Water Redistribution in The Body and Muscles of Young Athletes After a Single Workout Complemented by Water Intake

Anthropometric assessments of athletes, including the determination of body composition, are commonly performed since a certain array of physical characteristics were found to be statistically relevant to obtaining the best competitive achievements in sports settings [1]. Anthropometry has been regarded as a valuable means in sports settings to identify suitable skills, abilities, and ergonomic conditions of athletes and to obtain the best-fitting athlete-sport alignment and pursue optimal performances [2]. Body composition monitoring of fat/muscle alterations it is also applied in healthcare settings to the general population to maintain well-being and health, and to prevent pathologies [3-5]. Human prospective studies have addressed the relevance of body composition as the clinical endpoint of the outcome of pharmacological/physical management of healthy or pathological conditions [6]. Intended shaping of anthropometry of a body towards an increment and better functioning of skeletal muscle (lean mass), while reducing fat mass, is the ideal way to achieve significant improvements in sports performance and to prevent injuries, too [7, 8]. Measuring of fat and lean masses of the body, including water itself as the main constituent, is easily achievable through Bioelectrical Impedance Vector Analysis (BIVA or BIA). This is a non-invasive and low-cost methodology that allows to simultaneous measurement of multiple and physiologically interrelated body anthropometric components, in real time. Water, the most valuable nutrient and component of the human body, can be timely monitored through BIA [9]. Either total body or extracellular water are directly measurable entities in BIA [10, 11], while intracellular water, which correlates to whole muscle hydration, can be derived by calculations. Awareness of actual body water dislodgment and relative quantities can advise on hydration and electrolyte strategies to optimize protocols for training and post-workout recovery. For example, addressing fluid imbalances with appropriate intake of water and electrolytes as nutritional strategies can sustain reparation and reduce discomfort of the muscle. Evaluating intracellular water after an intense workout provides insights into the physiological state of the muscle and how it responds to exercise. ICW measurement can help differentiate between true muscle growth and transient changes due to fluid shifts, as for extracellular oedema. Low ICW levels after a workout may show excessive fatigue or insufficient recovery. The analysis of such parameters could provide important insights for nutritionists and trainers who guide athletes to adjust (individual) training requirements timely. In our opinion, BIA is the best method to rapidly evaluate ICW [12], in terms of lowest discomfort, ease of execution and no invasiveness, speed of acquisition and analysis, and low-cost. By evaluating ICW, athletes, trainers, and researchers can better understand the physiological impact of exercise, enhance recovery strategies, and improve overall performance and muscle health.

Notably, acute changes in total body intracellular and extracellular water occurring within 3 hours could be accurately measured through (multifrequency) BIA [11]. Instead, to our knowledge, no data on short-term changes in the body composition because of physical exercise are available, except for a study on a small cohort of extreme sport athletes losing total body water due to sweat and/or diuretic substances [9]. Hence, the primary aim of the present study was to show whether and how the redistribution of body compartments would occur in fully hydrated athletes during one single session of training. Our secondary goal was to elucidate whether body compartments were differently affected in male and female athletes.

Study design

Pre-training. Participants were instructed to refrain from exercise starting the night before and to empty their bladder before examination. Then, they were instructed to consume a light breakfast and to carry out a session of sporting activity in the following 1 hour. Hydrated training. The training session was performed in the gymnasium of the Institute to make logistically feasible the rapid post-training measure. 1.5 L of water was assumed after the first BIA evaluation, during the training course. The training was performed according to previously published guidelines [13, 14]. Post-training. At the end of the training, within the next 30 minutes, a second BIA measurement was carried out for each participant. Measurements of the body composition were conducted following the manufacturer’s instructions, as described in the next paragraph (Figure 1).

Subjects: The whole cohort of participants was enrolled between February 2023 and February 2024 voluntarily among the athletes referring to our O.U. Exercise and Sports Sciences of the University Chieti-Pescara to undergo the sports medicine visit to obtain the certificate of fitness for agonistic practice. They included 100 consecutive young adult athletes, 62 males with an average age of 27.17±5.25 years and 38 females with an average age of 26.52±5.37. On this occasion, the athletes were asked to take part in the study and were informed of the meaning, the implication and the protocol and the use, storage and sharing of personal data (informed consent). Exclusion criteria: use of medications that might affect body water compartments. Anthropometric data were gathered using standard methods. Participants were weighed while wearing only underwear after emptying their bladders. Body height was measured without shoes using a scale (Table 1).

Table 1: Age and BMI (mean +/-SD) of the two male and female cohorts and their changes between pre- and post-training (percentual and absolute values).

