Operation Everest III: energy and water balance

We hypothesized that hypoxia decreases energy intake and increases total en ergy requirement and, additionally, that decreased barometric pressure incr eases total water requirement. Energy and water balance was studied over 31 days in a hypobaric chamber at 452-253 Torr (corresponding to 4,500-8,848 m altitude), after 7 days acclimatization at 4,350 m. Subjects were eight m en, age 27+/-4 years (mean+/-SD), body mass index 22.9+/-1.5 kg/m(2). Food and water intake was measured with weighed dietary records, energy expendit ure and water loss with labelled water. Insensible water loss was calculate d as total water loss minus urinary and faecal water loss. Energy intake at normoxia was 13.6+/-1.8 MJ/d. Energy intake decreased from 10.4+/-2.1 to 8 .3+/-1.9 MJ/d (P<0.001) and energy expenditure from 13.3+/-1.6 to 12.1+/-1. 8 MJ/d (P<0.001) over the first and second 15-day intervals of progressive hypoxia, Absolute insensible water loss did not change (1.67+/-0.26 and 1.6 6+/-0.37 l/d), however, adjusted for energy expenditure it increased with a mbient pressure reduction (P<0.05). In conclusion, hypoxia induced a negati ve energy balance, mainly by a reduction of energy intake. Overall insensib le water loss was unchanged because the increase in respiratory evaporative water loss was counterbalanced by a decrease in metabolic rate that probab ly limited the hypoxia-induced increase in ventilation.