Effects on regional brain metabolism of high-altitude hypoxia: a study of six US marines

Previous studies of brain glucose metabolism in people indigenous to high-a ltitude environments uncovered two response patterns: Quechuas native to th e high Andes of South America sustained modest hypometabolism in most brain regions interrogated, whereas Sherpas, native to the Himalayas and conside red by many biologists to be most effectively high-altitude adapted of all humans, showed brain metabolic patterns similar to lowlanders, with no accl imation effects noted. In the present study, the database was expanded to i nclude hypoxia acclimation effects in lowlanders. Positron emission tomogra phy (PET) and [F-18]-2-deoxy-2-fluro-D-glucose (FDG) imaging techniques wer e used to assess regional cerebral glucose metabolic rates (rCMR(glc)) in s ix US marines (Caucasian lineage) before and after a 63-day training progra m for operations at high altitudes ranging from 10,500 to 20,320 ft. Signif icant changes in rCMR(glc) were found for 7 of 25 brain regions examined. S ignificant decreases in absolute cerebral glucose metabolism after high-alt itude exposure were found in five regions: three frontal, the left occipita l lobe, and the right thalamus. In contrast, for the right and left cerebel lum significant increases in metabolism were found. The magnitudes of these differences, in terms of absolute metabolism, were large, ranging from 10 to 18%. Although the results may not be solely the result of lower oxygen l evels at high altitude, these findings suggest that the brain of healthy hu man lowlanders responds to chronic hypoxia exposure with precise, region-sp ecific fine tuning of rCMR(glc). The observed short-term hypoxia acclimatio n responses in these lowlanders clearly differ from the long-term hypoxia a daptations found in brain metabolism of people indigenous to high-altitude environments.