Impact of body composition on physical fitness components in the Mexican Navy: Is overweight an issue?

Abstract Overweight and inadequate physical fitness are critical variables that impact directly on the health, wellbeing of service members and military readiness, characteristics required to overcome dangerous situations during sea rescues and in the first line of combat in the Mexican Navy. Here we determined the impact of body composition (BC) on physical fitness components (PFC) [cardiorespiratory fitness (CRF) and muscle strength-resistance (MSR)] of 110 marines, rescue swimmers, and divers of the Mexican Navy classified as overweight. We found significant differences in medians of BC and PFC among groups (p < 0.001). The WHO and military BMI showed 59 (54%) and 16 (15%) individuals as overweight, respectively. All groups showed CRF adequate for their age, but only 26 (51%) and 14 (24%) classified as normal and overweight (WHO), respectively, had adequate CFR (METs ˃12) (p = 0.003) for physical demands of military tasks. BMI in overweight at expenses of high BFP, without cardiovascular risk represents an operational task advantage that will lead to reach the balance between physical fitness, health, and military readiness.

Our group is focused on studying the influence of exercise, diet, kynurenine metabolites and bacterial diversity of the intestinal microbiota on body composition, cardiorespiratory fitness and muscle strength of individuals subjected to highintensity exercise. The research work of this article determines for the first time the body composition and physical fitness components of naval personnel from three different disciplines, considering that evaluating changes in anthropometrics and body composition is very important because these changes could affect fitness, performance, safety, and the success of maritime rescue operations, diving and underwater works, and completion of infantry tasks.

PUBLIC INTEREST STATEMENT
Obesity's prevalence is an important concern in the Armed Forces worldwide. In the Mexican Navy, 23% of their personnel are currently classified with overweight and obesity. Our results reinforce the importance of analyzing body composition plus BMI in military personnel for a better overweight/obesity classification and to determine the best physical fitness components for each naval discipline. We concluded that BMI in overweight with adequate muscle mass distribution and high body fat without metabolic risk could be considered adequate for marines, rescue swimmers, and divers because do not significantly affect naval task performance. However, although, these conditions compromise cardiorespiratory fitness and muscle-resistance force at the beginning of their training courses, these conditions improved at the end of the training.

Introduction
Overweight, obesity, and inadequate physical fitness in active naval personnel are critical variables that impact directly on the health and wellbeing of service members, military readiness, and the costs associated with medical care and early attrition; being now the leading medical reason for failing to qualify for military service (Christeson et al., 2019). The objective of the admission process of civilians to naval discipline is to select recruits that can meet the body composition and physical fitness standards of the Mexican Navy; however, most of them are misclassified and not recruited due to their apparent overweight (Directiva 03, 2019).
The Mexican Marine Infantry (MMI) and the School of Search and Rescue and Diving (ESBUSREB, for its acronyms in Spanish) are two Mexican Navy operative units whose mission is to carry out special amphibious operations, by executing high-risk practices (military diving, parachuting, vertical descent, urban combat, sniping, and marine rescues) and physically demanding tasks, such as lifting, handling, and carrying additional loads (Knapik et al., 2004).
The high variability of external and internal factors that intervene in the actions of rescue in the marine environment or as the first line of combat requires professionals highly qualified physically, technically, and psychologically to reduce likelihood of injury and mortality and to successfully overcome a wide range of dangerous situations, since human life depends on them (Sargent et al., 2017). Therefore, it is necessary to carry out regular physical examinations and fitness tests to ensure that naval personnel are prepared for the physical and mental demands of military service (Afari et al., 2019).
The Annual Medical Exam of the Mexican Navy is an official evaluation of physical health that monitors naval personnel to determine their physical fitness and body composition (Directiva 03, 2019). The Annual Medical Exam 2019 reported 7320 (13%) cases as overweight and 5459 (10%) with obesity. This classification was made by calculating the BMI by bioelectrical impedance analysis measures only, taking as reference military BMI cutoff points, which establish that personnel with a BMI < 28 is to be classified as normal weight, BMI of 28 to 29.9 as overweight, and BMI ≥ 30 as obese (grade I-III) (Directiva 03, 2019). These cutoff points are broad if we compare them with those established by the WHO and underestimates cases of overweight with good body composition; therefore, although high BMI has been negatively associated with cardiorespiratory fitness (Grier et al., 2015;Nogueira et al., 2016) does not significantly affect the performance of military tasks, which do not justify the exclusion of physically capable individuals from military operational tasks (Pierce et al., 2017).
Considering that the strict military body composition standards are aimed to ensure a physically capable and readily deployable force in military personnel (Department of Defense Instruction 1308.1, 2004; Department of the Army, 2013; Directiva 03, 2019; Friedl, 2012) and that there are certain physical attributes that could represent an advantage in operative tasks, this study was aimed at determining the body composition of Mexican naval personnel classified as overweight, and to analyze its impact on physical fitness components such as cardiorespiratory fitness and muscle strength-resistance, characteristics that have not been determined in this population and that are important determinants of health and performance to successfully complete sea rescues and infantry operations, and appropriate military appearance. In this study, we recruited 110 men in which physical fitness criteria were determined, evaluating cardiovascular risk, cardiorespiratory fitness (VO 2 max/METs), and muscle strength-resistance in upper and central train; as well as body composition to determine body fat and muscle mass percentage by anthropometrical measurements.

