Acclimatization to 4,300-m altitude decreases reliance on fat as a substrate

Roberts, A. C., G. E. Butterfield, A. Cymerman, J. T. Reeves, E. E. Wolfel, and G. A. Brooks. Acclimatization to 4,300-m altitude decreases reliance on fat as a substrate. J. Appl. Physiol. 81(4): 1762-1771, 1996.We testedthe hypothesis that exposure to altitude decreases reliance on freefatty acids (FFA) as substrates and increases dependency on bloodglucose. Therefore, the effects of exercise, hypobaric hypoxia, andaltitude acclimatization on FFA, glycerol and net glucose uptake andrelease [ = 2(leg blood flow)(arteriovenous concentration)]and on fatty acid (FA) consumption by the legs (= 3 × glycerolrelease + FFA uptake) were measured. Because sympathetic responses havebeen implicated, we utilized nonspecific -blockade and observedresponses to exercise, altitude, and altitude acclimatization. Westudied six healthy -blocked men () and five matched controls (C)during rest and cycle ergometry exercise (88 W) at 49% of sea-level(SL) peak O₂ uptake at the sameabsolute power output on acute altitude exposure (A1; barometric pressure = 430 Torr) and after 3 wk of chronic altitude exposure to4,300 m (A2). During exercise at SL, FA consumption rates increased (P < 0.05). On arrival at 4,300 m,resting leg FFA uptake and FA consumption rates were not significantlydifferent from those at SL. However, after acclimatization to altitude,at rest leg FA consumption decreased to essentially zero in both C and groups. During exercise at altitude after acclimatization, leg FAconsumption increased significantly, but values were less than at SL orA1 (P < 0.05), whereas glucoseuptake increased relative to SL values. Furthermore, -blockadesignificantly increased glucose uptake relative to control. We concludethat 1) chronic altitude exposure decreases leg FA consumption during rest and exercise;2) relative to SL, FFA uptakedecreases while glucose uptake increases during exercise at altitude;and 3) -blockade potentiatesthese effects.