Estimating energy expenditure using heat flux measured at a single body site.

INTRODUCTION: The Personal Calorie Monitor (PCM) is a portable direct calorimeter that estimates energy expenditure (EE) from measured heat flux (i.e., the sum of conductive, convective, radiative, and evaporative heat). PURPOSE: The primary aim of this study was to compare EE estimated from measures of heat flux with those measured using indirect calorimetry in a thermoneutral environment (26°C). A secondary aim was to determine whether exposure to ambient temperature below thermoneutral condition (19°C) influences the accuracy of the PCM. METHODS: Thirty-four adults (mean ± SD: age, 28 ± 5 yr; body mass index, 22.9 ± 2.6 kg · m(-2)) were studied for 5 h in a whole-room indirect calorimeter (IC) in thermoneutral and cool conditions. Participants wore the PCM on their upper arm and completed two 20-min treadmill walking bouts (0% grade, 3 mph). The remaining time was spent sedentary (e.g., watching television, using a computer). RESULTS: In thermoneutral conditions, EE values (mean (95% confidence interval)) measured by IC and PCM were 560.0 (526.5-593.5) and 623.3 (535.5-711.1) kcal, respectively. In cool conditions, EE values measured by IC and PCM were 572.5 (540.9-604.0) and 745.5 (668.1-822.8) kcal, respectively. Under thermoneutral conditions, mean PCM minute-by-minute EE tracked closely with IC, resulting in a small nonsignificant bias (63 kcal (-5.8 to 132.4)). During cool conditions, mean PCM minute-by-minute EE did not track IC, resulting in a large bias (173.0 kcal (93.9-252.1)) (P <; 0.001). CONCLUSIONS: This study demonstrated the validity of using measured heat flux to estimate EE. However, accuracy may be impaired in cool conditions possibly because of excess heat loss from the exposed limbs.