Nutrition and hydrated training protocol: Starting from 1 week before evaluation, the athletes followed a balanced diet based on the nutritional principles of the Mediterranean Diet characterized by calorie intake and nutritional components correct for gender and sport and focussing on hydration (Figure 2). The first BIA measurements were taken in the morning after an overnight fast. The training session was performed according to previously published guidelines [13, 14] in the gymnasium of the Institute to make logistically feasible the rapid post-training measure. The athletes started to drink water (1.5 L) after the first BIA evaluation, during the training course.

Anthropometry and Body Composition Assessment: Bodyweight was assessed using a portable digital scale with values approximated to 0.1 Kg. The BIA evaluation for the assessment of BC were performed according to [16] using the bioimpedance meter “Medical Body Composition Analyzer SECA model 515” (Strumedical, Montecassiano, MC, Italy), in the foot-to-hand and 4 electrodes configuration, according to the manufacturer’s instructions. This device provides an alternating electric current of low intensity throughout the body and allows showing the composition of the body based on a method able to detect changes of measurable electrical entities (resistance/reactance/capacitance) which correlate with the absolute or relative mass of water and fat in the body. In our setting, BIA parameters, R and Xc, were assessed by applying an electrical current of 800 μA to the body and by setting the signal frequency at 50 kHz (accuracy 1.0 Ohm) [16]. Body components - Fat Mass (FM), Free Fat Mass (FFM), Skeletal Muscle Mass (SSM), Total Body Water (TBW), and Extracellular Water (ECW) (Figure 1) [17] were calculated from impedance parameter based on the dilution method deuterium/bromide in a young lean athletic sample [18]. Reference values for normal ranges of mass indices (FMI, FFMI) and phase angle (φ) of BIA were also set electronically. Each parameter was shown as an absolute value or percent of the whole-body mass. Linear correlation between the anthropometric parameters and the BIA output signal has been established previously [19]. To normalize the data, body composition indexes were computed as parameters (Kg/%) divided by the square of height in meters (m²). Comparability and accuracy of these formulas are ensured by the manufacturer’s validation studies performed with constant reference methods and homogeneous reference Caucasian male or female populations of different age ranges, which were appropriately set in this study for each participant. Results are shown as normalized bioimpedance indices since body capacitive index [BCI = capacitance (C) / (BMI)] overcomes the gender effect, in healthy subjects (BCI=2.0 nF *kg/mexp2, for both genders), besides being found to be better predictors of outcome in peritoneal dialysis patients [20]. The maximal yield of total body water was promoted by making the participants drink 1.5 L of water during the exercise session and before the post-workout measurement.

Statistics

Pre- and post-training BC data were gathered anonymously from each participant and analysed as grouped data. Student’s t-tests was used to compare the means +/- SD of the two groups of pre-workout and post-workout assessments. The data were analysed using JASP software (Jasp Stats, v.1.0; Amsterdam, Netherlands), A p< 0.05 was considered as statistically significant.

Female athletes’ cohort: The female group was characterized as shown in Table 2. No significant differences were found after training when compared with the pre-training baseline values, in any of the anthropometric values or body composition BIA measurements.

Male athletes’ cohort: The male cohort anthropometric characteristics are reported in Table 2. No significant differences were found after training in any of the anthropometric values between pre-and post-training. All the other (hydration-related) body composition parameters evaluated through BIA resulted in significantly increased post-training values.

Table 2: Body composition of male and female cohorts and their percentual and absolute changes between pre-and post-training.

Comparative analysis between genders: At baseline and pre-training, the BMI was lower in males than in females (0.04 absolute points) but this difference was not significant. Fat Mass (FM) was significantly higher in the female cohort, compared to the male cohort. In both cohorts, FM did not change post-training, compared with the pre-training measure. Fat-free Mass (FFM) (i.e. SK+ECW+ICW) was higher in the male cohort at baseline and increased significantly after the hydrated training, compared to the female cohort. Accordingly, Skeletal muscle (SK) was higher in the male cohort at baseline and increased post-exercise, compared to the female cohort. Extracellular water (ECW) was significantly higher in the male cohort at baseline and increased significantly, too, compared to the female cohort. Post-training, and after water intake, the male cohort showed a higher lean mass (FFM) (+1.56 %), skeletal muscle mass (SMM) (+1.65 %), Total body water (TBW) (+1.96 %), and Extracellular water (ECW) (+2.25 %), compared to the female cohort. Intracellular water (ICW) (i.e. TBW-EC) was higher in the male cohort at baseline, compared to the female cohort, and increased significantly after the hydrated training, too (Table 2, Table 3, Figure 3).

Table 3: Percentual difference of the mean BIA values between the male and female cohorts of athletes for each body component.

