Menstrual Cycle Modulates the Contribution of Dry Heat Loss to Total Heat Loss During Exercise in Warm‐Dry Conditions in Young, Recreationally Active Females: Preliminary Findings

The mid‐luteal phase of the menstrual cycle is characterized by an upward shift in basal body core temperature secondary to elevated circulating estradiol and progesterone levels as compared to the early‐follicular phase. This elevation in body core temperature, perhaps together with increased estradiol‐mediated cutaneous vasodilation, may increase convective and radiative (dry) heat loss by improving core‐to‐skin heat transfer. This increase in dry heat loss may, in turn, reduce the need for sweat secretion (evaporative heat loss) without necessarily altering the relative contribution of these avenues to total heat exchange. Thus, the purpose of this study was to evaluate the hypothesis that females in the mid‐luteal phase would exhibit a greater contribution of dry heat loss to total heat loss as compared to the early‐follicular phase during exercise in warm‐dry conditions. Seven young, recreationally active, ovulating females (3 using hormonal intrauterine devices, mean (SD), 24 (3) years, V̇O 2peak 40.5 (3.4) mL kg ‐1 min ‐1 ) completed two 45‐min bouts of semi‐recumbent cycling at a low (175 W m ‐2 ; ~40% V̇O 2peak ) and high (275 W m ‐2 ; ~65% V̇O 2peak ) rate of metabolic heat production, interspersed by 15‐min rest, in warm‐dry conditions (30.0 (0.2) °C, (25 (9) % relative humidity) in the early‐follicular (cycle days 2‐6) and mid‐luteal phase (days 19‐23). Metabolic heat production and dry and evaporative heat loss were measured via indirect and direct calorimetry, respectively. The contribution of dry heat loss was expressed as a percentage of total heat loss. Body core temperature (esophageal, n =6) and mean skin temperature (8 sites) were measured continuously. Averages of the final 5‐min of exercise in each heat load were compared between phases using dependent, two‐tailed t‐tests. As anticipated, resting body core temperature in the mid‐luteal phase was 0.2 °C [95% CI: 0.1, 0.3] higher than in the early‐follicular phase ( p <0.001). Core temperature remained 0.2 °C [0.1, 0.5] higher at the low heat load (37.8 (0.2) vs. 37.6 (0.3) °C, p <0.01), but initial differences in core temperature were not observed in the high heat load (38.4 (0.3) vs. 38.3 (0.4) °C, p =0.83). Skin temperature and core‐to‐skin gradient were similar between phases throughout (all p ≥0.21). Total heat loss was not different between phases at the low (162 (8) vs. 162 (13) W m ‐2 , p =0.93) or high (255 (10) vs. 252 (7) W m ‐2 , p =0.39) heat loads. However, the contribution of dry heat loss to total heat loss was 3% [1, 4] greater in the mid‐luteal phase at the low heat load (20 (8) vs. 17 (9) %, p <0.01). This difference was not observed at the high heat load (12 (8) vs. 10 (9) %, p =0.16). In this preliminary analysis, females in the mid‐luteal phase exhibited a higher body core temperature and greater contribution of dry heat loss to total heat loss at low exercise‐induced heat loads in warm‐dry conditions. However, these differences were not observed at higher heat loads. These data provide novel mechanistic insight into the shifts in the contribution of the main avenues of heat loss in ovulating females across the menstrual cycle.

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