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Department of Human Biology, Nutrition Research Centre, University Maastricht, NL-6200 MD Maastricht, The Netherlands
Received 8 April 1996; accepted in final form 1 July 1996.
Jeukendrup, Asker E., Lars B. Borghouts, Wim H. M. Saris,
and Anton J. M. Wagenmakers. Reduced oxidation rates of ingested glucose during prolonged exercise with low endogenous CHO availability. J. Appl. Physiol. 81(5):
1952-1957, 1996.
This study investigated the effect of endogenous
carbohydrate (CHO) availability on oxidation rates of ingested glucose
during moderate-intensity exercise. Seven well-trained cyclists
performed two trials of 120 min of cycling exercise in random order at
57% maximal O2 consumption. Preexercise glycogen concentrations were manipulated by
glycogen-lowering exercise in combination with CHO restriction
[low-glycogen (LG) trial] or CHO loading
[moderate-to-high-glycogen (HG) trial]. In the LG and HG
trials, subjects ingested 4 ml/kg body wt of an 8% corn-derived
glucose solution of high natural
13C abundance at the start,
followed by boluses of 2 ml/kg every 15 min. The third trial, in which
potato-derived glucose was ingested, served as a control test for
background correction. Exogenous glucose oxidation rates were
calculated from the 13C enrichment
of the ingested glucose and of the breath
CO2. Total CHO oxidation was lower
in the LG trial than in the HG trial during 60-120 min of exercise
[84 ± 7 (SE) vs. 116 ± 8 g;
P < 0.05]. Exogenous CHO
oxidation in this period was 28% lower in the LG trial compared with
the HG trial. Maximal exogenous oxidation rates were also lower
(P < 0.05) in the LG trial (0.64 ± 0.05 g/min) than in the HG trial (0.88 ± 0.04 g/min). This
decreased utilization of exogenous glucose was accompanied by increased plasma free fatty acid levels (2-3 times higher) and lower insulin concentrations. It is concluded that glycogen-lowering exercise, performed the evening before an exercise bout, in combination with CHO
restriction leads to a reduction of the oxidation rate of ingested
glucose during moderate-intensity exercise.
exogenous glucose oxidation; carbon-13; breath test; stable isotopes; substrate utilization; carbohydrate
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