To determine if such low-cost, harmless and fast biomonitoring could be of use to improve training plans and/or long-term athletic achievements, either at the individual or at team level. Reducing fat mass significantly in a single training session is not physiologically possible. Body fat loss is a gradual process that requires a negative calorie balance (i.e., consuming more calories than you take in) over several days or weeks. However, there are exercises that, thanks to their high-calorie consumption, can promote the calorie deficit necessary to reduce fat mass over time. Types of effective exercises for fat loss are high-intensity workouts (HIIT) and short intervals of maximum effort alternating with active recovery (sprints, burpees, jumping rope). High-calorie consumption in a brief time and prolonged increase in metabolism (EPOC) for several hours after training is generated by long-lasting cardiovascular exercises (steady state), such as moderate and prolonged aerobic activities (e.g., running, cycling, swimming). However, they burn calories during training, but without the post-workout effect of HIIT. Weight training (squats, deadlifts, multi-joint weightlifting) stimulates the basal metabolic rate (higher muscle mass = more calories burned at rest). It does not burn fat directly in one sitting, but it does contribute to overall calorie consumption. Circuit training such as the combination of aerobic and strength exercises without extended breaks is effective for burning calories and keeping your heart rate up. During an intense workout, the body predominantly burns carbohydrates as an energy source, but with a prolonged calorie deficit, it will begin to use stored fat as well. Immediate weight loss after a session is mainly due to the loss of water, not fat. Key aspects to reducing fat mass are the calorie deficit, that is the need to consume more calories than you ingest, which is achieved through diet, playing a crucial role, and training being only one part of the process. Consistency in reducing fat mass requires regular workouts and a sustainable approach over time. A diet-workout combination of a balanced diet combined with targeted exercises is the most effective method of reducing body fat. In summary, while it is not possible to reduce fat mass significantly with a single training session, high-intensity or cardiovascular exercises contribute to creating the calorie deficit necessary to lose fat in the long term. Instead, intense exercise causes micro-damage to muscle fibres, and intracellular water can show the level of cellular repair or swelling due to inflammation. Proper hydration is critical for cellular functions, including protein synthesis and nutrient transport, both of which are essential for recovery, and keeping the best ICW levels is associated with improved muscle contractility and performance. A decrease in ICW might signal dehydration or impaired muscle function, both of which can hinder performance and increase injury risk. Muscle growth (hypertrophy) involves not only an increase in contractile proteins but also an increase in intracellular fluid volume. ICW measurement can help differentiate between true muscle growth and temporary changes due to fluid shifts (e.g., extracellular oedema). Low ICW levels after a workout may show excessive fatigue or insufficient recovery. This can guide athletes and trainers in adjusting training intensity or recovery protocols to prevent overtraining. Intracellular hydration influences cell volume, which plays a role in cell signalling pathways that regulate metabolism and anabolic processes. Monitoring ICW can offer insights into overall muscle cell health and metabolic activity. Such measurement can inform hydration and electrolyte strategies to perfect post-workout recovery. For example, addressing fluid imbalances with proper intake of water and electrolytes can support muscle repair and reduce soreness. Water is the fundamental nutrient for living cells and organisms, being the solvent allowing chemical reactions of life to take place in the body. Loss of water, or dehydration, is linked to detrimental effects at cellular and tissue and organism levels, particularly clear in the elderly. Hyperosmotic stress, inflammation, cell shrinkage and damage to intracellular protein structure and function have been described as severe effects associated with dehydration, leading to catabolism, anabolic resistance, and muscle wasting as well as impaired muscle contractile ability. Muscle intracellular hydration could be an indicator of muscle quality and contractile capacity, particularly important in the elderly and workers who rather display ECW increase, especially if disabled or affected by dysfunctions. In the elderly, reduced muscle quality (reduced strength, speed and mass) is associated with increased ECW/ICW, ratio independent of age, sex, BMI, functional ability and muscle mass [21]. Notably, a BIA-based study on people aged over 75 years showed that ICW was strongly related to muscle functionality. Hence, intracellular water protects the muscle from functional decline and better cell hydration corresponds to greater muscle. ICW content per unit of lean mass (mL/kg) is considered an indicator of muscle quality showing that this ratio decreases with age, is positively related to strength and functional ability and is negatively related to frailty risk, regardless of age, sex and the number of co-morbidities. Our findings (Figure 4) suggest a role in intracellular hydration in muscle performance in young (trained) subjects, too. These findings point to the importance of early detection and correction of water loss during sports training or muscle-challenging activities. The above-mentioned effects of water on protein folding and function and on cell volume and metabolism would therefore suggest that more attention needs to be paid to the hydration status of athletes [21].

