OBJECTIVES:Despite cross-sectional evidence supporting raw bioelectrical impedance analysis (BIA) parameters as informative markers of body composition in youth, their role from a longitudinal perspective remains unclear. We examined how 1-y changes in phase angle (PhA) and other raw BIA parameters were associated with changes in body components at multiple levels in children and adolescents. METHODS:In a 1-y prospective cohort investigation, including 562 healthy and on-time matured youth (51.6% females), we assessed raw BIA parameters with a phase-sensitive 50 kHz BIA device and examined body composition through dual-energy X-ray absorptiometry and related models. The association between changes in raw BIA parameters and changes in body composition by sex and peak height velocity (PHV) stage was analyzed using multiple linear regression. RESULTS:PhA was positively associated with changes in fat-related components, fat-free mass, extracellular water, fluids and solids, osseous minerals, and muscle tissue in females (β = 0.088-0.150; P ≤ 0.05) and with alterations in most body components in males (β = 0.098-0.300; P ≤ 0.05). Similar trends to those of males were observed in both sexes during post-PHV (females: β = 0.149-0.297; males: β = 0.185-0.352; P ≤ 0.05). Other series and parallel BIA parameters exhibited even higher explanatory power of body composition than PhA, with the magnitude of the associations depending on each raw BIA parameter, sex, and PHV stage (females: β =|0.099|-|0.429|; males: β = |0.106|-|0.706|; P ≤ 0.05). Among all raw BIA parameters, the highest associations were observed in resistance adjusted to stature (RI) (total sample and pre-PHV), parallel capacitive reactance adjusted to stature (XcpI), and capacitance (Cap) (both post-PHV). CONCLUSION:PhA emerged as a modest marker of body composition dynamics in youth, particularly in males and during adolescence (post-PHV), whereas other raw BIA parameters, particularly RI, XcpI, and Cap, showed even greater promise in both sexes and across development stages.

BACKGROUND:Bromide (Br-) dilution is the criterion to determine extracellular water. While 3 h equilibrium is typically used in athletes, the optimal equilibrium time has not been assessed. The study aimed to determine the optimal time of equilibrium of Br- in athletes before and after increasing water intake (WI). METHODS:Thirty-one athletes [21.5 (0.8) years; 41.9% females] from the H2OAthletes randomized controlled trial (clinicaltrials.gov ID: NCT05380089) were included in this longitudinal sub-analysis. Athletes were assessed before and after a 4-day intervention to increase WI (experimental group) or maintain WI (control group). Br- dilution was assessed through saliva before and 3, 4, and 5 h post-dose. Total body water (TBW) and water turnover (rH2O) was measured through deuterium dilution. Body composition was determined through the 4-compartment model. RESULTS:At baseline, most athletes achieved equilibrium at 3 h (45.2%) or 4 h (48.4%). After the intervention, most athletes (70.4%) achieved peak concentration at 3 h, with fewer reaching at 4 h (14.8%) or 5 h (14.8%). No differences in body composition, TBW, or rH2O were observed between athletes grouped according to the time of peak, both before and after the intervention. Time of peak was not associated with the intervention group [x2 (1) = 1.439; p = 0.487] or sex both before [x2 (1) = 0.230; p = 0.891] and after the intervention [x2 (1) = 0.728; p = 0.695]. CONCLUSION:Br- equilibrium appears to occur 3 h-4 h post-dosing, suggesting it is unnecessary to extend the collection to 5 h, reducing the burden on athletes. Future studies should include larger samples to confirm the current findings.

BACKGROUND:Aging is associated with progressive declines in skeletal muscle mass (SMM) and substantial interindividual variability in adaptations to resistance training (RT). However, whether training load intensity influences hypertrophic responsiveness in older adults remains unclear. OBJECTIVE:To investigate the effects of two commonly prescribed RT loading schemes (15RM and 10RM) on estimated SMM adaptations and responsiveness in older women. METHODS:Twenty-seven older women (67.7 ± 4.9 years; 65.7 ± 10.6 kg; 154.5 ± 5.8 cm) completed two 8-week RT interventions performed at 15RM and 10RM in a randomized crossover design, separated by a 12-week detraining period. Estimated SMM was assessed using dual-energy X-ray absorptiometry. Responsiveness was operationally defined using the standard error of measurement, and participants were classified as responders (RP) or non-responders (N-RP) according to changes in estimated SMM. RESULTS:Both loading schemes promoted significant and comparable increases in estimated SMM (15RM: +0.62 ± 0.4 kg vs. 10RM: +0.43 ± 0.3 kg). Overall, 61.1% of participants were classified as responders in at least one condition, whereas 38.9% were classified as non-responders. Responsiveness status was consistent across conditions for most participants, with 48.1% classified as responders under both loading schemes and 25.9% as non-responders under both conditions. Only seven participants (25.9%) changed responsiveness status between interventions. No clear pattern indicated a greater likelihood of responsiveness with either loading scheme. CONCLUSIONS:Results suggest that both the 10RM and 15RM RT protocols significantly increase estimated SMM in older women, with no significant differences between training conditions. Although substantial interindividual variability in adaptive responses exists, training load intensity does not appear to meaningfully influence responsiveness status in most individuals. These findings suggest that both loading schemes can be effectively prescribed to promote SMM gains in older women.

