Carbohydrate utilization during exercise after high-altitude acclimation: A new perspective
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abstract
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
2max
). HA rats run under hypoxia (F
I
O
2
= 0.12) showed a decrease in V̇
O
2max
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
2max
, 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
2max
, 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.
