Body water content increases during carbohydrate loading because 2.7-4-g water binds each 1 g of glycogen. Bioelectrical impedance spectroscopy (BIS) allows separate assessment of extracellular and intracellular water (ECW and ICW, respectively) in the whole body and each body segment. However, BIS has not been shown to detect changes in body water induced by carbohydrate loading. Here, we aimed to investigate whether BIS had sufficient sensitivity to detect changes in body water content and to determine segmental water distribution after carbohydrate loading. Eight subjects consumed a high-carbohydrate diet containing 12 g carbohydrates·kg body mass⁻¹·day⁻¹ for 72 h after glycogen depletion cycling exercise. Changes in muscle glycogen concentration were measured by ¹³C-magnetic resonance spectroscopy, and total body water (TBW) was measured by the deuterium dilution technique (TBW_D2O). ICW and ECW in the whole body (wrist-to-ankle) and in each body segment (arm, trunk, and leg) were assessed by BIS. Muscle glycogen concentration [72.7 ± 10.0 (SD) to 169.4 ± 55.9 mmol/kg wet wt, P < 0.001] and TBW_D2O (39.3 ± 3.2 to 40.2 ± 3.0 kg, P < 0.05) increased significantly 72 h after exercise compared with baseline, respectively. Whole-body BIS showed significant increases in ICW (P < 0.05), but not in ECW. Segmental BIS showed significant increases in ICW in the legs (P < 0.05), but not in the arms or trunk. Our results suggest that increase in body water after carbohydrate loading can be detected by BIS and is caused by segment-specific increases in ICW.

Muscle fiber composition is associated with physical performance, with endurance athletes having a high proportion of slow-twitch muscle fibers compared to power athletes. Approximately 45% of muscle fiber composition is heritable, however, single nucleotide polymorphisms (SNP) underlying inter-individual differences in muscle fiber types remain largely unknown. Based on three whole genome SNP datasets, we have shown that the rs236448 A allele located near the cyclin-dependent kinase inhibitor 1A (CDKN1A) gene was associated with an increased proportion of slow-twitch muscle fibers in Russian (n = 151; p = 0.039), Finnish (n = 287; p = 0.03), and Japanese (n = 207; p = 0.008) cohorts (meta-analysis: p = 7.9 × 10⁻⁵. Furthermore, the frequency of the rs236448 A allele was significantly higher in Russian (p = 0.045) and Japanese (p = 0.038) elite endurance athletes compared to ethnically matched power athletes. On the contrary, the C allele was associated with a greater proportion of fast-twitch muscle fibers and a predisposition to power sports. CDKN1A participates in cell cycle regulation and is suppressed by the miR-208b, which has a prominent role in the activation of the slow myofiber gene program. Bioinformatic analysis revealed that the rs236448 C allele was associated with increased CDKN1A expression in whole blood (p = 8.5 × 10⁻¹⁵) and with greater appendicular lean mass (p = 1.2 × 10⁻⁵), whereas the A allele was associated with longer durations of exercise (p = 0.044) reported amongst the UK Biobank cohort. Furthermore, the expression of CDKN1A increased in response to strength (p < 0.0001) or sprint (p = 0.00035) training. Accordingly, we found that CDKN1A expression is significantly (p = 0.002) higher in the m. vastus lateralis of strength athletes compared to endurance athletes and is positively correlated with the percentage of fast-twitch muscle fibers (p = 0.018). In conclusion, our data suggest that the CDKN1A rs236448 SNP may be implicated in the determination of muscle fiber composition and may affect athletic performance.

Muscle glycogen is a crucial energy source for exercise, and assessment of muscle glycogen storage contributes to the adequate manipulation of muscle glycogen levels in athletes before and after training and competition. Muscle biopsy is the traditional and gold standard method for measuring muscle glycogen; alternatively, ¹³C magnetic resonance spectroscopy (MRS) has been developed as a reliable and non-invasive method. Furthermore, outcomes of ultrasound and bioimpedance methods have been reported to change in association with muscle glycogen conditions. The physiological mechanisms underlying this activity are assumed to involve a change in water content bound to glycogen; however, the relationship between body water and stored muscle glycogen is inconclusive. In this review, we discuss currently available muscle glycogen assessment methods, focusing on ¹³C MRS. In addition, we consider the involvement of muscle glycogen in changes in body water content and discuss the feasibility of ultrasound and bioimpedance outcomes as indicators of muscle glycogen levels. In relation to changes in body water content associated with muscle glycogen, this review broadens the discussion on changes in body weight and body components other than body water, including fat, during carbohydrate loading. From these discussions, we highlight practical issues regarding muscle glycogen assessment and manipulation in the sports field.