Participants
This was an analytical and transversal study to determine the physical fitness components and body composition of marines, rescue swimmers, and divers of the Mexican Navy and to analyze the impact of overweight on cardiorespiratory fitness, muscle strength-resistance, and body composition. Individuals in this study were those who started the Training Course for Recruits of the Mexican Navy (CCRAM, for its acronym in Spanish) from the MMI, enrolled in the courses of surface rescue and helicopter rescue swimmers; as well as divers and underwater works from the ESBUSREB. We recruited 110 men, aged 18 to 49 years, distributed as follows: 50 participants from MMI [25 marine instructors (MI) and 25 marine recruits (MR)], and 60 from ESBUSREB [10 surface rescue swimmers (SRS), 7 surface rescue and helicopter swimmers (SRHS), 8 helicopter rescue swimmers (HRS), 10 advanced and 14 beginner divers/underwater works (ADUW and BDUW, respectively) and 11 swimming and diving instructors (SDI)].
Physical fitness criteria were determined prior to initial training, evaluating cardiovascular risk, cardiorespiratory fitness (CRF) (VO 2 max/METs), and muscle strength-resistance (MSR) in upper and central train, as well as body composition (BC) calculating body fat percentage (BFP) and muscle mass percentage (MMP) by anthropometrical measurements.
This study was approved by the Research and Ethics Committees of the Instituto Nacional de Rehabilitación "Luis Guillermo Ibarra Ibarra" (CONBIOETICA-09-CEI-03120171207). All naval personnel were informed of the benefits and risks of the investigation prior to signing an institutional informed consent approval document to participate in the study.

Stratification of cardiovascular risk
This was done through exercise and clinical evaluation including spirometry and electrocardiogram performed prior to the initial training on the same day.

Cardiorespiratory fitness
It was evaluated by the Course-Navette test, which is an indirect estimate of the maximum oxygen consumption (VO 2 max in mL kg −1 min −1 ) (Léger et al., 1988). All tests were done in groups of eight participants and were performed on the same day and the same running track to improve the test's precision and achieve standardization among the participants. Cardiorespiratory fitness was converted to METs when necessary, dividing the VO 2 max values by 3.5 (Garber et al., 2011). We used the US CRF standards for firefighters to categorize CRF of naval personnel because there are no CRF standard values for the Mexican naval personnel and because the 12 METs threshold has been already reported (Baur et al., 2012). . The test consists of one event of 1-minute maximal effort of push-ups, sit-ups, and bench press.

Upper train
Participants were asked to perform as many push-ups and bench press as possible within 1 min in a continuous pattern with no more than two seconds of rest between repetitions. A successful push-up was defined as lowering the body so that elbows reached 90° followed by a return to the starting plank position. A successful bench press was defined as pressing the bar back up until your arms are straight, holding the weight for a second at the top, with straight arms.

