Women oxidize more lipid and less carbohydrate and protein compared with men during endurance exercise. The increase in fat oxidation is associated with higher intramyocellular lipid content and use as well as greater adipocyte lipolysis. Glucose rates of appearance and disappearance are lower for women than for men, with no change in basal muscle glycogen, and some evidence for muscle glycogen sparing during endurance exercise. Women oxidize less protein compared with men and show lower leucine oxidation during exercise. The consistent and robust finding of higher mRNA abundance for most components of fat-oxidation pathways in women compared with men is directionally consistent with the substrate-oxidation data. A lack of directional consistency between mRNA species involved in carbohydrate and protein metabolism and the known sex differences during exercise implies that fat oxidation is regulated and that carbohydrate and protein oxidation follow by metabolic demand. Administration of 17-beta-estradiol to men recapitulates most of the described sex differences in metabolism and mRNA content. The greater fat oxidation for women during submaximal endurance exercise compared with men seems to occur partly through a sex hormone-mediated enhancement of lipid-oxidation pathways.
Department of Pediatrics and Medicine, McMaster University, Hamilton, CANADA
Editor's Note: This paper is an Editor-in-Chief-invited contribution from ACSM's conference on Integrative Physiology of Exercise held in Indianapolis, Indiana, September 27-30, 2006.
Address for correspondence: Mark Tarnopolsky, M.D., Ph.D., FRCPC, Department of Pediatrics and Medicine, McMaster University, 1200 Main St. W., HSC-2H26, Hamilton, Ontario, L8N 3Z5; E-mail: firstname.lastname@example.org.
Submitted for publication May 2007.
Accepted for publication September 2007.