Soccer is a variable intensity endurance sport with an overall intensity of 70-80% [VO.sub.2max], dependent in part on playing position and performance level (Bangsbo et al., 2006). The intensity of soccer is sufficient to elevate core body temperature and initiate a sweating response, which may generate a net loss of body water unless this water is replaced via fluid intake (Edwards and Noakes, 2009).
Several studies have investigated fluid balance in soccer players during laboratory exercise protocols (Drust et al., 2000; Maxwell et al., 2009; Nicholas et al., 1995). However, the laboratory environment may not adequately mimic conditions faced by soccer players undertaking habitual training and competition in the field, such as variability in environmental temperature, humidity, and wind strength, and differences in the intensity of training and competition. Existing field-based research in adult soccer players reports large variations in sweat rate (SR) and sweat losses between players (Shirreffs et al., 2006), and incidences of players beginning training in a hypohydrated state (Maughan et al., 2005).
Existing field-based research into fluid balance and hydration status in soccer players has largely focused on measuring adult athletes on a single day, and data on the fluid balance, sweat loss, and hydration status of young soccer players during training and competition is sparse (Silva et al., 2011). Factors such as chronological age, biological maturation, morphology, and anthropometry can influence cardiovascular and thermoregulatory responses to exercise (Havenith, 2001; Rowland, 2008). Therefore, it is important to investigate fluid balance, hydration status, and associated fluid intake practices in young soccer players (Silva et al., 2011). Recently, Silva et al. (2011) showed that elite Brazilian soccer players (17.2 [+ or -] 0.5 years) began each of three consecutive training sessions (mean temperature 30.1[degrees]C, relative humidity 59.6%) mildly hypohydrated and did not voluntarily ingest sufficient fluid during training to offset sweat losses. Comparable findings have been reported during a competitive soccer match with similar participants (elite Brazilian players, 17.0 [+ or -] 0.6 years) and environmental conditions (31.0[degrees]C, 48.0% relative humidity; Da Silva et al., 2012). These findings indicate that young soccer players may be at risk of developing hypohydration during soccer training and competition.
In the studies of Silva et al. (2011) and Da Silva et al. (2012), urine specific gravity (USG) was used to quantify hydration status pre- and post-training/competition, but neither study measured USG from first-morning urine samples. Hydration status can be estimated by analysing the first urine excretion of the day as urine concentration in this sample is sufficiently sensitive to detect deviations in fluid balance (Sawka et al., 2007; Shirreffs and Maughan, 1998). Furthermore, the first morning urine sample is advantageous over urine samples taken at other times as it is not affected by acute fluid ingestion, which can mask true hydration status (Oppliger and Bartok, 2002). For these reasons, guidelines emphasise the need to use first morning urine samples to allow discrimination between euhydration and dehydration (Sawka et al., 2007). Additionally, neither Silva et al. (2011) nor Da Silva et al. (2012) quantified the intensity of the training sessions/competition, making it difficult to interpret the hydration data within the context of the exercise demands.
The data from Silva et al. (2011) indicates that young soccer players exhibit voluntary hypohydration during consecutive training sessions in a hot and humid environment. However, the data is not generalisable beyond the sample used and the environmental conditions in which the study was conducted. Data for elite young European soccer players undertaking typical training in a temperate environment is not available; therefore, further study into hydration status and fluid balance in this sample is warranted. The aim of this study was to assess first morning, pre- and post-training hydration status and fluid balance of elite European youth soccer players during three consecutive days of soccer training.
Fourteen male soccer players volunteered for the study, all of whom were members of the same elite youth academy team (mean time at the academy 3.9 [+ or -] 2.9 years). The characteristics of the participants were: age 16.9 [+ or -] 0.8 years, height 1.79 [+ or -] 0.06 m, body mass (BM) 70.6 [+ or -] 5.0 kg. To be eligible for the study participants had to be free from injury and medication that affected the ability to exercise, and free from medication or medical conditions that influenced body fluid balance. Prior to inclusion, comprehensive written and verbal explanation of the study was provided to participants and parents, written parental informed consent and child assent was received, and a medical questionnaire was completed. The study received ethical approval from the Abertay University School of Social and Health Sciences Research Ethics Committee.
On three separate occasions, a sample of the first urine output upon waking was collected from each participant. Participants were instructed to refrain from strenuous exercise, other than the normal prescribed training sessions, for at least 24 h before providing the sample, but were otherwise permitted to go about their normal activity and dietary routine. Participants brought the samples into the laboratory the same morning they were deposited, whereupon they were analysed for USG using a clinical refractometer (model A300, ATAGO Co., Tokyo, Japan) calibrated according to manufacturer guidelines. These samples provided baseline hydration data for each participant that served as a comparison for the pre- and post-training hydration data (Shirreffs and Maughan, 1998). For all analyses, USG values
Data collection took place over three consecutive training sessions near the end of the competitive season (April/May 2012), beginning with the first training session of the week. Sessions one and two were separated by 72 hours, and sessions two and three by 24 hours. The goal was to quantify hydration status during normal training sessions; therefore, every effort was made to prevent interference in the normal training-day practices and logistical decisions of the participants and coaching staff. To assist this aim, the...