Emily B. Kloss

Background/Objectives: United States Marine Corps (USMC) recruit training (“boot camp”) is a physically demanding environment in which recruits often experience significant changes in body composition. Energy balance (EB) plays a critical role in shaping these outcomes, yet how EB is associated with longitudinal fat and muscle changes remains unclear. This study evaluated the relationship between periodic EB and changes in body composition in male and female recruits. Methods: One hundred eighty-two USMC recruits (122 females, 60 males) volunteered; 158 recruits (106 females, 52 males) were included in the analysis. Energy intake was assessed on 10 distinct sampling days, energy expenditure was tracked via wearable devices, and body composition was obtained using bioelectrical impedance analysis. Statistical analyses included t-tests to compare sex differences in EB and general linear models to assess associations between EB, sex, and changes in body composition. Results: Across seven sampled days, females experienced periods of greater (more negative) daily energy deficit (−649 ± 775 kcal/day, 17%) compared to males (−388 ± 673 kcal/day, 8%, bootstrap p = 0.035). More negative estimated EB was associated with greater fat mass loss, with no evidence that this association differed by sex. In the 7-day model, the association between estimated EB and skeletal muscle mass differed by sex (sex × EB interaction, bootstrap p = 0.015); this interaction was not significant in the 10-day model that included the Crucible (bootstrap p = 0.792). Conclusions: USMC recruits experienced periods of low-moderate energy deficits during boot camp. Estimated EB during sampled training days was associated with fat mass change, whereas the association with skeletal muscle mass change differed by sex only in the 7-day model.

Military personnel routinely complete stressful training exercises in harsh environmental conditions to prepare for intense operational demands. Purpose: This study determined the effect of environmental conditions on salivary hormone profiles in Marines during a mountain warfare training exercise (MTX). Methods: Two cohorts of Marines (age 22 ± 4, height 174 ± 7 cm, body mass 79.2 ± 11.5 kg) completed an MTX (elevation 2100 to 3500 m) in the Fall (n = 63, temperature 11 ± 2 °C) and Winter (n = 64, temperature −5 ± 4 °C). Saliva samples were provided before (PRE), during (MID), and after (POST) the MTX, and were assayed for α-amylase, cortisol, DHEA, testosterone, and osteocalcin. Results: Linear mixed models were used to determine significant interactions (time × season) and found differences in DHEA, testosterone, and osteocalcin. Testosterone and DHEA were lower at MID compared to PRE and POST during the Fall MTX. Testosterone was higher at MID compared to PRE and POST during the Winter MTX, while DHEA remained stable. Osteocalcin was higher in Fall participants compared to Winter but demonstrated a similar trend to increase at MID and decrease at POST in both groups. Cortisol was higher during the Winter MTX compared to the Fall. Conclusions: These findings highlight the differential physiological stress responses in varying seasonal conditions, suggesting the need for tailored training strategies to enhance military readiness and prevent hormonal dysregulation. Further research is needed to elucidate the mechanisms underlying these seasonal effects.

Wearables are lightweight, portable technology devices that are traditionally used to monitor physical activity and workload as well as basic physiological parameters such as heart rate. However recent advances in monitors have enabled better algorithms for estimation of caloric expenditure from heart rate for use in weight loss as well as sport performance. can be used for estimating energy expenditure and nutritional demand. Recently, the military has adopted the use of personal wearables for utilization in field studies for ecological validity of training. With popularity of use, the need for validation of these devices for caloric estimates is needed to assist in work-rest cycles. Thus the purpose of this effort was to evaluate the Polar Grit X for energy expenditure (EE) for use in military training exercises. Polar Grit X Pro watches were worn by active-duty elite male operators (N = 16; age: 31.7 ± 5.0 years, height: 180.1 ± 6.2 cm, weight: 91.7 ± 9.4 kg). Metrics were measured against indirect calorimetry of a metabolic cart and heart rate via a Polar heart rate monitor chest strap while exercising on a treadmill. Participants each performed five 10-minute bouts of running at a self-selected speed and incline to maintain a heart rate within one of five heart rate zones, as ordered and defined by Polar. Polar Grit X Pro watch had a good to excellent interrater reliability to indirect calorimetry at estimating energy expenditure (ICC = 0.8, 95% CI = 0.61-0.89, F (74,17.3) = 11.76, p < 0.0001) and a fair to good interrater reliability in estimating macronutrient partitioning (ICC = 0.49, 95% CI = 0.3-0.65, F (74,74.54) = 2.98, p < 0.0001). There is a strong relationship between energy expenditure as estimated from the Polar Grit X Pro and measured through indirect calorimetry. The Polar Grit X Pro watch is a suitable tool for estimating energy expenditure in free-living participants in a field setting and at a range of exercise intensities.