METABOLISM DURING EXERCISE - AEROBIC VS. ANAEROBIC

Metabolically, there are two types of exercise, aerobic and anaerobic. Aerobic exercise uses oxygen as energy substrate to metabolize food to adenosine triphosphate (ATP) (see box below, METABOLIC CHANGES DURING AEROBIC AND ANAEROBIC EXERCISE). When the supply of oxygen is no longer sufficient to meet the needs of exercising muscles, anaerobic metabolism begins. In anaerobic metabolism, glucose is converted to ATP without oxygen, and lactic acid is generated as a by-product. A healthy person can perform aerobic exercise for several hours; in contrast, pure anaerobic exercise can only be sustained for a few minutes before severe dyspnea and fatigue set in.

During short bursts of activity, such as sprinting, energy may be obtained only anaerobically. Otherwise, anaerobic metabolism occurs in addition to ongoing aerobic metabolism. Typically, anaerobic metabolism begins approximately midway between resting and maximal oxygen consumption. The point at which anaerobic metabolism begins is called the anaerobic threshold (AT). AT can be identified by a typical pattern of changes in the blood and in expired gases (see the next section).

METABOLIC CHANGES DURING AEROBIC AND ANAEROBIC EXERCISE During aerobic exercise, both glucose and fatty acids are metabolized. One molecule of glucose utilizes 6 molecules of oxygen and produces 6 molecules of carbon dioxide, for a metabolic respiratory quotient (RQ) of I.O. For fatty acids, 23 molecules of oxygen are used for every 16 molecules of carbon dioxide produced, giving an RQ of 0.71. The average RQ during mild to moderate exercise (before anaerobic threshold) is approximately 0.85. By contrast, anaerobic metabolism produces only 2 molecules of ATP per molecule of glucose; at the same time 2 molecules of lactic acid are produced, which, when buffered, generate carbon dioxide in excess of that from aerobic metabolism. AEROBIC METABOLISM C6H12O6 + 6 02 ----> 6 CO2 + 6 H2O + 36 ATP (RQ = 1.0) (Glucose) C16H32O2 + 23 02 ----> 16 CO2 + 16 H2O + 130 ATP (RQ = 0.71) (Fatty acid) ANAEROBIC METABOLISM Glucose + 2 ADP ----> 2 H+ lactate + 2 ATP (Lactic acid) H+ lactate- + Na+HCO-3 ---> Na+ lactate- + H2CO3 H2CO3 ---> H2O + CO2

Chapter 12: Exercise Physiology

from Pulmonary Physiology in Clinical Practice, copyright 1999 by

Lawrence Martin, M.D.