Pubblicato: novembre 17, 2012 in health, nutrition, training

Edward F. Coyle, Ph.D.
Professor, Department of Kinesiology and Health Education
The University of Texas at Austin
Member, GSSI Sports Medicine Review Board

1. People store large amounts of body fat in the form of triglycerides within fat (adipose) tissue as well as within muscle fibers
(intramuscular triglycerides).When compared to carbohydrate stored as muscle glycogen, these fat stores are mobilized and oxidized
at relatively slow rates during exercise.
2. As exercise progresses from low to moderate intensity, e.g., 25-65% VO2max, the rate of fatty acid mobilization from adipose
tissue into blood plasma declines, whereas the rate of total fat oxidation increases due to a relatively large use of intramuscular
triglycerides. Intramuscular triglycerides also account for the characteristic increase in fat oxidation as a result of habitual
endurance-training programs.
3. Dietary carbohydrate intake has a large influence on fat mobilization and oxidation during exercise; when dietary carbohydrate
produces sufficient carbohydrate reserves in the body, carbohydrate becomes the preferred fuel during exercise. This is especially
important during intense exercise because only carbohydrate(not fat) can be mobilized and oxidized rapidly enough to
meet the energy requirements for intense muscular contractions.

Snap 2012-11-17 at 18.22

Eliminating Carbohydrate From theDiet of Endurance-Trained People

Recognizing that even smallamounts of dietary carbohydrate mightinfluence fat metabolism, a study was
performed by Phinney et al. (1983) duringwhich they fed endurance-trained
men a high-fat diet containing almostno carbohydrate, i.e., less than 20 g/d
for 4 wk. This diet reduced the concentrationof muscle glycogen by one-half,
and it markedly increased fat oxidation
during exercise at moderate intensitiesof 62-64% VO2max. However, the dietdid not increase the length of time that
exercise could be maintained, despitethe fact that fat oxidation was increased.Furthermore, these subjects were notcapable of exercising at higher intensities.
Even with this extreme diet, itseems clear that fat oxidation cannot beincreased sufficiently to fully replace
muscle glycogen as a source of energyfor intense exercise. Furthermore, high
fat intake is a risk factor for cardiovascularand other diseases.
Snap 2012-11-17 at 18.22Snap 2012-11-17 at 18.23Snap 2012-11-17 at 18.23



People store large amounts of bodyfat in the form of triglyceride within
adipose tissue as well as within musclefibers. These stores must be mobilized
into FFA and transported to musclemitochondria for oxidation during exercise.
Fatty acids from adipose tissueare mobilized into plasma and carried
by albumin to muscle for oxidation. Asexercise intensity increases from low
(25% VO2max) to moderate (65%VO2max) to high (85% VO2max), plasma
FFA mobilization declines.However, total fat oxidation increases
when intensity increases from 25% to
65% VO2max, due to oxidation of intramusculartriglycerides, which provide
about one-half of the fat for oxidation.Endurance training characteristically
increases fat oxidation during moderateintensity exercise by accelerating the
oxidation of intramuscular triglyceridewithout increasing the mobilization or
oxidation of plasma FFA. Similarly,during low-intensity exercise with little
intramuscular triglyceride oxidation,the increased fat oxidation of trained
people does not appear to be caused byincreased mobilization of FFA into
plasma, but rather by a greater rate ofoxidation of the FFA removed from the
blood during exercise. Therefore, itseems that untrained people have
greater abilities to mobilize FFA thanthey do to oxidize it when they exercise
in the fasted state. Carbohydrate ingestionduring the hours before exercise,even in relatively small amounts,
reduces fat oxidation during exercise
largely through the action of insulin.Fat supplementation and special dietshave limited ability to increase fat oxidation
in people, especially duringsport competitions. Therefore, fat frombody stores and/or dietary supplementation
cannot adequately replace muscleglycogen and blood glucose as fuels for intense exercise.

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