1. The influence of ethanol administration on the metabolism of non-esterified fatty acids (NEFA) and on the utilization of blood-borne substrates and stored glycogen by leg muscles was examined in nine young healthy subjects at rest and during 40 min bicycle ergometer exercise at 50% of maximal capacity. Ethanol was administered by a constant-rate intravenous infusion (6 mmol/min), giving an arterial concentration within the range of 9·3–15·6 mmol/l. Turnover of NEFA and regional exchange were evaluated with [14C]oleic acid, and muscle metabolites were analysed from needle biopsy samples.

2. Leg blood flow was not altered by ethanol. Alanine in arterial blood was reduced by approximately 25% after ethanol administration during exercise. A slight decrease in the concentration of NEFA in arterial plasma was observed after ethanol, associated with an increase in the fractional turnover of oleic acid, indicating an increased removal of NEFA from the plasma. The rate of uptake of NEFA in the leg was only slightly decreased.

3. The total glycogen depletion in leg muscle during exercise was not influenced by ethanol. The pattern of glycogen utilization in different muscle fibres was changed by ethanol and found to be dependent on the fibre composition of the respective subject. In the control experiments, but not after ethanol administration, a negative correlation was observed between glycogen utilization during exercise and the fraction of type I fibres.

4. It is concluded that ethanol administration during exercise (a) decreases leg uptake of NEFA due to a lower concentration of NEFA in plasma, although this lower uptake is compensated by an increased fractional oxidation, (b) does not influence total glycogen depletion in leg muscle, (c) changes the pattern of glycogen utilization in different muscle fibres, (d) influences only to a small extent the energy supply to working muscles, the most predominant isolated effect being on inhibition of the lactate release.

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