Determinants of the energy costs of light activities: inferences for interpreting doubly labeled water data

OBJECTIVE: To investigate the controversy regarding the means of adjusting the energy cost of physical activity measured by doubly labeled water for differences in body size. DESIGN: We performed a cross-sectional study of the energy costs of carefully reproduced light activities with careful control for fidgeting and other unnecessary movement. SUBJECTS: The study was performed in 23 healthy, young to middle aged adults with body masses between 39 and 118 kg. MEASUREMENTS: Energy expenditure was measured by respiratory gas exchange while subjects performed controlled light activities representative of activities of daily life. Body composition was measured by bioelectrical impedance analysis. RESULTS: The energy costs of individual controlled light activities were proportional to body weight except for mock vacuum cleaning. When the energy costs of all five activities were taken together, allometric regression analysis indicated that the energy cost of these light activities was proportional to body weight (slope = 0.88 +/- 0.07), but not to fat-free mass (1.24 +/- 0.10), fat mass (0.27 +/- 0.03) or resting metabolic rate (1.43 +/- 0.12). CONCLUSION: Normalization of energy expenditure of physical activity by division by body weight is an appropriate means for comparing the volume (intensity x time) of physical activity between individuals of different body size.