Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada.
Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, Ontario, Canada.
J Physiol. 2024 Mar;602(5):875-890. doi: 10.1113/JP285977. Epub 2024 Feb 17.
Synthetic progestins in oral contraceptives are thought to blunt heat dissipation by reducing skin blood flow and sweating. However, whether progestin-releasing intrauterine devices (IUDs) modulate heat loss during exercise-heat stress is unknown. We used direct calorimetry to measure whole-body total (dry + evaporative) heat loss in young, physically active women (mean (SD); aged 24 (4) years, 39.3 (5.3) ml/kg/min) with (IUD; n = 19) and without (Control; n = 17) IUDs in the follicular and luteal phases of the menstrual cycle during light- and moderate-intensity exercise at fixed rates of heat production (∼175 and ∼275 W/m ) in 30°C, ∼21% relative humidity. Between-group and -phase differences were evaluated using traditional hypothesis testing and statistical equivalence testing within pre-determined bounds (±11 W/m ; difference required to elicit a ±0.3°C difference in core temperature over 1 h) in each exercise bout. Whole-body total heat loss was statistically equivalent between groups within ±11 W m (IUD-Control [90% CIs]; Light: -2 [-8, 5] W/m , P = 0.007; Moderate: 0 [-6, 6] W/m , P = 0.002), as were dry and evaporative heat loss (P ≤ 0.023), except for evaporative heat loss during moderate-intensity exercise (equivalence: P = 0.063, difference: P = 0.647). Whole-body total and evaporative heat loss were not different between phases (P ≥ 0.267), but dry heat loss was 3 [95% CIs: 1, 5] W/m greater in the luteal phase (P ≤ 0.022). Despite this, all whole-body heat loss outcomes were equivalent between phases (P ≤ 0.003). These findings expand our understanding of the factors that modulate heat exchange in women and provide valuable mechanistic insight of the role of endogenous and exogenous female sex hormones in thermoregulation. KEY POINTS: Progestin released by hormonal intrauterine devices (IUDs) may negatively impact heat dissipation during exercise by blunting skin blood flow and sweating. However, the influence of IUDs on thermoregulation has not previously been assessed. We used direct calorimetry to show that IUD users and non-users display statistically equivalent whole-body dry and evaporative heat loss, body heat storage and oesophageal temperature during moderate- and high-intensity exercise in a warm, dry environment, indicating that IUDs do not appear to compromise exercise thermoregulation. However, within IUD users and non-users, dry heat loss was increased and body heat storage and oesophageal temperature were reduced in the luteal compared to the follicular phase of the menstrual cycle, though these effects were small and unlikely to be practically meaningful. Together, these findings expand our understanding of the factors that modulate heat exchange in women and have important practical implications for the design of future studies of exercise thermoregulation.
口服避孕药中的合成孕激素被认为通过减少皮肤血流量和出汗来减弱散热。然而，孕激素释放宫内节育器（IUD）是否在运动-热应激期间调节热量损失尚不清楚。我们使用直接量热法测量了年轻、活跃的女性（平均（SD）；年龄 24（4）岁， 39.3（5.3）ml/kg/min）在卵泡期和黄体期的全身总（干+蒸发）热量损失，在 30°C、相对湿度约为 21%的环境下，以固定的产热率（约 175 和约 275 W/m ）进行轻和中度强度的运动，IUD（n=19）和没有 IUD（对照组；n=17）。使用传统的假设检验和在每个运动回合内预先确定的界限内的统计学等效性检验（±11 W/m ；需要引起核心温度在 1 小时内相差 0.3°C 的差异）评估组间和组内差异。全身总热量损失在±11 W/m 内统计学等效于组间（IUD-对照组[90% CIs]；轻度：-2[-8, 5]W/m ， P=0.007；中度：0[-6, 6]W/m ， P=0.002），干热和蒸发散热也是如此（P≤0.023），除了中度运动时的蒸发散热（等效性：P=0.063，差异：P=0.647）。全身总热量和蒸发散热在不同阶段没有差异（P≥0.267），但黄体期干热损失高 3[95% CIs：1, 5]W/m （P≤0.022）。尽管如此，所有的全身热量损失结果在不同阶段都是等效的（P≤0.003）。这些发现扩展了我们对调节女性热量交换的因素的理解，并为内源性和外源性女性性激素在体温调节中的作用提供了有价值的机制见解。 关键点： 由激素宫内节育器（IUD）释放的孕激素可能通过减弱皮肤血流和出汗来负面影响运动时的散热。然而，IUD 对体温调节的影响以前尚未得到评估。我们使用直接量热法表明，在温暖、干燥的环境中，IUD 用户和非用户在中高强度运动期间显示出统计学上等效的全身干热和蒸发散热、体热储存和食管温度，表明 IUD 似乎不会损害运动时的体温调节。然而，在 IUD 用户和非用户中，与卵泡期相比，黄体期的干热损失增加，体热储存和食管温度降低，尽管这些影响很小，不太可能具有实际意义。总的来说，这些发现扩展了我们对调节女性热量交换的因素的理解，对未来运动体温调节研究的设计具有重要的实际意义。