Skeletal muscle mass (SMM) is a central determinant of metabolic health, physical function, and resilience across the lifespan, yet its assessment is often confounded by the assumption that fat-free mass is compositionally uniform and stably hydrated. This narrative review synthesizes discussions from the 12th International Symposium on In Vivo Body Composition Studies and integrates evidence from aging, chronic energy deficit, disuse/microgravity, resistance training, and anabolic interventions to examine how intracellular and extracellular fluid compartments influence the interpretation of muscle mass and quality. We highlight that declines in muscle quality, reflected by loss of intracellular water and reduced cellular integrity, frequently precede measurable losses in muscle quantity and are more closely linked to functional impairment than bulk skeletal mass. Dual-energy X-ray absorptiometry, while valuable for population studies, cannot distinguish water from protein or intracellular from extracellular compartments and therefore may overestimate anabolic gains in fluid-retentive states or underestimate clinically meaningful muscle loss when extracellular water is preserved or expanded. Multi-frequency bioelectrical impedance techniques, including MF-BIA and BIS, complement structural methods by resolving total, extracellular, and intracellular water and by providing indices of cellular integrity, such as phase angle, that reflect muscle quality. We conclude that hydration-sensitive interpretation is essential for accurate assessment of skeletal muscle under physiological stress.

BACKGROUND:Water turnover (rH2O) is related to water balance including preformed water intake (WI), metabolic, inspired, and transcutaneous water. Literature is scarce on athletes. This study aimed to estimate and compare rH2O and its components during one athletic season (preparatory and competition phases), adjusting for body composition and energy expenditure (EE). METHODS:This longitudinal observational study included 112 athletes (19.8 ± 4.9 years, 39 females). Doubly labeled water determined total EE, rH2O, and the remaining components through specific equations. Total body water was determined by deuterium dilution and fat-free mass by the four-compartment model. Resting EE was assessed by indirect calorimetry. RESULTS:During the preparatory phase, male athletes showed higher rH2O [5.03 (0.14) kg/day] compared with females [3.64 (0.19) kg/day, p < .001], even adjusting for body mass (p = .024), but not for fat-free mass, total body water, and EE. Triathletes showed higher values of rH2O after adjustments (p < .05). Over the athletic season, no changes occurred for rH2O or preformed WI within or between sexes or sports, even adjusting for body composition while some changes occurred adjusting for EE (p < .05). Over the season, metabolic water increased (p < .001), while inspired and transcutaneous water showed both increases and reductions (p < .05). A small prevalence (∼16%) of low WI (WI ≤ 35 ml/kg of body mass/day) was persistent in both timepoints. CONCLUSION:While rH2O and preformed WI did not differ over the season, cross-sectional differences were observed between sports but not between sexes, highlighting the variability of rH2O and impact of contributing factors. Low preformed WI was present throughout the athletic season.

