Exercise and energy metabolism.

Most of the energy required for muscle contraction is generated by the splitting of ATP. Catabolism of carbohydrates to pyruvate and lactate by glycolysis (anaerobic) produces ATP needed during exercise of high intensity and short duration but the energy release is incomplete. The first step in the release of the remaining energy is the conversion of pyruvate to acetyl-CoA which is also formed from the oxidation of fatty acids particularly during sustained exercise. Acetyl-CoA is next oxidized in the tricarboxylic acid cycle producing the reduced forms of pyridine and flavin nucleotides. The hydrogens of the reduced nucleotides are transported in the form of electrons through the electron transport (respiratory) chain to molecular O2 to form H2O. The resulting free energy change is used to phosphorylate ADP to ATP. Enzymes of fatty acid oxidation, tricarboxylic acid cycle and respiration are located inside mitochondria. Energy status of cells can be described by the relative concentrations of adenine nucleotides (ATP, ADP and AMP) since the nucleotides act as allosteric modifiers of several key enzymes of energy metabolism. Energy is expended to synthesize and store fuel nutrients in utilizable forms (glycogen, triglycerides and proteins) and to maintain the storage forms in a dynamic steady state. The energy cost of such postprandial metabolic processes is defined as the "thermic" effect of food. Storage of dietary carbohydrates as adipose tissue triglycerides expends more energy than that of dietary fats. Overfeeding of certain individuals is known to burn more energy over and above that expected from the "thermic" effect and the process is referred to as "luxus consumption."(ABSTRACT TRUNCATED AT 250 WORDS)