Exercise Metabolism: Fuels for the Fire

Abstract

During exercise, the supply of adenosine triphosphate (ATP) is essential for the energy-dependent processes that underpin ongoing contractile activity. These pathways involve both substrate-level phosphorylation, without any need for oxygen, and oxidative phosphorylation that is critically dependent on oxygen delivery to contracting skeletal muscle by the respiratory and cardiovascular systems and on the supply of reducing equivalents from the degradation of carbohydrate, fat, and, to a limited extent, protein fuel stores. The relative contribution of these pathways is primarily determined by exercise intensity, but also modulated by training status, preceding diet, age, gender, and environmental conditions. Optimal substrate availability and utilization before, during, and after exercise is critical for maintaining exercise performance. This review provides a brief overview of exercise metabolism, with expanded discussion of the regulation of muscle glucose uptake and fatty acid uptake and oxidation.

Figure 1.

Schematic overview of skeletal muscle metabolism. Hb, Hemoglobin; FFA, free fatty acid; FABP_pm and FABP_c, fatty acid binding protein–plasma membrane and cytoplasm; FAT/CD36, fatty acid translocase; FATP, fatty acid transport protein; GLUT1 and 4, glucose transport proteins 1 and 4; PM, plasma membrane; ATG, HS, and MG lipases, adipocyte glyceride, hormone-sensitive, and monoglyceride lipases; mtOM and mtIM, outer and inner mitochondrial membranes; CPT-I and -II, carnitine palmitoyl transferase I and II; ACT, acylcarnitine transferase; G-1-P and G-6-P, glucose 1 and 6 phosphate; HK, hexokinase; PFK, phosphofructokinase; LDH, lactate dehydrogenase, MCT, monocarboxylate transport proteins; PDH, pyruvate dehydrogenase; TCA, tricarboxylic acid; ANT, adenine nucleotide transport protein; Cr and PCr, creatine and phoshocreatine; CK and mtCK, creatine kinase and mitochondrial CK.

Figure 2.

Contributions of phosphocreatine (PCr), glycolysis, and oxidative phosphorylation to adenosine triphosphate (ATP) turnover during 30 sec of maximal isokinetic cycling exercise. (From Parolin et al. 1999; reprinted, with permission, from the authors.)

Figure 3.

Relative contribution of carbohydrate and fat fuel sources to energy metabolism during exercise of increasing intensity. FFA, Free fatty acid. (From Romijn et al. 1993; reprinted, with permission, from the American Physiological Society © 1993.)

Figure 4.

Sarcolemmal vesicle glucose transporter type 4 (GLUT4) expression and glucose transport during exercise. (From Kristiansen et al. 1997; reprinted, with permission, from the authors.)

Figure 5.

Fatty acid translocase (FAT/CD36) protein expression in and palmitate uptake by giant sarcolemmal vesicles obtained from control and chronically stimulated rat skeletal muscle. (From Bonen et al. 1999; reprinted, with permission, from the American Physiological Society © 1999.)