Central train
Participants performed as many sit-ups as possible in 1 min in a continuous pattern with no more than two seconds of rest between repetitions. A successful sit-up required participants to curl up to a 90° position (vertical) to the floor and then return to the starting position.
The results of both tests were expressed as a number of consecutive successful repetitions for 1 min.

Body composition
Body composition was assessed prior to initial training and for each group, through three different indices calculated by anthropometric measurements: (a) BMI with the cutoff points proposed by WHO (Technical Report Series 894, 2000) and the Military classification (Léger,l988); b) body fat percentage and muscle mass percentage calculated by the 25-skinfold protocol according to the guidelines of the International Society for the Advancement of Kinanthropometry (ISAK); and (c) waist circumference (WC).

Anthropometric measurements
Anthropometric measurements were used to assess weight, height, diameters, circumference, and skinfolds to determine the BC according to the ISAK guidelines (Marfell-Jones et al., 2012). The measurements were obtained, on the same day, in the same session to avoid technical measurement errors. Naval personnel had a 12-hour fast, without lotions or body oils on their bodies, and without having done any type of exercise, and using only shorts. Twenty-five anthropometric variables were measured, of which 21 were part of the restricted profile of ISAK, and 4 additional variables (maximum waist circumference, forearm fold, chest fold, and left abdominal fold). Skinfold thicknesses were measured using calibrated skinfold calipers (Slim Guide) and a short-branch anthropometer (Cescorf). The measurements were made by two anthropometrists certified with level 2 by ISAK.

Body mass index
Height was measured using a portable stadiometer (Seca Model 213, Hamburg, Germany) and the body mass (kg) using a calibrated digital flat scale (Omron Brand model Hbf-514 C). Body mass index was calculated by dividing weight (in kilograms) by height (in square meters) (kg/m 2 ). Naval personnel was classified according to a) the BMI cutoff points of WHO as low weight (<18.5 kg/m 2 ), normal weight (18.5-24.9 kg/m 2 ), overweight (25-29.9 kg/m 2 ), obesity grade I (30-34.9 kg/m 2 ), obesity grade II (35-39.9 kg/m 2 ), and obesity grade III (≥40 kg/m 2 ); and b) the BMI cutoff points described in articles 226 and 226-Bis of the Mexican military classification, which establish that personnel with a BMI of 28 to 29.9 will be classified within the list of conditions (overweight), BMI of 30-34.9 in third category (obesity I), BMI of 35-39.9 second category (obesity II), and BMI ≥ 40 in the first category (obesity III) (Directiva 03, 2019).

Muscle mass and body fat percentage
Muscle mass percentage was calculated using the Drinkwater and Ross equation (Drinkwater & Ross, 1980;Ross & Kerr, 1993). The MMP reference used was from 46.7 to 51.5% for men who practice triathlon (Garber et al., 2011), because a triathlon combines extreme physical and psychological resistance, aspects that are also very important in naval discipline. Body fat percentage was calculated by the Siri formula from the 3-fold Jackson and Pollock formula for body density (Pons et al., 2015). The body fat percentage reference used was that established by the American College of Sports Medicine, whose values are: 7.9-10.5% (18-29 years), 12.4-14.9%

Waist circumference
Waist circumference was measured with a measuring tape (Rosscrafty Lufkin) according to the ISAK protocol for the diagnosis of metabolic diseases for men (healthy ≤ 90 cm; at-risk > 90 cm) (Macias et al., 2014).

Statistical analysis
Descriptive analysis was carried out estimating the median of age, BMI, BFP, MMP, and CRF. The distribution of overweight (WHO and military cutoff points), WC BFP, MMP, WC, CRF, and muscular strength (percentage of individuals above and below the average number of sit-ups, bench press and push-ups) between overweighed and normal-weighed individuals among groups was performed by Fisher's exact exam. We assessed the association between physical fitness components and body composition by linear and logistic regression models. Best BFP, BMI, MMP, and WC cut-off points, which differentiate fit and unfit individuals, were estimated by receiver-operating characteristic (ROC) curves. All statistical analyses were performed with the STATA v.14 software (College Station, TX, USA) considering a significance level of 0.05.

