Sodium Ingestion Improves Groundstroke Performance in Nationally-Ranked Tennis Players: A Randomized, Placebo-Controlled Crossover Trial.

This study examined the dose-response effects of ingesting different sodium concentrations on markers of hydration and tennis skill. Twelve British nationally-ranked tennis players (age: 21.5 ± 3.1 years; VO2peak: 45.5 ± 4.4 ml.kg.min-1) completed four identical in-door tennis training sessions in a cluster randomized, single-blind, placebo-controlled, crossover design. Twenty-minutes prior to each training session, participants consumed a 250 ml sodium-containing beverage (10, 20, 50 mmol/L) or a placebo (0 mmol/L), and continued to consume 1,000 ml of the same beverage at set periods during the 1-h training session. Tennis groundstroke and serve performance, agility, urine osmolality, fluid loss, sodium sweat loss and perceptual responses (rating of perceived exertion (RPE), thirst, and gastrointestinal (GI) discomfort) were assessed. Results showed that ingesting 50 mmol/L sodium reduced urine osmolality (-119 mOsmol/kg; p = 0.037) and improved groundstroke performance (5.4; p < 0.001) compared with placebo. This was associated with a reduction in RPE (-0.42; p = 0.029), perception of thirst (-0.58; p = 0.012), and GI discomfort (-0.55; p = 0.019) during the 50 mmol/L trial compared with placebo. Linear trend analysis showed that ingesting greater concentrations of sodium proportionately reduced urine osmolality (ß = -147 mOsmol/kg; p = 0.007) and improved groundstroke performance (ß = 5.6; p < 0.001) in a dose response manner. Perceived thirst also decreased linearly as sodium concentration increased (ß = -0.51; p = 0.044). There was no evidence for an effect of sodium consumption on fluid loss, sweat sodium loss, serve or agility performance (p > 0.05). In conclusion, consuming 50 mmol/L of sodium before and during a 1-h tennis training session reduced urine osmolality and improved groundstroke performance in nationally-ranked tennis players. There was also evidence of dose response effects, showing that ingesting greater sodium concentrations resulted in greater improvements in groundstroke performance. The enhancement in tennis skill may have resulted from an attenuation of symptomologic distracters associated with hypohydration, such as RPE, thirst and GI discomfort.

Interpretation of Dual-Energy X-Ray Absorptiometry-Derived Body Composition Change in Athletes: A Review and Recommendations for Best Practice.

Dual-energy X-ray absorptiometry (DXA) is a medical imaging device which has become the method of choice for the measurement of body composition in athletes. The objectives of this review were to evaluate published longitudinal DXA body composition studies in athletic populations for interpretation of "meaningful" change, and to propose a best practice measurement protocol. An online search of PubMed and CINAHL via EBSCO Host and Web of Science enabled the identification of studies published until November 2016. Those that met the inclusion criteria were reviewed independently by 2 authors according to their methodological quality and interpretation of body composition change. Twenty-five studies published between 1996 and November 2016 were reviewed (male athletes: 13, female athletes: 3, mixed: 9) and sample sizes ranged from n = 1 to 212. The same number of eligible studies was published between 2013 and 2016, as over the 16 yr prior (between 1996 and 2012). Seven did not include precision error, and fewer than half provided athlete-specific precision error. There were shortfalls in the sample sizes on which precision estimates were based and inconsistencies in the level of pre-scan standardization, with some reporting full standardization protocols and others reporting only single (e.g., overnight fast) or no control measures. There is a need for standardized practice and reporting in athletic populations for the longitudinal measurement of body composition using DXA. Based on this review and those of others, plus the official position of the International Society for Clinical Densitometry, our recommendations and protocol are proposed as a guide to support best practice.

Effect of Hand Positioning on DXA Total and Regional Bone and Body Composition Parameters, Precision Error, and Least Significant Change.