Fat-free mass (FFM) is a key component of body composition that contributes moderately to water turnover (WT). However, FFM consists of multiple components, and the extent of its contribution to WT, especially in relation to skeletal muscle mass (SMM) in athletes, remains unclear. This study aimed to examine the association between WT and body composition in athletes. Thirty male participants (20 ± 2 years old) were members of college sports clubs or athletic organizations. WT was determined using the deuterium dilution method for 1 week. Total body water (TBW), intracellular water (ICW), and extracellular water (ECW) were measured using deuterium and sodium bromide dilution. Bone mineral content was measured using DXA, and four-component models were used to calculate FFM and total body protein (TBP). MRI and MRS were used to evaluate SMM and muscle glycogen. WT was positively associated with TBW (r = 0.563), ICW (r = 0.549), SMM (r = 0.548), and ambient temperature (r = 0.519). To overcome mathematical coupling and multicollinearity, regularized regression analyses (LASSO and Elastic Net) were performed. Because TBW and TBP are algebraically dependent on the deuterium-derived water space from which WT was calculated, they were excluded from the candidate predictors. Both models retained SMM, ICW, and ambient temperature as predictors of WT, whereas ECW, bone mineral content, muscle glycogen, and relative humidity were not retained. The modest cross-validated performance (leave-one-out R2 = 0.20-0.23) suggests that WT in athletes is also influenced by behavioral and unmeasured environmental factors. Functional body composition compartments, specifically SMM and ICW, were associated with WT in athletes beyond the mathematical dependency between WT and TBW; SMM, measured independently by MRI, was an independent correlate, whereas the association of ICW-derived in part from TBW-was not fully independent of the WT measurement.

BACKGROUND/OBJECTIVES:The purpose of this investigation was (i) to assess the intra- and inter-tester reliability of delineating regions of interest (ROIs) in Dual-energy X-ray Absorptiometry (DXA) for measuring regional fat mass (FM), lean soft tissue (LST), and bone mineral content (BMC) and (ii) to investigate the prevalence of inconsistencies between automatically defined ROIs and manufacturer guidelines. A secondary aim was to assess whether discrepancies in the delineation of ROIs by different testers could bias regional body composition changes over time. METHODS:DXA measurements were performed with a total body scan in 86 athletes (64.0% females) from 12 sports. Two independent testers conducted post-scan analysis following the manufacturer's guidelines. Intra- and inter-tester reliability was evaluated using the intra-class correlation coefficient (ICC), mean differences, paired samples t-tests and Bland-Altman plots. RESULTS:Intra- and inter-tester reliability was nearly perfect (ICC: 0.998-1.000) across all regions and components (FM, LST, and BMC). Mean differences were consistently <1% for all regions and components, except for left arm FM (1.4%; SD = 4.6%) in inter-tester reliability. Inconsistencies between automatically defined ROIs and manufacturer guidelines were frequent, with chest line corrections (which separate the arm from the trunk) required in 91.9% of scans. Inter-tester ICC for regional changes (n = 52) ranged from 0.668 to 0.981, with wider limits of agreement than cross-sectional estimates. CONCLUSION:DXA post-scan regional analysis can be performed with very high reliability and low measurement error, but manual review is essential. When monitoring changes over time, baseline and follow-up scans should be analyzed by the same evaluator.

PURPOSE:This study aimed to develop and validate Elastic Net-based predictive models using simple anthropometric measurements to estimate dual-energy X-ray absorptiometry (DXA)-derived body composition variables in older women. METHODS:Body mass, skinfolds, circumferences, and DXA measures of total and regional fat mass (FM) and lean soft tissue (LST) were obtained from 523 older women. Elastic Net regression developed models for total body, upper- and lower-limb FM and LST, and android and gynoid FM. The sample was split into training, validation, and independent testing sets. RESULTS:Elastic Net models demonstrated high predictive accuracy for DXA-derived body composition. Coefficients of determination were strong for total FM and LST (R² ≥ 0.884), and good for regional compartments (R²≥ 0.711). Root mean square errors were low across outcomes (0.272-1.858 kg). Bland-Altman analyses showed minimal mean bias for most variables, except for lower-limb FM and android FM, which presented small but significant biases (≤ 2.78 %). Proportional bias was identified in some models, although the explained variance remained low (≤22%). Limits of agreement were narrow for total body estimates (≤11.73%), while wider limits were observed for segmental measures. CONCLUSIONS:Elastic Net-based models provide accurate estimates of total body composition and acceptable estimates of regional compartments, although regional predictions showed greater individual variability than total body estimates. However, these prediction equations should be interpreted as internally validated models intended primarily for screening and research applications.