Results
We recruited 110 men with a median age of 28 (18-49)  The characteristics of body and physical components according to operative groups are shown in Table 1. When analyzing body composition, we found differences in BMI, WC and MMP except BFP (p = 0.30) in all groups. The analysis of physical fitness components shows significant differences in METs and muscular strength in all groups except in upper train (bench press) (p = 0.031). According to the BMI cutoff points of WHO, 59 (54%) individuals had overweight, but only 16 (14%) according to the military classification. The marine instructors with a median age of 36.8 years showed the highest percentage of men in overweight, 24 (41%) and 12 (75%) according to WHO and military classification, respectively, at expense of high BFP and adequate MMP however, they showed an excellent muscular strength in upper train (push-ups) but decreased in central train (sit-ups). Rescue swimmers and divers also presented an important percentage of men in overweight (WHO) at the expense of high BFP and adequate muscle mass but with decreased muscular endurance performance in both, upper and central train (Table 1).
In general, the population showed a CRF adequate for their age but is still below the ideal value for military physical activity; however, this condition improved at the end of the training (data not shown). Only 26 (51%) in normo-weight and 14 (24%) classified as overweight (WHO) had METs ≥12 (p = 0.003). Figure 1 shows the cardiorespiratory fitness among groups in normal and overweight (WHO) individuals; being MR the only group that had METs ≥12 in both normal and overweight individuals, which implies that high body fat percentage without cardiometabolic risk, may not be compromising their physical fitness.
The analysis of the risk of being unfit according to physical fitness components showed that individuals between 40 and 49 years were 11.24-times more likely to be unfit (p = 0.023); those classified as overweight by the ISSFAM were 10.64-times (p = 0.025), and those with cardiometabolic risk (WC > 90 cm) were 6.58-times (p = 0.015) more likely to be unfit. However, in the multivariate analysis, only BFP represented a risk to be unfit (OR 1.17, p < 0.038) ( Table 2).
We observed that military classification based on BMI underestimates cases of overweight in the Mexican Navy; however, it is important to highlight that the analysis of body composition allowed us to identify cases of overweight with good distribution of muscle mass but high body fat percentage, which, depending on the naval operative tasks, could be considered adequate. Therefore, it is important to analyze the impact of body composition in infantry, maritime rescue operations, and diving/underwater works.
Mexican marines presented a high BFP, a characteristic that could contribute to increase the thermal isolation of the body and make metabolic adaptations to obtain energy in cases of extended semistarvation during operative activities (Barquera-Cervera et al., 2010). In line with  this idea, in infantry soldiers of the US Army, the BMI and body fat are strongly correlated (r = 0.86) (Grier et al., 2015) and, in the military environment, a higher fat percentage is advantageous in a starvation situation and for sensitivity, performance, and survival in the cold ( Barquera-Cervera et al., 2010). Besides, our MR presented a low muscle mass percentage (43.4%), a condition that represents a disadvantage because it is not the most appropriate to meet the physical demands of CCRAM and because many of the operational tasks in infantry are often performed wearing combat gear and a body armor, which increase the weight of the load; thus, they require a lot of energy, adequate muscle mass, and muscle strength to accelerate the lower limbs to displace the total body weight (Jaworski et al., 2015). It is important to consider that this group is formed by young civilians who had just recently joined the Mexican Navy and although having met the entry standards (BMI ≤ 24.9 kg/m 2 ), their body composition (high BFP and low MMP) is not impacting negatively their physical fitness because they had the highest number of fit individuals (METs ≥12). However, it is important for this group to work in improving muscle quality and muscular strength in order to improve their physical performance and avoid muscle injuries; considering that increasing the weight of the load has been reported to influence negatively the physical performance of soldiers during tasks of longer and shorter duration (Jaworski et al., 2015).
Our rescue swimmers and divers also had a high BFP, with 78 (71%) of them without metabolic risk, these facts could represent a physical advantage because a high BFP decreases the body's need to release more energy for flotation, providing a greater buoyancy, and increasing thermal resistance to cold (Pendergast et al., 2003), not hampering swimmers performance (López-Cerón, 2009;Lowensteyn et al., 1994;Mameletzi et al., 2003). Reports indicate that during a maritime rescue operation, the distances covered by the swimmers towards the victims depend on the currents, tides, waves, wind, and water temperature, representing greater energy expenditure per distance unit for them than for terrestrial locomotion (López-Cerón, 2009;Pendergast et al., 2003). When search and rescue swimmers descend into the water, they not only have to overcome the impediments of the aquatic environment to reach the victim, but they must also carry their own weight, that of the victim, and their protective components and equipment, so they require an optimal level of muscular strength and physical fitness (López-Cerón, 2009).
