Quote from Clarke: “While the fuel for exercise is thought to be composed primarily of carbohydrates, it is becoming increasingly evident that fat metabolism is important as an energy source, and that the fat that is used comes from free fatty acids (FFA) hydrolized from the triglycerides of the adipose tissue by the enzyme lipase…”
Metabolism can be divided into anaerobic and aerobic phases, depending upon atmospheric oxygen. Glycolysis involves the degradation of glucose into pyruvic acid. A number of intermediate products are involved, with the formation of some ATP and the transferrance of two hydrogen atoms to the coenzyme NAD, forming NAD2H. If oxygen is not present in sufficient quantities, the hydrogens are passed to pyruvic acid, converting it to lactic acid.
Pyruvic acid is condensed aerobically in the Krebs cycle by first yielding acetyl CoA and combining with oxaloacetic acid, yielding 3 molecules of CO2, one ATP, and removal of 5 pairs of hydrogens and passed by means of a coenzyme to the respiratory chain. Eventually they undergo electron transfer and combine with oxygen to form water. The acetyl CoA is formed by the degradation of pyruvic acid to carbon dioxide and hydrogen, accomplished initially by having a carbon dioxide molecule cleaved from pyruvic acid as it combines with coenzyme A.
FFA are hydrolized from the triglycerides of the adipose tissue, where they are transported by the blood to the muscles, entering the Krebs cycle to yield ATP. Carbohydrates are the preferred fuel for short, strenuous bouts of activity, and fats are used in longer endurance events. If the exercise is severe, lactic acid will be formed. Moreover, the amount of energy a person uses reflects a dynamic time lag before the requisite circulatory factors can deliver sufficient oxygen. Both will result in oxygen debt.
FFA, like carbs, enter the cycle as 2C acetyl fragments, but FFA passes through an oxidation pathway. Most amino acids also may enter the cycle, half entering as acetyl CoA and 1/3d as alpha ketoglutaric or oxaloacetic acid.
Most of the energy needed for synthesizing ATP is obtained through electron transport, wherein a pair of electrons is removed from organic molecules and passed along a series of cytochromes from a high energy level to a lower energy level. Energy is released in the process and the electrons, together with a pair of hydrogen ions, combine with an oxygen atom.