Increased dependence on blood glucose after acclimatization to 4,300 m

Abstract

To evaluate the hypothesis that altitude exposure and acclimatization result in increased dependency on blood glucose as a fuel, seven healthy males (23 +/- 2 yr, 72.2 +/- 1.6 kg, mean +/- SE) on a controlled diet were studied in the postabsorptive condition at sea level (SL), on acute altitude exposure to 4,300 m (AA), and after 3 wk of chronic altitude exposure to 4,300 m (CA). Subjects received a primed continuous infusion of [6,6-2D]glucose and rested for a minimum of 90 min, followed immediately by 45 min of exercise at 101 +/- 3 W, which elicited 51.1 +/- 1% of the SL maximal O2 consumption (VO2 max; 65 +/- 2% of altitude VO2 max). At SL, resting arterial glucose concentration was 82.4 +/- 3.2 mg/dl and rose significantly to 91.2 +/- 3.2 mg/dl during exercise. Resting glucose appearance rate (Ra) was 1.79 +/- 0.02 mg.kg-1.min-1; this increased significantly during exercise at SL to 3.71 +/- 0.08 mg.kg-1.min-1. On AA, resting arterial glucose concentration (85.8 +/- 4.1 mg/dl) was not different from sea level, but Ra (2.11 +/- 0.14 mg.kg-1.min-1) rose significantly. During exercise on AA, glucose concentration rose to levels seen at SL (91.4 +/- 3.0 mg/dl), but Ra increased more than at SL (to 4.85 +/- 0.15 mg.kg-1.min-1; P less than 0.05). Resting arterial glucose was significantly depressed with CA (70.8 +/- 3.8 mg/dl), but resting Ra increased to 3.59 +/- 0.08 mg.kg-1.min-1, significantly exceeding SL and AA values.(ABSTRACT TRUNCATED AT 250 WORDS)

Similar carbohydrate but enhanced lactate utilization during exercise after 9 wk of acclimatization to 5,620 m

“…On the basis of the present study and the study of Roberts et al (27), one could argue that the enhanced utilization of carbohydrates and blood glucose seen in acute hypoxic conditions and in the 1st wk of acclimatization is overcome by a prolonged period of acclimatization and that glucose homeostasis is reestablished to the sea-level condition. However, we cannot exclude the possibility that energy imbalance (5,8) and change in dietary carbohydrate and fat intake may have affected our findings. Noteworthy, however, is the much higher blood lactate R a and net lactate release from the active leg at altitude compared with sea level, indicating that muscle glycogen stores were sufficient despite reduced body weight.…”

“…Lowlanders acutely exposed to high altitude showed a shift toward increased blood glucose utilization relative to normoxic conditions (5,28). After 3 wk of acclimatization, reliance on blood glucose increased even further (5) or decreased (28).…”

“…Thus any increase in the percentage of energy derived from carbohydrate sources will result in a more economical use of oxygen. Indeed, results from studies in high-altitude natives (17) and lowlanders exposed to high altitude (5,27) show a shift toward increased blood glucose utilization relative to normoxic conditions. However, McClelland et al (23) reported that rats acclimatized to severe hypoxia did not increase carbohydrate utilization compared with normoxic conditions.…”

“…Thus there is no consensus in the literature with regard to metabolic fuel selection under hypoxic conditions. Some of the differences observed in fuel utilization after altitude exposure have been attributed to cachexia and energy deficiency (5,8), gender (2), or limited carbohydrate stores (23).…”

“…After several weeks of acclimatization to hypoxia, the blood lactate response to a given workload is blunted, i.e., the "lactate paradox" (33), although this concept has been challenged (22,32). So far, only one study is available describing in more detail whole body and muscle carbohydrate and lactate metabolism at rest and during exercise after a relatively short period of 3 wk of acclimatization to altitude (3,4,5). In addition, a careful evaluation of the data reveals that the field study suffered from variability in the measurements, which potentially might explain the "disbalance" in glucose and lactate whole body vs. leg glucose and lactate utilization/production (3).…”