While rapid four-compartment (4C) models have demonstrated cross-sectional validity in diverse populations, including athletes, their longitudinal validity remains unverified. This study aims to evaluate the longitudinal validity of utilising dual-energy X-ray absorptiometry (DXA)-derived estimates of body volume (BV) and bioelectrical impedance spectroscopy (BIS)-derived total body water (TBW) in rapid 4C models in athletes. Criterion 4C used DXA for bone mineral content, air displacement plethysmography for BV and 2H dilution for TBW. Analyses of longitudinal changes in fat mass (FM), in both kilograms and percentage, were performed in participants who experienced changes greater than 1 % (n 60) and in a subgroup exceeding the least significant change (1·97 kg, n 25). All alternative estimates underestimated changes relative to criterion 4C, with 4C TBWBIS providing the smaller mean difference (MD) (0·41 kg) and DXA the larger (0·94 kg). The MD for 4C BVSilvaTBWBIS and 4C BVHeymsfieldTBWBIS were 0·48 kg and 0·50 kg, respectively. Bland-Altman analyses showed wide limits of agreement (LOA) for all methods. Even the alternative with the smaller 95 % LOA had a wide LOA (4C TBWBIS: -2·9 %, 4·0 %, n 60). This investigation demonstrates that these rapid 4C models exhibit precision superior to or equivalent to DXA alone while offering reduced assessment times and broader accessibility than the criterion 4C. Regarding accuracy for evaluating changes over time, despite acceptable results at the group level, interpretation should be careful at the individual level.

Skinfold thickness assessment is one of the most widely used anthropometric methods for estimating subcutaneous adipose tissue and overall body adiposity in clinical, epidemiological, and sports settings. Despite its long-standing application, substantial heterogeneity persists across measurement protocols and standardization systems, potentially affecting reproducibility and comparability across studies. This narrative review aims to critically synthesize the historical, conceptual, methodological, and technical development of the main skinfold measurement protocols and standards, systematically compare their characteristics, and examine their methodological and practical implications for research and clinical applications. A targeted bibliographic synthesis of classic textbooks, consensus manuals, and original peer-reviewed studies on surface anthropometry was conducted. Seventeen anatomical skinfold sites and 31 distinct technical descriptions were identified, including 22 site-selection protocols and 9 measurement standards. Five established standards were comparatively analyzed, with emphasis on anatomical landmarking, fold orientation, spatial pinching procedures, and temporal reading control. Similarities were observed in general technical principles, including right-side assessment, perpendicular caliper application, controlled reading time, and maintenance of fold elevation. However, persistent divergences were identified in anatomical definitions, fold orientation, landmark precision, spatial positioning of pinch and caliper jaws, and recommended reading time, with greater heterogeneity at trunk sites than limb sites. Although the reviewed protocols and standards were developed for broad application across age groups, this review primarily reflected their use in adolescents and adults, as skinfold assessment in infants and young children involves additional considerations related to growth dynamics and rapid changes in adipose tissue. Skinfold measurement protocols and standards are not interchangeable and reflect pluralistic, complementary international initiatives rather than a single institutional standardization process.

Dehydration's adverse impact on athletic performance is a critical concern in sports science, yet its investigation remains challenging due to interindividual variability and methodological inconsistencies. Traditional reductionist approaches, which isolate single variables, are insufficient to capture the multifactorial nature of dehydration, which emerges from dynamic interactions among physiological and behavioral factors. These include the method of inducing dehydration, protocol duration, type of fluid loss, environmental conditions, participant characteristics, assessment techniques, and potential nocebo effects. Despite the complexity and relevance of this issue, standardized guidelines for designing sport-specific dehydration studies are lacking. This article addresses this gap by cataloging key interacting variables and offering structured, adaptable guidance rather than proposing a rigid framework. It introduces a dual framework of 'challenges and considerations' and 'recommendations' for each factor, aiming to support researchers in developing rigorous, context-specific study designs. Ultimately, this approach will promote a more nuanced understanding of dehydration and facilitate reproducible, sport-relevant research advancement.