Genetic polymorphisms and sex differences are suggested to affect muscle fiber composition; however, no study has investigated the effects of genetic polymorphisms on muscle fiber composition with respect to sex differences. Therefore, the present study examined the effects of genetic polymorphisms on muscle fiber composition with respect to sex differences in the Japanese population. The present study included 211 healthy Japanese individuals (102 men and 109 women). Muscle biopsies were obtained from the vastus lateralis to determine the proportion of myosin heavy chain (MHC) isoforms (MHC-I, MHC-IIa, and MHC-IIx). Moreover, we analyzed polymorphisms in α-actinin-3 gene (ACTN3; rs1815739), angiotensin-converting enzyme gene (ACE; rs4341), hypoxia-inducible factor 1 α gene (rs11549465), vascular endothelial growth factor receptor 2 gene (rs1870377), and angiotensin II receptor, type 2 gene (rs11091046), by TaqMan single-nucleotide polymorphism genotyping assays. The proportion of MHC-I was 9.8% lower in men than in women, whereas the proportion of MHC-IIa and MHC-IIx was higher in men than in women (5.0 and 4.6%, respectively). Men with the ACTN3 RR + RX genotype had a 4.8% higher proportion of MHC-IIx than those with the ACTN3 XX genotype. Moreover, men with the ACE ID + DD genotype had a 4.7% higher proportion of MHC-I than those with the ACE II genotype. Furthermore, a combined genotype of ACTN3 R577X and ACE insertion/deletion (I/D) was significantly correlated with the proportion of MHC-I (r = −0.23) and MHC-IIx (r = 0.27) in men. In contrast, no significant correlation was observed between the examined polymorphisms and muscle fiber composition in women. These results suggest that the ACTN3 R577X and ACE I/D polymorphisms independently affect the proportion of human skeletal muscle fibers MHC-I and MHC-IIx in men but not in women. NEW & NOTEWORTHY In men, the RR + RX genotype of the α-actinin-3 gene (ACTN3) R577X polymorphism was associated with a higher proportion of myosin heavy chain (MHC)-IIx. The ID + DD genotype of the angiotensin-converting enzyme gene (ACE) insertion/deletion (I/D) polymorphism, in contrast to a previous finding, was associated with a higher proportion of MHC-I in men. In addition, the combined genotype of these polymorphisms was correlated with the proportion of MHC-I and MHC-IIx in men. Thus ACTN3 R577X and ACE I/D polymorphisms influence the muscle fiber composition in Japanese men.

To determine energy density for rapid weight loss (RWL) of weight-classified sports, eight male elite wrestlers were instructed to lose 6% of body mass (BM) within 53 h. Energy deficit during the RWL was calculated by subtracting total energy expenditure (TEE) determined using the doubly labeled water method (DLW) from energy intake (EI) assessed with diet records. It was also estimated from body composition change estimated with the four-component model (4C) and other conventional methods. BM decreased significantly by 4.7 ± 0.5 kg (6.4 ± 0.5%). Total body water loss was the major component of the BM loss (71.0 ± 7.6%). TEE was 9446 ± 1422 kcal, and EI was 2366 ± 1184 kcal during the RWL of 53-h; therefore, the energy deficit was 7080 ± 1525 kcal. Thus, energy density was 1507 ± 279 kcal/kg ∆BM during the RWL, comparable with values obtained using the 4C, three-component model, dual energy X-ray absorptiometry, and stable isotope dilution. Energy density for RWL of wrestlers is lower than that commonly used (7400 or 7700 kcal/kg ΔBM). Although RWL is not recommended, we propose that commonly practiced extreme energy restriction such as 7400 or 7700 kcal/kg ΔBM during RWL appears to be meaningless.

Estimated energy requirements (EERs) are important for sports based on body weight classifications to aid in weight management. The basis for establishing EERs varies and includes self-reported energy intake (EI), predicted energy expenditure, and measured daily energy expenditure. Currently, however, no studies have been performed with male wrestlers using the highly accurate and precise doubly labeled water (DLW) method to estimate energy and fluid requirement. The primary aim of this study was to compare total energy expenditure (TEE), self-reported EI, and the difference in collegiate wrestlers during a normal training period using the DLW method. The secondary aims were to measure the water turnover and the physical activity level (PAL) of the athletes, and to examine the accuracy of two currently used equations to predict EER. Ten healthy males (age, 20.4±0.5 y) belonging to the East-Japan college league participated in this study. TEE was measured using the DLW method, and EI was assessed with self-reported dietary records for ~1 wk. There was a significant difference between TEE (17.9±2.5 MJ•d-1 [4,283±590 kcal•d-1]) and self-reported EI (14.4±3.3 MJ•d-1 [3,446±799 kcal•d-1]), a difference of 19%. The water turnover was 4.61±0.73 L•d-1. The measured PAL (2.6±0.3) was higher than two predicted values during the training season and thus the two EER prediction equations produced underestimated values relative to DLW. We found that previous EERs were underestimating requirements in collegiate wrestlers and that those estimates should be revised.