The effect of environmental temperature and humidity on 24 h energy expenditure in men.
The effects of environmental temperature and humidity and their interaction on 24 h energy expenditure were measured using whole-body indirect calorimetry in eight normal-weight young men who wore standardized light clothing and followed a controlled activity regimen. A randomized-block experimental design was used, with temperature effects assessed by measurements at 20, 23, 26 and 30 degrees, while humidity was altered from ambient (50-65% relative humidity) to high (80-93% relative humidity) at 20 and 30 degrees only. There was no significant effect of humidity on 24 h energy expenditure at the two extreme temperatures in this range, though when periods of sleep and exercise were excluded the energy expenditure at high humidity was significantly higher than that ambient humidity (P < 0.02). The effect of temperature at ambient humidity levels showed lower values at 23 and 26 degrees than at 20 and 30 degrees (P < 0.02). The effect of temperature was not equally apparent in all components of the 24 h energy expenditure, as sleeping metabolic rate and the energy cost of walking and cycling showed no significant effect of temperature over this range. This raises the possibility that the effects of temperature are attributable to behavioural changes during the waking portion of the day rather than any non-shivering thermogenic mechanisms at tissue level.