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Fuel preference during exercise after altitude acclimation McClelland, Grant Bryce


At high altitude, carbohydrate (CHO) is thought to be the fuel of choice because of its higher yield of ATP per mole of O2. Alternatively, the increased use of lipids spares valuable glycogen. We used indirect calorimetry to determine total fuel oxidation rates during exercise in high-altitude acclimated (HA) and sea level (SL) rats. D-(6-³H) glucose and 1-[14C] palmitate infusions quantified circulatory glucose utilization and nonesterified fatty acid (NEFA) turnover. We hypothesized that the % contribution of fuels to total metabolism (VO2) is determined by exercise intensity relative to an aerobic maximum (%VO2max). VO2max was lower at HA (FIO2=0.12) compared to SL (68 ± 1 vs 89 ± 1 ml kg-1min-1). When exercised at 60 % Vo2max, both groups showed the same relative use of CHO [38 ± 3% (HA) and 38 ± 5% (SL) of Vo2] and lipids [62 ± 5% (HA) and 61 ± 3% (SL) of VO2]. Circulatory glucose accounted for ~ 20%, while muscle glycogen was ~ 18% of VO2. Plasma [NEFA] were higher in HA but rate of turnover was the same as SL [38 ± 9 (HA) vs 49 ± 6 μmol kg-1 min-1 (SL)]. Muscle triacylglycerol (TAG) decreased by 64% and 90% in soleus and red gastrocnemius at HA only. At 80 % Vo2max, 54 ± 5 (HA) and 59 ± 4 % (SL) of VO2was accounted for by CHO. Lipid accounted for 46 ± 6 (HA) and 41 ± 4 % (SL) of VO2. Measurements of the TAG-FA cycle showed an increase at rest and exercise with HA acclimation. We conclude that: 1) the relative contributions of total CHO, circulatory glucose, and muscle glycogen do not increased after HA acclimation because the O2-saving advantage of CHO is outweighed by limited CHO stores, 2) the relative contributions of total lipid are also the same, contrary to differences in [NEFA] because the relationship between flux rate and [NEFA] is modified at HA, 3) muscle TAG may play a more important role at HA, 4) TAG - FA cycling is increased with acclimation and 5) relative exercise intensity is the major determinant of metabolic fuel selection at high altitude and sea level. [Scientific formulae used in this abstract could not be reproduced.]

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