Dual-energy X-ray absorptiometry (DXA) body composition measurements are performed in both clinical and research settings for estimations of total and regional fat mass, lean tissue mass, and bone mineral content. Subject positioning influences precision and positioning instructions vary between manufacturers. The aim of the study was to determine the effect of hand position and scan mode on regional and total body bone and body composition parameters and determine protocol-specific body composition precision errors. Thirty-eight healthy subjects (men; mean age: 27.1 ± 12.1 yr) received 4 consecutive total body GE-Lunar iDXA (enCORE v 15.0) scans with re-positioning, and scan mode was dependent on body size. Twenty-three subjects received scans in standard mode and 15 received scans in thick scan modes. Two scans per subject were conducted with subject hands prone and 2 with hands mid-prone. The precision error (root mean squared standard deviation; percentage coefficient of variation) and least significant change for each protocol were determined using the International Society for Clinical Densitometry calculator. Hands placed in the mid-prone position increased arm bone mineral density (BMD) (standard mode: 0.185 g*cm-2, thick mode: 0.265 g*cm-2; p < 0.05), total body BMD (standard mode: 0.051 g*cm-2, thick mode: 0.069 g*cm-2; p < 0.001), and total body BMD Z-score (standard mode: 0.5. thick mode: 0.7; p < 0.001). This was due to reductions in bone area and bone mineral content. In standard mode, hands mid-prone reduced fat mass (0.05 kg, p < 0.05) and increased lean mass (0.11 kg, p < 0.05). There were no differences in body composition for thick mode scans. Hands mid-prone reduced lean mass precision error at the arms, trunk, and total body (p < 0.01). DXA clinical and research centers are advised to maintain consistency in their hand positioning and scan mode protocols, and consideration should be given to the hand positioning used for reference data. As a best practice recommendation, published DXA-based studies and reports for clinic-based total body assessments should ensure that subject positioning is fully described.

Effect of the glycaemic index of a pre-exercise meal on metabolism and cycling time trial performance.

This study investigated the effects of low or high glycaemic index (GI) foods consumed prior to a 40 km time trial (TT) on metabolism and subsequent endurance performance. Ten male cyclists consumed high GI or low GI meals, providing 1 g kg(-1) body mass of carbohydrate, 45 min prior to the TT. The TT performance was significantly (p=0.009) improved in the low (93+/-8 min) compared to the high GI trial (96+/-7 min). Low GI carbohydrate oxidation rate (2.51+/-1.71 g min(-1)) was higher (p=0.003) than the HGI carbohydrate oxidation rate (2.14+/-1.5 g min(-1)). Fat oxidation rate was significantly higher (p=0.002) for the high (0.27+/-0.17 g min(-1)) than the low GI trial (0.16+/-0.14 g min(-1)). Insulin rose significantly following the high compared to the low GI meal (p=0.008) but dropped significantly to similar values throughout the TT. No significant differences in either TGA or FFA concentration were observed between the trials. The low GI meal led to an increase in the availability of carbohydrate and a greater carbohydrate oxidation throughout the exercise period, which may have sustained energy production towards the end of exercise and led to the improved TT performance observed.

The effects of low- and high-glycemic index meals on time trial performance.

PURPOSE: The aim of this work was to determine whether the consumption of pre-exercise high- or low-glycemic index (GI) meals has a beneficial effect on time trial performance. METHODS: Eight male cyclists were provided with either a high-GI or low-GI meal, providing 1 g x kg(-1) body mass of carbohydrate, 45 min before performing a 40-km time trial on a Velotron cyclePro. RESULTS: Time trial performance was significantly improved in the low-GI trial (92.5 +/- 5.2 min) compared with the high-GI trial (95.6 +/- 6.0 min) (P = .009). Blood glucose concentrations at the point of exhaustion were significantly higher in the low-GI trial (5.2 +/- 0.6 mmol x L(-1)) compared with the high-GI trial (4.7 +/- 0.7 mmol x L(-1)) (P = .001). There was no significant difference in estimated carbohydrate oxidation data between the low-GI (2.51 +/- 1.74 g x min(-1)) and high-GI (2.18 +/- 1.53 g x min(-1)) meals (P = .195). No significant difference in estimated fat oxidation was observed between the low-GI (0.15 +/- 0.15 g x min(-1)) and high-GI (0.29 +/- 0.18 g x min(-1)) diets (P = .83). CONCLUSIONS: The improvement in time trial performance for the low-GI trial may be associated with an increased availability of glucose to the working muscles, contributing additional carbohydrate for oxidation and possibly sparing limited muscle and liver glycogen stores.