The HRS, SRS, SDI, had the highest MMP compared to SRHS, and despite their training during 3 (SRS and HRS) to 6 months (SRHS) to strengthen the upper trunk and core muscles; only SDI had an excellent upper and central train strengthening. Hence, it is imperative that these groups work on improving muscular strength and power, particularly in their arms and central train because these characteristics are essential components of the explosiveness needed to reach and drag their victims during rescue operations at the sea (Sharp et al., 1982). It is important to mention that the rescue swimmers group presented the smallest percentage of individuals with good physical fitness. The BC and physical fitness level of swimmers and divers corresponds to Navy personnel that previously had military training, but with a low CRF (METs<12) and MMP (45%) at the beginning of their training courses; however, at the end of it (6 months to 1 year later, respectively), they were lean and fit men who gained muscle mass (MMP 46.7-51.5%), had less fat mass and reached a good physical fitness level (METs ≥12), indicating that the tactical and physical training course taught by the ESBUSREB was beneficial for those overweight young men in this group (data not shown). The benefits of BC and aerobic performance due to military training has been reported also in other military populations (Vaara et al., 2015). The physiological demands of swimming are quite different from those of diving; thus, body composition may differ between these two disciplines. Unfortunately, there are few studies that have analyzed the relationships between body composition and swimming and diving performance in athletes or military personnel (Lowensteyn et al., 1994;Mameletzi et al., 2003); therefore, additional research on this field is needed to help determine optimal levels to maximize performance, prevent injuries, and ensure overall health in rescue swimmers and divers from ESBUSREB of the Mexican Navy, as well as to warrant the success of rescue missions at sea and underwater works.
It is important to emphasize that swimmers, divers, and marines that did not meet BC Mexican Navy standards at the beginning of their training courses, but were in the process of improving physical fitness and BC during the training, could be able to excel in naval-relevant tasks, which would make it inappropriate to exclude them from naval services. Additionally, it is necessary to implement tests focused on evaluating other important aspects, in addition to those already evaluated by the Mexican Navy (aerobic conditioning, muscle strength, and endurance, and body composition) such as: flexibility, muscular power and agility, balance, coordination, speed and time of reaction (Nindl et al., 2015); finally, it should be considered to include anthropometry measures in the Annual Medical Exam to assess body composition to correctly classify military personnel with overweight and for future naval population studies (Shams-White et al., 2020) as well as to explore other measurement methods as alternative diagnostic criteria to assess body composition (Agsalda-Garcia et al., 2020; Baran et al., 2019;Zhanga et al., 2019).
Finally, BFP and WC were the most important predictors for having poor physical fitness; therefore, the recommendation to improve weight and fitness in the Mexican Navy's training programs is to increase CRF and muscle resistance-strength, with changes in lifestyles associated with specific nutritional demands that would favor the success in maritime rescue operations, diving and underwater works, and completion of infantry tasks.
Our study has some limitations. First, the number of participants was limited because only few of them request to take the courses. Second, women could not be included because of the small number of participants with operational tasks in the Mexican Navy. Third, there are few studies that have analyzed the body composition and evaluated the physical fitness of rescue swimmers and divers in military personnel, which limits the benchmarks to compare our population.
Our major strength is that it is the first study that highlights important variables, such as body composition and physical fitness components among Mexican naval personnel, and determined that BMI in overweight with high BFP (without a cardiometabolic risk) represents a physical advantage that not significantly affects naval task performance and cardiorespiratory fitness. There is also a gap in the literature between body composition and swimming/diving performance, which opens the door for further studies.

Conclusion
Actual marines, rescue swimmers and divers body composition standards should consider a BMI in overweight at expenses of high BFP, without cardiovascular risk in current military body composition policies, because represent an operational task advantage that will led to reach the balance between physical fitness, health and military readiness.