United States Army Research Institute of Environmental Medicine

Exertional heat stroke (EHS) affects
500 Military Personnel and over 100 Soldiers every year, and it is believed that these numbers are underreported. EHS is the most severe form of exertional heat illness (EHI) and can result in substantial, long-lasting organ damage, and even death in severe cases. Based on current knowledge and training needs, it is impossible to prevent every EHS - which shifts the focus from exclusively prevention to a combination of prevention, treatment, and enhancing recovery as much as possible. While many risk factors have been identified and there are adequate treatments available, biomarkers associated with heat stroke risk, recovery, and return-to-duty (RTD) remain largely unclear. The purpose of the proposed study is to enhance knowledge surrounding biomarkers of EHS and long-term health consequences that result from EHS. The investigators will recruit research volunteers for a field study in order to collect pre-, post-, and follow-up measures from a high-risk EHS event (i.e. ruck marches, timed runs) this will allow us to have a basis for comparison between Soldiers who collapse with EHS (from previously collected data) and those that complete high-risk events, but do not collapse. This will allow for comparison between the groups to identify EHS-specific biomarkers that could aid in recovery and RTD decisions for Soldiers.

This longitudinal study will examine the effects of repeated bouts of operational stress and limited recovery on integrated MPS, whole-body protein balance, iron absorption, and aerobic performance. Following baseline characterization measures, active adults (n=24) representative of normal weight phenotype (NW; n=12) and overweight phenotype (OW; n=12) will complete a 48h balance phase preceding two rounds of repeated 72h energy deficit exposure each immediately followed by a 48h recovery phase. NW cutoff will be defined ≤ 22% body fat for males and ≤ 32% body fat for females. OW cutoff will be defined as >22% body fat for males and >32% body fat for females. These body composition cutoffs are informed by the maximum allowable percent body fat standards outlined in current Army Regulation 600-9. Additional details for determining % body fat are outlined in the experimental procedures section of the protocol.

Over recent years, military service women have shown increasing interest in utilizing long-acting reversible contraceptives (LARCs; i.e., implants or intrauterine devices, IUDs). While clinically, LARC have been proven safe and effective at preventing pregnancy & decreasing menstrual symptoms, it is unclear what impact this type of contraceptive may have on physiological responses to extreme environments (heat, cold and high altitude). Additionally, iron deficiency and iron deficiency anemia are prevalent in U.S. military women which likely affect health and performance. There is evidence to suggest that the prevalence of iron deficiency and iron deficiency anemia is lower in women utilizing LARCs, which preserve iron stores, likely due to reduced menstrual blood loss that compounds with each cycle over the course of months. Due to the increasing interest in the utilization of LARCs it is important to understand what, if any, impact these methods of contraception may have on a female Soldier's physiological responses.
The overall goal of the present study is to characterize iron status and physiological responses (including but not limited to sweating, skin blood flow, ventilation, and heart rate) to extreme environmental stressors of heat, cold and high altitude (i.e. hypobaric hypoxia) in women utilizing LARCs. Up to thirty-three individuals (n=18 LARCs, n=15 monophasic oral contraceptives) will complete testing visits separated by at least 48 hours for environmental testing visits. During heat stress testing, measures of core body temperature, skin temperature, and sweat rate will be measured during a standardized protocol in hot, humid conditions. During cold stress, measures of body core temperature, metabolic heat production, skin blood flow and temperature (at both distal extremity and proximal body sites) will be assessed in order to better quantify the impact of exogenous sex hormones via LARCs and OCs in volunteers exposed to the cold. Measurements will be obtained during a standardized protocol in cold air (
90 min; 10°C). For altitude, cardiopulmonary responses will be assessed during rest and exercise during an acute altitude exposure (4300m). Blood samples will be collected throughout the investigation in order to quantify sex hormone concentrations, iron status, and inflammatory biomarkers in response to environmental stressors. The investigators will also utilize laboratory tests (i.e., carbon monoxide (CO) rebreathing technique) and collect blood samples to quantity iron stores in these groups of women. To characterize iron status, the Soldier Performance Health and Readiness (SPHERE) epidemiological data repository will be utilized to quantify the prevalence of iron deficiency and iron deficient anemia diagnoses between women using LARCs and women not utilizing LARCs.