Substrate source use in older, trained males after decades of endurance training.
OBJECTIVE
The purpose of this study was to compare substrate source use in older, long-term exercising, endurance-trained males with sedentary controls.


METHODS
[U-C]palmitate and [6,6-H2]glucose tracers were applied to assess plasma free fatty acid (FFA) and glucose oxidation rates, and to estimate muscle- and/or lipoprotein-derived triacylglycerol (TG) and muscle glycogen use. Subjects were 10 long-term exercising, endurance-trained males and 10 sedentary controls (age 57 +/- 1 and 60 +/- 2 yr, respectively). Muscle biopsy samples were collected before and after exercise to assess muscle fiber type-specific intramyocellular lipid and glycogen content.


RESULTS
During exercise, plasma palmitate Ra, Rd, and Rox were significantly greater in the trained subjects compared with the controls (Ra: 0.36 +/- 0.02 and 0.25 +/- 0.02; Rd: 0.36 +/- 0.03 and 0.24 +/- 0.02; Rox: 0.31 +/- 0.02 and 0.20 +/- 0.02 mmol.min, respectively, P < 0.01). This resulted in greater plasma FFA and total fat oxidation rates in the trained versus sedentary subjects (P < 0.001). Muscle- and/or lipoprotein-derived TG use contributed 10 +/- 2 and 11 +/- 3% in the trained and control groups, respectively (NS). No significant net changes in muscle fiber lipid content were observed.


CONCLUSIONS
Older, endurance-trained males oxidize more fat during moderate-intensity exercise than do sedentary controls. This greater total fat oxidation rate is attributed to a higher plasma FFA release, uptake, and oxidation rate. In contrast, intramyocellular triacylglycerol does not seem to represent a major substrate source during 1 h of moderate-intensity exercise in older trained or sedentary men.
Boon
Hanneke
H
Department of Human Biology, Nutrition and Toxicology Research Institute Maastricht, Maastricht University, Maastricht, The Netherlands. H.Boon@HB.unimass.nl
Jonkers
Richard A M
RA
Koopman
Rene
R
Blaak
Ellen E
EE
Saris
Wim H M
WH
Wagenmakers
Anton J M
AJ
VAN Loon
Luc J C
LJ
D000375:Aging / Q000502:physiology; D003430:Cross-Sectional Studies; D004734:Energy Metabolism / Q000502:physiology*; D005080:Exercise Test; D005230:Fatty Acids, Nonesterified / Q000378:metabolism; D006003:Glycogen / Q000378:metabolism; D006801:Humans; D050356:Lipid Metabolism / Q000502:physiology; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000378:metabolism; D010807:Physical Endurance / Q000502:physiology; D010809:Physical Fitness / Q000502:physiology*; D012146:Rest / Q000502:physiology
D005230:Fatty Acids, Nonesterified; D006003:Glycogen