Carbohydrate utilization during exercise after high-altitude acclimation: A new perspective

At high altitude (HA), carbohydrate (CHO) is thought to be the preferred fuel because of its higher yield of ATP per mole of O(2). We used indirect calorimetry and d-[6-(3)H]glucose infusions to determine total CHO and circulatory glucose utilization during exercise in HA-acclimated and sea level (SL) rats. We hypothesized that the percent contribution of CHO to total metabolism (V̇(O(2))) is determined by exercise intensity relative to an aerobic maximum (% V̇(O(2))(max)). HA rats run under hypoxia (FIO(2) = 0.12) showed a decrease in V̇(O(2))(max) compared with SL (67.55 ± 1.26 vs. 89.30 ± 1.23 ml kg(−1) min(−1)). When exercised at 60% of their respective V̇(O(2))(max), both groups showed the same relative use of CHO (38 ± 3% and 38 ± 5% of V̇(O(2)), at the beginning of exercise, in HA and SL, respectively). In both HA and SL, circulatory glucose accounted for ≈20% of V̇(O(2)), the balance was provided by muscle glycogen (≈18% of V̇(O(2))). After 20 min at a higher intensity of 80% V̇(O(2))(max), 54 ± 5% (HA) and 59 ± 4% (SL) of V̇(O(2)) was accounted for by CHO. We conclude the following: (i) the relative contributions of total CHO, circulatory glucose, and muscle glycogen do not increase after HA acclimation because the O(2)-saving advantage of CHO is outweighed by limited CHO stores; and (ii) relative exercise intensity is the major determinant of metabolic fuel selection at HA, as well as at SL.