The metabolic upper critical temperature of the human thermoneutral zone

The thermoneutral zone (TNZ) defines the range of ambient temperatures at which resting metabolic rate is at a minimum without sensible dry heat loss; the body not needing to defend its core temperature. The TNZ has been defined in a number of species yet surprisingly, in humans only its lower limit has been well characterised; indeed, it is not yet clear if there is an upper limit at which metabolic rate increases.

To evaluate the evidence for a metabolic upper critical temperature to the thermoneutral zone in humans.

We synthesised current evidence about an upper limit to the human TNZ, highlighting the contradictions in the literature, and then discussed likely explanations for those contradictions.

The data from relevant studies differ in terms of whether they indicate that the TNZ has an upper limit, and this was related to the fundamental type of heat exposure protocol employed. Those studies showing evidence for an upper limit associated that limit with a wide range of temperatures.

We offer suggestions for future studies that should clarify the presence/absence of an upper limit to the TNZ and if present, where it lies.