Energy expenditure in underweight free-living adults: impact of energy supplementation as determined by doubly labeled water and indirect calorimetry.

The effect of energy supplementation was evaluated in six underweight adults under free-living conditions. Customary energy intake (EI) over 4 wk and estimated body composition were defined. Then for 12 wk subjects were fed their customary EI under controlled conditions to assure stable energy reserve. Finally, intake was increased by a mean of 720 kcal/d for 8 wk adjusted to increase body fat from 9 to 18%. Body weight (means +/- SD) increased from 55.2 +/- 3.4 to 57.0 +/- 4.2 kg. Body fat gain was highly variable ranging from 0.6 to 3.8 kg. Energy expenditure (EE) by the factorial and doubly labeled water methods did not change. Fat storage accounted for 66% of the supplemental energy; 237 kcal/d remained unaccounted for. EE by the factorial method, which uses indirect calorimetry or isotopic measurements, were highly correlated (r = 0.8; p less than 0.01). Under the conditions of this study EI does not affect EE. The labeled water method permits the evaluation of energy expenditure in free-living conditions.