Influences of Female Sex and Reproductive Hormones on Physiological Aspects of Heat Acclimation

Women are often understudied in thermal physiology research, leaving recommendations for Soldier safety and performance in hot conditions based largely on data collected in men. Female sex hormones estradiol and progesterone clearly have non-reproductive physiological effects, including influences on thermoregulatory and cardiovascular function. However, mechanisms of differing physiological adaptations to repeated heat exposure (i.e., heat acclimation) as a function of reproductive hormone status have yet to be investigated in a systematic way. Understanding possible sex differences in adaptation or mechanisms for adaptation during heat acclimation is important to ultimately optimize interventions to maximize soldier health and safety during training and deployment in the heat. Our goals in the present study are to evaluate physiological and biophysical responses to a standard heat acclimation protocol in a group of young, healthy men and women. Thirty individuals (n=10 males, n=10 women with a low hormonal status (i.e. early follicular phase), n=10 women with a high hormonal status (i.e. midluteal phase)) will complete 10 consecutive days of exercise (treadmill walking: 3.1 mph/2% grade) in the heat (40°C /40% relative humidity) up to 3hr per day. Changes in core temperature, heart rate, and sex hormones will be assessed to examine differences in thermoregulatory response to heat acclimation.

Study Overview

• Status: Completed
• Conditions: Heat Stress, Exertional
• Intervention / Treatment: Other: Heat acclimation

• Study Type: Interventional
• Enrollment (Actual): 27
• Phase: Not Applicable

Contacts and Locations

Study Locations

• United States
  • Massachusetts
    • Natick, Massachusetts, United States, 01760
      • U.S. Army Research Institute of Environmental Medicine

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: 18 years to 40 years (Adult)
• Accepts Healthy Volunteers: Yes

Description

Inclusion Criteria:

• • Males and females, age 18-40 y (17 y for active duty military)

  • Eumenorrheic females (menstrual cycle length between 24-35 days) or females taking oral contraceptives
  • In good health as determined by OMSO (Office of Medical Support and Oversight) General Medical Clearance and willing to report all medications and dietary supplements (i.e. cold medication, Tylenol. Aleve) to the PI and OMSO.
  • Passed his/her most recent Army Combat Fitness Test (ACFT) (military volunteers only) or exercise at least 2 times per week (civilian volunteers)
  • Willing to not exercise, or drink alcoholic beverages for 24 hours before each testing session
  • Willing to not consume caffeine 12 hours prior to any testing sessions
  • Willing to refrain from additional moderate to strenuous physical activity throughout testing period
  • Males and females with BMI <30

Exclusion Criteria:

• • Females who are pregnant or planning to become pregnant during the study

  • Females utilizing implantable contraception (intrauterine device, implantable bar)
  • Tested positive for COVID-19 within the past 30 days.
  • Taking prescription or over the counter medication, other than a contraceptive (unless approved by OMSO and Principal Investigator (PI))
  • Tobacco users, or anyone who has used tobacco within the last four months
  • Physical problems/injuries that would affect walking or running on a treadmill
  • Allergy to skin adhesive
  • Heart, lung, kidney, muscle, endocrine, or nerve disorder(s)
  • History of heat intolerance or orthostatic intolerance
  • Diagnosed and/or treated for fluid/electrolyte imbalance within the last 30 days
  • History of obstructive disease of the gastrointestinal tract including (but not limited to) diverticulosis, diverticulitis and inflammatory bowel disease, peptic ulcer disease, Crohn's disease, ulcerative colitis.
  • Scheduled MRI during testing

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Prevention
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: None (Open Label)

Number of Arms

3

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Males; 10 consecutive day heat acclimation in males	Other: Heat acclimation; 10 consecutive days of heat acclimation
Experimental: Females + high hormones; 10 consecutive day heat acclimation in females with high hormonal dose	Other: Heat acclimation; 10 consecutive days of heat acclimation
Experimental: Females + low hormones; 10 consecutive day heat acclimation in females with low hormonal dose	Other: Heat acclimation; 10 consecutive days of heat acclimation

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Time Frame
Heart rate	10 days
Core temperature	10 days

Collaborators and Investigators

• Sponsor: United States Army Research Institute of Environmental Medicine

Study record dates

Study Major Dates

• Study Start (Actual): October 1, 2021
• Primary Completion (Actual): April 30, 2024
• Study Completion (Actual): April 30, 2024

Study Registration Dates

• First Submitted: March 14, 2022
• First Submitted That Met QC Criteria: March 14, 2022
• First Posted (Actual): March 23, 2022

Study Record Updates

• Last Update Posted (Actual): August 7, 2024
• Last Update Submitted That Met QC Criteria: August 5, 2024
• Last Verified: August 1, 2024

More Information

Terms related to this study

• Additional Relevant MeSH Terms: Wounds and Injuries; Heat Stress Disorders
• Other Study ID Numbers: 21-07Hc

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No