Temperature-related overestimation of energy expenditure, based on heart-rate monitoring in obese boys

An equation has been developed to reduce the error in predicting energy expenditure (EE) from heart rate (HR) monitoring, by correcting for climate-related HR increases. To evaluate the effects of such a correction in a "real life" situation, a group of 9- to 14-year-old obese boys [mean (SD) 36.6 (3.3)% body fat; n=14] was monitored for 6 days during the summertime. An activity interview was taken daily. During times of outdoor activities, measurements of HR were corrected for the influence of ambient temperature (T) using the aforementioned equation. Time spent outdoors was 196.7 (21.1) min/day at a T of 17–34°C. Temperature correction reduced the time spent at HR>140 beats/min and >160 beats/min by 20–25% during outdoor activity. Estimated outdoor EE corrected for the influence of temperature was 2.46 (2.37) MJ; the uncorrected value was 2.68 (2.45) MJ. The respective values for the waking hours and over 24 h were 7.34 (2.77) MJ versus 7.55 (2.81) MJ and 10.83 (2.72) MJ versus 11.05 (2.77) MJ, respectively. The excessive HR measured in a warm summer climate could explain 8.8 (3.5)% of EE during outdoor activities, which resulted in a 2.9 (2.7)% overestimate of EE during daily waking hours and a 1.9 (1.8)% overestimate of the total 24-h EE. Thus, the influence of temperature on HR may, at least in part, explain the previously reported overestimate of EE calculated using the HR/oxygen consumption relationship versus that calculated using the doubly labelled water method. We recommend correcting HR for the influence of climate in studies using HR profiles to determine outdoor physical activity levels and/or EE during summertime. However, when total daily EE or waking hours EE is assessed, even in subjects who are engaging in long intervals of outdoor play, the temperature correction required is minimal.