Fluid Balance is Unlikely Modified by the Menstrual Cycle when Ad Libitum Drinking is Permitted During Physical Work in the Heat

Abstract

PurposeFederal and local entities provide recommendations aiming to limit dehydration during occupational heat stress. These recommendations are largely founded on the premise that during physical work in the heat humans under drink, which increases the likelihood of dehydration. Nearly all these studies have been conducted in men. The female menstrual cycle is characterized by fluctuations in estrogen and progesterone, such that elevations in these ovarian sex hormones may modify fluid regulation by both reducing urine output and promoting fluid intake. However, it is unknown if body fluid balance (i.e., both fluid intake and output) during occupationally relevant heat stress scenarios is altered across the menstrual cycle. Therefore, we tested the hypothesis that during physical work in the heat women will be more dehydrated during the early follicular (EF) phase (when ovarian sex hormones are low), compared to the late follicular (LF) and mid‐luteal (ML) phases of the menstrual cycle (when ovarian sex hormones are elevated).MethodsEight healthy, eumenorrheic, unacclimated women (26 ± 5 y) completed three trials consisting of 4 h (half a workday) exposure to 33.8 ± 0.8°C, 54 ± 1% relative humidity (WBGT = 28.3 ± 0.2°C, the average heat stress encountered in outdoor occupations) during the EF, LF, ML phases of their menstrual cycle. All visits were completed using a block‐randomized, crossover design. Each hour participants walked on a treadmill for 30 min at an intensity evoking a rate of metabolic heat production (Hprod) of ~430 W (the average Hprod for outdoor workers in the U.S.), while the work‐rest ratio is consistent with the NIOSH heat stress recommendations. Participants drank a cool (12 ± 1°C) flavor preferred non‐caloric sport drink (Gatorade Zero) ad libitum. Nude body weight was measured pre‐ and post‐ exposure, and percent changes in body weight loss (%BWL) were interpreted as an index of changes in total body water. Total fluid intake and urine output were also measured and sweat loss was estimated from changes in body weight corrected for fluid intake and urine loss. Rectal temperature is reported as a change from pre‐ to post‐ exposure. Data are presented as mean ± SD.ResultsChanges in rectal temperature did not differ between menstrual cycle phases (EF: 0.3 ± 0.2°C, LF: 0.5 ± 0.5°C, ML: 0.3 ± 0.3°C, p = 0.544). Sweat loss (EF: 2628 ± 1504 mL, LF: 2804 ± 1204 mL, ML: 2817 ± 1267 mL, p = 0.762) and urine output (EF: 864 ± 494 mL, LF: 955 ± 540 mL, ML: 925 ± 480 mL, p = 0.725) also did not differ between phases. Likewise, there were no differences in fluid intake between phases (EF: 1764 ± 1039 mL, LF: 1849 ± 694 mL, ML: 1892 ± 883 mL, p = 0.785). Thus, %BWL did not differ between phases (EF: ‐0.3 ± 0.7%, LF: ‐0.1 ± 0.7%, ML: ‐0.3 ± 0.8, p = 0.530) and %BWL did not statistically differ from zero (p ≥ 0.247).ConclusionWhen cool, flavor preferred fluids are freely accessible, fluid balance in women likely does not differ across three distinct phases of the menstrual cycle and thus, is unlikely to result in a net loss of body water during occupationally relevant heat stress scenarios.