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Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1; and Department of Medicine, McMaster University, Hamilton, Ontario, Canada L8N 3Z5
The study
examined the existence and regulation of fat-carbohydrate interaction
during low- and moderate-intensity exercise. Eight males cycled for 10 min at 40% and 60 min at 65% maximal O2 uptake
(
O2 max) while
infused with either Intralipid and heparin (Int) or saline (Con).
Before exercise, plasma arterial free fatty acid (FFA) was 0.69 ± 0.04 mM (Int) vs. 0.25 ± 0.04 mM (Con). Muscle biopsies were taken
at rest and at 10, 20, and 70 min of exercise. Arterial and femoral
venous blood samples and expired gases were collected simultaneously
throughout exercise, and blood flow was estimated from pulmonary
O2 uptake and the leg
arterial-venous O2 difference.
Respiratory exchange ratio was higher in Con (0.94 ± 0.01) compared
with Int (0.91 ± 0.01). Mean net leg FFA uptake was higher in Int
(0.16 ± 0.03 vs. 0.04 ± 0.01 mmol/min), and net lactate efflux
was reduced (Int, 1.55 ± 0.36 vs. Con, 3.07 ± 0.47 mmol/min).
Leg net glucose uptake was unaffected by Int. Muscle glycogen
degradation was 23% lower in Int [230 ± 29 vs. 297 ± 36 mmol glucosyl units/kg dry muscle (dm)]. Pyruvate dehydrogenase
activity in the a form
(PDHa) was lower during Int (1.61 ± 0.17 vs. 2.22 ± 0.24 mmol · min
1 · kg
wet muscle1), and muscle
citrate was higher (0.59 ± 0.04 vs. 0.48 ± 0.04 mmol/kg dm).
Muscle lactate, phosphocreatine, ATP, acetyl-CoA, acetyl-carnitine, and
Pi were unaffected by Int.
Calculated free AMP was significantly lower in Int compared with Con at
70 min of exercise (3.3 ± 0.8 vs. 1.5 ± 0.3 µmol/kg dm). The
high FFA-induced reduction in glycogenolysis and carbohydrate oxidation
at 65% O2 max
appears to be due to regulation at several sites. The reduced flux
through phosphorylase and phosphofructokinase during Int may have been
due to reduced free AMP accumulation and increased cytoplasmic citrate.
The mechanism for reduced PDH transformation to the
a form is unknown but suggests reduced
flux through PDH.
glucose-fatty acid cycle; glycogen phosphorylase; pyruvate dehydrogenase; citrate; acetyl-coenzyme A; free adenosine monophosphate; phosphofructokinase
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