ENERGY-EXPENDITURE USING ISOTOPE-LABELED WATER (H-2(2)-O-18), EXERCISE PERFORMANCE, SKELETAL-MUSCLE ENZYME-ACTIVITIES AND PLASMA BIOCHEMICAL PARAMETERS IN HUMANS DURING 95 DAYS OF ENDURANCE EXERCISE WITH INADEQUATE ENERGY-INTAKE

Two men, R.F. and M.S., pulled sledges each with starting masses of 22 2 kg, 2300 km across Antarctica. Exercise was performed for approximat ely 10 h each day for 95 days. Despite an average energy intake of 21. 3 MJ . day(-1) both subjects lost more than 25% of body weight. Energy expenditure was measured using energy balance data (EB) and isotape-l abelled water (H-2(2) O-18). Isotope doses were taken on day 0 and day 50 of the expedition. During the first 50 days both methods gave reas onable agreement, giving energy expenditures of 38.3 (EB) and 35.5J2 ( (H2O)-H-2-O-18) MJ . day(-1) in R.F. and 28.6 (EB) and 29.1 ((H2O)-H-2 -O-18) MJ . day(-1) in M.S. The isotope data for days 20-30 yielded ex ceptional values of 44.6 MJ . day(-1) in R.F. and 48.7 MJ . day(-1) in M.S. Estimates of energy expenditure between day 51 and day 96 were m uch lower and although the methods were in agreement for R.F. -24.1 (E B) and 23.1 ((H2O)-H-2-O-18) MJ. . day(-1), there was poor agreement f or MS -26.8 (EB) and 18.8 ((H2O)-H-2-O-18) MJ . day(-1). However, some practical difficulties occurred during this second period and there w ere also problems arising from marked increases in body water that mad e estimates of body mass and composition change difficult to interpret . The latter problems were probably due to malnutrition, which may hav e also been responsible for surprising increases in urinary excretion of H-2 and O-18 observed in both men at around day 81. These increases may reflect the release of label incorporated into molecules other th an water which do not normally freely exchange with the body water poo l under the circumstances of marked malnourishment. Following the expe dition, both men showed declines in maximal O-2 consumption ((V)O-2max , 53.6 to 41.2 mi O-2 kg(-1) . min(-1) in R.F., 58.1-46.0 ml O-2 kg(-1 ) . min(-1) in M.S.); maximal voluntary isometric force production in different muscle groups (up to 19.9% in R.F. and 55.8% in M.S.) and bo th cytoplasmic and mitochondrial skeletal muscle enzyme activities (up to 56% in R.F. and 63% in M.S.). Plasma samples taken during the expe dition showed low glucose levels, inappropriately high insulin levels, and declines in testosterone and luteinizing hormone. Thyroxine, chol esterol, albumin and triglyceride levels remained normal.