Centre for Research in Ecology, Evolution and Behaviour, Life Sciences Department, Whitelands College, University of Roehampton, London, United Kingdom.
Dubowitz Neuromuscular Centre, UCL, Great Ormond Street Institute of Child Health, Holborn, London, United Kingdom.
Physiol Rep. 2021 Aug;9(16):e14973. doi: 10.14814/phy2.14973.
The thermoneutral zone (TNZ) defines the range of ambient temperatures at which resting metabolic rate (MR) is at a minimum. While the TNZ lower limit has been characterized, it is still unclear whether there is an upper limit, that is, beyond which MR during rest increases, and if so, what physiological upregulations explain this. We take the first step to fill this knowledge gap by measuring MR and multiple physiological variables in participants exposed to ambient heat stress while resting. Thirteen participants were exposed for an hour to 28℃-50% relative humidity (RH) air, and both 40 and 50℃ each in 25% RH and humid (50% RH) conditions. Core and skin temperatures, blood pressure, sweat-, heart-, and breathing-rate, minute ventilation, and movement levels were recorded throughout each condition. MR increased 35% (p = .015) during exposure to 40℃-25% RH compared to baseline and a further 13% (p = .000) at in 50℃-50%RH. This was not explained by increased fidgeting (p = .26), suggesting physiological upregulation. However, while greater heat stress invoked increases in heart rate (64%, p = .000), minute ventilation (78%, p = .000), and sweat rate (74%. p = .000) when comparing 50℃-50% RH with baseline, the exact size of their relative energy cost is unclear and, therefore, so is their contribution to this increase in MR. Our study shows clear evidence that resting MR increases in humans at high temperature-there is a metabolic upper critical temperature, at least as low as 40℃. Further studies should pinpoint this value and fully explain this increased MR.
热中性区（TNZ）定义了休息代谢率（MR）最低时的环境温度范围。虽然已经确定了 TNZ 的下限，但尚不清楚是否存在上限，也就是说，超过这个温度，休息时的 MR 是否会增加，如果是，那么是什么生理调节解释了这种情况。我们通过测量暴露于环境热应激下休息的参与者的 MR 和多个生理变量，迈出了填补这一知识空白的第一步。13 名参与者在 28℃-50%相对湿度（RH）的空气中暴露了一个小时，然后分别在 25% RH 和湿热（50% RH）条件下暴露在 40℃和 50℃的环境中。在每个环境中，都记录了核心和皮肤温度、血压、汗水、心率和呼吸率、分钟通气量、运动水平。与基线相比，暴露在 40℃-25% RH 时，MR 增加了 35%（p=0.015），而在 50℃-50% RH 时则增加了 13%（p=0.000）。这不是由于增加的烦躁不安（p=0.26）解释的，这表明存在生理调节。然而，虽然当比较 50℃-50% RH 与基线时，更大的热应激引起了心率（64%，p=0.000）、分钟通气量（78%，p=0.000）和汗水率（74%，p=0.000）的增加，但它们的相对能量消耗的确切大小尚不清楚，因此，它们对 MR 增加的贡献也不清楚。我们的研究清楚地表明，在高温下，人类的休息代谢率确实会增加——至少有一个代谢的上限临界温度，低至 40℃。进一步的研究应该确定这个值，并充分解释这种增加的 MR。