PURPOSE:This study aimed to examine the effects of two distinct repetition ranges in resistance training (RT) on cognitive function and mental health outcomes in older women. METHODS:A total of 120 older women (age = 68 ± 5 years; body mass = 69.2 ± 12.6 kg; stature = 155 ± 5 cm; BMI = 30 ± 2 kg/m2) were randomly assigned to one of three groups: an RT group performing 8-12 repetition maximum (RM), an RT group performing 10-15 RM, or a non-exercise control group. The term RM refers to the maximum load that can be lifted with proper technique within a given repetition range. The 12-week RT program comprised eight whole-body exercises, each performed for three sets, three times per week. Cognitive function was assessed using the Montreal Cognitive Assessment (MoCA), Trail Making Test Parts A and B (TMT-A/B), Verbal Fluency Test, and Stroop Test. Mental health was evaluated using the Patient Health Questionnaire-9 (PHQ-9), the 15-item Geriatric Depression Scale (GDS-15), and the Beck Anxiety Inventory (BAI). RESULTS:Both RT groups demonstrated significant improvements in cognitive performance (p < 0.05). MoCA scores increased by 2.9 % (8-12 RM) and 5.2 % (10-15 RM), semantic verbal fluency by 9.7 % (8-12 RM) and 9.0 % (10-15 RM), and phonological fluency by 12.3 % (8-12 RM) and 12.1 % (10-15 RM). Mental health outcomes also improved markedly: PHQ-9 scores decreased by 34.2 % (8-12 RM) and 24.4 % (10-15 RM); GDS-15 scores declined by 21.7 % (8-12 RM) and 17.4 % (10-15 RM); and BAI scores were reduced by 41.6 % (8-12 RM) and 41.5 % (10-15 RM). CONCLUSION:A 12-week RT program performed at either 8-12 RM or 10-15 RM promotes significant improvements in cognitive function and mental health among older women. These findings reinforce the role of RT as an effective non-pharmacological strategy to support mental and cognitive well-being in aging populations.

Bioelectrical impedance vector analysis (BIVA) and phase angle (PhA) are important for monitoring hydration, muscle function, and quality of life in older adults. Resistance training (RT) can counteract aging’s adverse effects on cellular integrity and function. This study compared the effects of RT volume reduction on BIVA and PhA in physically independent older women. Sixty-seven participants (>60 years) underwent a 20-week standardized whole-body RT program (eight exercises, three sets each, 8–12 repetitions, three non-consecutive days per week in the morning). They were then randomly assigned to one of the three eight-week training conditions following the training model described above: reduced volume to one set (RV1, n = 22), two sets (RV2, n = 24), or maintained volume (MV, n = 21). Bioimpedance spectroscopy measured total body water (TBW), intracellular (ICW), and extracellular (ECW) water, BIVA, and PhA based on resistance (R), impedance (Z), reactance (Xc), and height (H). All groups showed significant increases in TBW, ICW, and ECW during the volume reduction phase (P < 0.05). R, R/H, Z, and Z/H decreased across all groups, with significant changes from the pre-conditioning phase in the RV1 and RV2 groups (P < 0.05). Xc and Xc/H increased during the pre-conditioning phase across all groups and returned to baseline during the volume reduction phase (P < 0.05). PhA increased during the pre-conditioning and was maintained during the volume reduction phase (RV1 = +0.33°, RV2 = +0.50°, MV = +0.47°; P < 0.05). These results suggest that reducing RT volume by up to one-third can still improve PhA, BIVA, and hydration status in older women.

ABSTRACT:Stavinski, N, Cavazzotto, TG, Carneiro, MAS, Kunevaliki, G, Francsuel, J, Costa, B, Kassiano, W, Tricoli, I, Castro-E-Souza, P, Prado, A, Cyrino, LT, Rodrigues, RJ, Silva, DRP, Cavaglieri, CR, Silva, AM, Sardinha, LB, and Cyrino, ES. Synergistic relationship between muscle thickness, muscular strength, and functional fitness after resistance training in older women. J Strength Cond Res 39(12): e1455-e1466, 2025-This study examined the synergistic relationship between changes in muscle thickness (MT) and muscular strength and functional fitness after a resistance training (RT) program in older women. Sixty physically independent women (68.8 ± 5.9 years) participated in a 24-week RT program. The MT, muscular strength, and functional fitness were analyzed at baseline and after 12 and 24 weeks. The whole-body RT program was conducted in 3 weekly sessions (8 exercises, 3 sets of 8-15 repetitions). The repeated measures correlation analysis revealed significant correlations between MT and muscular strength ( rrm = 0.51 to 0.78; p < 0.05) and functional fitness tests ( rrm = 0.27 to 0.45; p < 0.05). Further analysis using repeated measures regression revealed significant associations between MT and muscular strength ( R2 = 0.16 to 0.54; p < 0.05) and functional fitness tests ( R2 = 0.10 to 0.32; p < 0.05). Furthermore, repeated measures mediation showed that 7.4 and 19.8% of the relationship between RT engagement and 1 repetition maximum (1RM) leg extension and 1RM preacher curl, respectively, was attributed to increases in MT. However, MT did not mediate the relationship between RT and 1RM chest press, handgrip strength, 4-m gait speed, timed-up-and-go, 5-repetition sit-to-stand (5STS), 30-second sit-to-stand, and 6-minute walk. Changes in MT induced by RT may account for a significant portion of the changes in muscular strength, but not in functional fitness.