Differences in BC between males and females after engaging in sports are influenced by biological, hormonal, and physiological factors. The key differences regard muscle mass and fat mass. Males tend to have greater muscle mass, both in absolute and percent terms due to their typical testosterone hormone-promoting muscle hypertrophy. Testosterone levels are typically low in females, who also show a lower percentage of muscle mass. Rather, females show greater fat mass, promoted by their typical hormone estrogen, mainly distributed through the body. Fat mass in males accounts for a lower percentage of the body mass (10-15%) and is mainly located in the abdomen, compared to females (20-25%) to whom it is spread subcutaneously (buttocks, thighs, breasts) and plays a role in reproduction. These differences in body composition correspond to different basal metabolisms, being lower in females than males. Despite this, training can increase muscle mass in both sexes. Female estrogen does not prevent muscle gain, but males often experience faster and more significant gains. Hormonal and genetic reasons appear to counteract sports efforts in females. Personalized training based on sex and goals is essential to refine results. Bioelectrical impedance analysis (BIA) is a safe and practical method for assessing the BC of athletes [22,23]. BIA is a method capable of making direct measurements and allowing a non-invasive evaluation of the hydration of soft tissues and the integrity of the cell membranes (hence, their functionality). The (indirect) assessment of FM, FFM and other parameters is considered a valuable means to provide practical evidence of the role of BC in sport [6]. Despite the fast and handy obtainment of ICW datasets from large numbers of subjects, BIA could vary in precision, compared with other methods [5]. this highlights how important it is to consider gender differences when designing and executing training programs and how individual assessment of BIA body composition can be crucial for personalizing training programs and monitoring progress over time. The reluctance in the reduce of fat index in females might be related to hormonal, metabolic/ glucose tolerance and insulin sensitivity, type of fat, diet, and lifestyle differences with males that need to be confirmed through addressed studies. Comparability and accuracy of these formulas must be regarded with a critical eye, as the validation studies were performed with differing reference methods and, in all respects, heterogeneous reference populations. In addition, the study results cannot necessarily be transferred to other manufacturers’ devices for technical reasons. Despite the fast and handy obtainment of ICW datasets from large numbers of subjects, BIA could vary in precision, compared with other methods.

The main limitation of the present study is the lack of a comparative methodology for body composition assessment, such as dual-energy x-ray absorptiometry. Indeed, body composition through BIA alone can be confounded by hydration status, which itself will be influenced by exercise and provide wrong results. In addition, the study results cannot necessarily be transferred to other manufacturers’ analysers for methodological reasons, and comparability and accuracy of the results must consider the heterogenicity and the different characteristics of reference populations. Validation studies are auspicious in this view. Nevertheless, our study was designed to maximalise total body water before and during the test workout by making the athletes follow a hydrating diet and drink a saturating amount of water. Furthermore, for the first time, the effects on body composition are unlinked from gender and nutritional effects, but rather solely related to acute effects of exercise (Figure 5).

The study evaluated the immediate effects of a fitness training session on the BC of 100 male and female athletes. The results show that the training led to a significant reduction in body fat and an increase in total body water in both sexes. However, the changes were more pronounced in men than in women. Specifically, the male subjects showed an average reduction in body fat of about 1 kg, while the female subjects exhibited an average reduction of about 0.5 kg. Total body water increased by an average of about 1 litre in men and about 0.5 litre in women. These sex differences may be due to a range of factors, including initial differences in body composition, hormonal levels, and metabolic responses to exercise. Overall, the study provides further evidence that fitness training can be an effective way to improve BC in both men and women. However, it is important to take sex differences into The study evaluated the immediate effects of a fitness training session on the BC of 100 male and female athletes. The results show that the training led to a significant reduction in body fat and an increase in total body water in both sexes. However, the changes were more pronounced in men than in women. Specifically, the male subjects showed an average reduction in body fat of about 1 kg, while the female subjects exhibited an average reduction of about 0.5 kg. Total body water increased by an average of about 1 litre in men and about 0.5 litre in women. These sex differences may be due to a range of factors, including initial differences in body composition, hormonal levels, and metabolic responses to exercise. Overall, the study provides further evidence that fitness training can be an effective way to improve BC in both men and women. However, it is important to take sex differences into account when designing training programs. For this reason, further scientific studies are needed to better understand the physiological mechanisms and adaptations to exercise. Moreover, it is important to note that the study examined the immediate effects of a single training session, and the long-term outcomes of fitness training may differ between the two groups.

Data are deposited at the “Sports Medicine Operative Unit” at G. d’Annunzio University of Chieti-Pescara and are available from the corresponding author upon reasonable request.

All authors gave consent for publication.

Ethical review and approval were waived for this study since the subjects did not participate in nutritional protocols.

AdA is supported by the Italian Ministry of University and Research (MUR). CUP D53C23002450003. The remaining authors did not receive any financial support for the research, authorship, and/or publication of this article.

None.

The authors declare no potential conflicts of interest concerning the research, authorship, and/or publication of this article.

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