1. It has been suggested previously that people in developed countries do not expose themselves to cold severe enough to induce a metabolic response. The energy expenditure, at both heat production and total heat loss, of nine women was therefore measured continuously while each lived for 30 h in a whole-body calorimeter on two occasions, one at 28 degrees and the other at 22 degrees. All subjects followed a predetermined pattern of activity and food intake. The environmental conditions were judged by the subjects to be within those encountered in everyday life. In the standard clothing worn, 28 degrees was considered to be comfortably warm but not too hot, while 22 degrees was judged to be cool but not too cold. 2. Heat production for 24 h was significantly greater at the lower temperature, by (mean +/- SE) 7 . 0 +/- 1 . 1%. The range was between 2 and 12%. Total heat loss was also significantly greater, by 6%, and there was a large change in the partition of heat loss. At the lower temperature sensible heat loss increased by 29% while evaporative heat loss decreased by 39%. 3. Resting metabolism measured in the morning 12--13 h after the last meal was significantly greater at 22 degrees than at 28 degrees, whereas there was no difference when the resting measurement was made for 2 . 5 h following a meal. 4.
In conclusion: (a) environmental temperature may play a more important role than was previously recognized in the energy balance of those living in this country, and (b) there is an indication of at least a partial replacement of cold-induced by diet-induced thermogenesis in man.