BACKGROUND AND AIMS:This study investigated the effects of a moderate caffeine dose on resistance (R), reactance (Xc), impedance (Z), capacitance (Cap), and phase angle (PhA) over four days in males. Thirty men aged 20-39 years, with low caffeine consumption (<100 mg/day), participated in this double-blind, randomized crossover trial (ClinicalTrials.gov: NCT01477294). The study consisted of two conditions - caffeine (5 mg/kg/day) and placebo (maltodextrin) - each lasting four days, separated by a three-day washout period. METHODS:Assessments were performed at baseline and on the fourth day of each condition. The R, Xc, Z, and PhA were measured through bioelectrical impedance spectroscopy using a frequency of 50 kHz. These were mathematically transformed in a parallel array. Fat mass (FM) and fat-free mass (FFM) were assessed by dual-energy X-ray absorptiometry, while urine specific gravity (USG) was determined using a refractometer. Water intake (WI) was estimated through dietary food records. RESULTS:A linear mixed model analysis, adjusting for FFM and WI and baseline values, found no significant differences in R, Xc, and Z in both series and parallel arrays, Cap, and PhA across conditions. Comparisons between baseline, placebo, and caffeine conditions did not reach statistical significance for any parameter (p > 0.05). USG values remained stable, indicating no changes in hydration status (p > 0.05). CONCLUSION:This study shows that caffeine intake does not significantly affect raw bioelectrical impedance parameters. These findings simplify research by removing caffeine restrictions, enhancing study feasibility, and improving participant adherence. Clinical and sports settings allow for more flexible assessments.

Dual-energy x-ray absorptiometry (DXA) has been suggested as an alternative method for estimating body volume (BV), a component of the reference four-compartment body composition model. This study reports the development and validation of DXA-BVSilva, a novel DXA-derived BV equation. An additional aim was to develop an estimate of lean soft tissue density (DLST), a key variable in the theory-based approach, by estimating total body protein with a six-compartment model. A sample of 332 athletes (36.7

Background Dysmenorrhea, characterized by painful menstruation, can influence body composition, muscle strength, and metabolic function. This study aimed to investigate differences and relationships between body composition variables, phase angle (PhA), muscle strength, and urine metabolites among Japanese female individuals with and without dysmenorrhea. Methods Thirty-four participants, divided into healthy menstruating control (n = 15) and dysmenorrhea (n = 19) groups, were included. PhA, whole body lean soft tissue (WB LST), visceral fat (VF) and subcutaneous fat (SF), hand grip strength (HGS), leg strength, and urine metabolites were measured using multifrequency bioelectrical impedance analysis, dual-energy X-ray absorptiometry, computed tomography, dynamometry, and liquid chromatography, respectively. Results No significant differences were found in the characteristic variables. BMI, PhA, and muscle strength showed a positive association only in the control group. However, menstrual status (control/dysmenorrhea, 0/1) did not affect muscle strength. Conversely, a negative coefficient (LASSO: -0.098; Elastic net: -0.109) demonstrated that the dysmenorrhea group had lower predictive PhA values than did the control group. PhA and WB LST were identified as meaningful predictors of HGS (R2 = 0.433) and leg strength (R2 = 0.251). Acylcarnitine metabolites were positively associated with VF and PhA in the dysmenorrhea and control groups, respectively. Conclusion The study findings highlight distinct associations among body composition, PhA, muscle strength, and urinary metabolites in these female groups. However, the study is limited by the absence of severity of dysmenorrhea symptoms, phases of menstruation, and no adjustment for lifestyle factors. A future longitudinal study is warranted to understand these physiological association between the control and dysmenorrhea groups.

BACKGROUND & AIMS:Raw bioelectrical impedance analysis (BIA) parameters, particularly phase angle (PhA), are essential clinical and performance markers. However, their potential as body composition markers remains underexplored in youth. This investigation examined the associations between PhA and other series- and parallel-transformed raw BIA parameters and multiple body components organized at the molecular, cellular, and tissue levels in youth. METHODS:A cross-sectional sample of 681 youth (10-17 years, 49 % females) was assessed using a phase-sensitive BIA device (50 kHz), dual-energy X-ray absorptiometry, and a freezing point depression osmometer. Multiple linear regression examined each raw BIA parameter's unadjusted and adjusted relationships with body components. RESULTS:PhA was associated with all body components in females (molecular level: β = |0.170| to |0.584|; cellular level: β = 0.409 to 0.582; tissue level: β = 0.170 to 0.569) and males (molecular level: β = |0.542| to |0.729|; cellular level: β = 0.612 to 0.729; tissue level: β = 0.714; p ≤ 0.05), independently of relevant confounders. No relationship was found between PhA and fat mass and adipose tissue in males (p > 0.05). Other raw BIA parameters were shown to be even more associated with body composition than PhA in females (molecular level: β = |0.172| to |0.827|; cellular level: β = |0.292| to |0.713|; tissue level: β = |0.635| to |0.821|) and males (molecular level: β = |0.555| to |0.926|; cellular level: β = |0.554| to |0.870|; tissue level: β = |0.827| to |0.916|; p ≤ 0.05). In both sexes, the association power of these BIA parameters was lower regarding fat-related parameters (β = non-significant to |0.460|). CONCLUSIONS:PhA was moderately associated with most body components in youth, except for fat-related components in males, independent of confounding variables. Resistance adjusted to stature consistently emerged as the most relevant marker, highlighting its potential to inform about body composition at an early age.

Isotope-based tracer methods allow the determination of total energy expenditure (TEE), water turnover (rH2O), and total body water (TBW) in free-living conditions. These methods have exciting applications in athletes. However, the limited number of available measurements constrains their applicability. The aim was to describe the application of isotope dilution techniques for measuring TEE, rH2O, and TBW in athletic populations. A comprehensive search (https://doi.org/10.17605/OSF.IO/7932T) was performed in three databases: PubMed, EBSCO (CINAHL, MEDLINE, and SPORTDiscus), and Cochrane Library. A total of 1,540 records were identified (564 excluded) and 174 through other sources. After excluding 53 duplicates, 1,097 articles were screened. A total of 121 studies were included, totaling 3,244 measurements from different types of sports, age range, and tier level, with 1,020 from female athletes and 139 measurements where sex was not reported. For TEE, 75 studies were included with values ranging from 1,939 to 10,070 kcal/day. For rH2O, 15 studies were included with values ranging from 2.7 to 13.4 L/day. For TBW, 77 studies were included with values ranging from 29.8 to 76.8 kg. Variability was observed across the studies among the variables of interest. Overall, males showed higher TEE, rH2O, and TBW values than females, with endurance sports showing the greatest variability in energy and water flux, and TBW values varying most in team and mixed sports. Future research should increase representation of females, athletes with disabilities, and Tier 5 "world-class" athletes to establish normative values across sports, age groups, and sex while applying standardized isotope dilution methodologies.

PURPOSE:This study investigated the relationship between muscle thickness and raw and derived bioelectrical impedance analysis (BIA) parameters following 12 wk of resistance training (RT) in older women. METHODS:Fifty-five older women were assigned to a control group ( n = 31) or an RT group ( n = 24). Muscle thickness was assessed using B-mode ultrasound, while BIA parameters, including reactance (Xc), resistance (R), phase angle (PhA), intracellular water, extracellular water (ECW), and total body water, were measured at 50 kHz. Total and appendicular lean soft tissue and fat mass were evaluated via dual-energy X-ray absorptiometry. RESULTS:Muscle thickness and total and appendicular lean soft tissue increased in the RT group (2.9%-6.2%; P < 0.001), whereas muscle thickness decreased in the control group (-4.1%; P < 0.001). Xc, PhA, and ECW also increased in the RT group (1.6%-5.8%; P ≤ 0.002) but decreased in the control group (-1.1% to -5.8%; P ≤ 0.022). In the RT group, muscle thickness was positively correlated with Xc, PhA, total body water, and ECW ( rrm = 0.40-0.52; P ≤ 0.023), whereas in the control group, muscle thickness correlated only with Xc, R, and PhA ( rrm = 0.43-0.46; P ≤ 0.015). Linear mixed models showed that muscle thickness was associated with all raw and derived BIA parameters in the RT group but only with PhA in the control group. CONCLUSIONS:Our findings suggest that increases in muscle thickness resulting from a 12-wk RT program are synergistically associated with changes in both raw and derived BIA parameters, which may play a crucial role in enhancing cellular health in older women.