Study design
Fifteen collegiate athletes from Merrimack College (NCAA Division I (ice hockey) and II (all other sports)) were recruited for this randomized, crossover study. Participants were eligible if they were between 18 and 24 years of age, injury-free, able to exercise at greater than 75% of their maximal heart rate for a minimum of 45 min, were on one of the college’s sports teams, and provided informed consent. Informed consent was also required by the participant’s head coach and the head athletic trainer on campus. Because the training sessions utilized in this study consisted of already-scheduled team training sessions, athletes were recruited from in-season sports that were currently engaged in heavy sports-specific training sessions. Once recruited, participants underwent a qualitative assessment for hydration habits and knowledge. Participants were interviewed one-on-one by researchers to gauge hydration habits and knowledge pertaining to dehydration and overhydration. This subjective questionnaire consisted of a combination of open-response and multiple-choice questions. The full list of questions are shown in the results section of this study. Following this, participants were assessed for sweat loss, then randomized to either a prescription hydration plan (PHP) or asked to continue with their normal hydration habits (NHP). Participants in each group underwent performance assessments prior, during, and immediately after a moderate to hard training session (goal average heart rate ≥ 75% of maximum for at least 45 min in duration). Maximum heart rate was estimated with the formula, 207-(0.7 x age in years) [17]. Heart rate (HR) was recorded remotely using the Zephyr PSM Training System (Zephyr Technology Corporation, Annapolis, MD, US) [18]. Mean and peak heart rate were recorded throughout the entire training session, including just prior to warm up, warm up, and 15-min cool down. All measurements took place immediately before, during, or after a sports-specific training session. For example, hockey players recruited for this study underwent assessments during a full-pad, on ice, practice. Similarly, Lacrosse players were assessed outdoors on the Lacrosse field during one of the teams harder practice sessions. Athletes were also weighed several times per week in the two weeks preceding the training session for determining fluid loss (see “Sweat Assessment”) as well as prior to the NHP and PHP training sessions in order to determine weight stability.
The overall design of this study is shown in Fig. 1. Each participant completed a training session with their NHP and PHP, separated by 7 days.
To determine the NHP for each participant, researchers observed the hydration habits of each athlete during at least one training session in addition to reviewing the results of the hydration survey noted earlier. No instruction was provided to athletes with regards to their NHP. Each participant was monitored during their NHP training session for compliance, particularly those who were randomized to follow a PHP first. Researchers also controlled for pre-training hydration status by monitoring fluid consumption beginning at 60 min prior to the start of the sweat assessment, NHP, and PHP training sessions. All fluids consumed during this study were kept at room temperature.
Sweat assessment
Both fluid loss and sweat sodium (Na+) concentration were assessed. Fluid loss from training was performed as described previously [9]. Briefly, nude weight was taken immediately prior to training. Fluid bottles (32 oz of water or sports drink of choice (lemon-lime flavored Gatorade®)) were measured out and provided to each participant. Participants were instructed to only drink from his or her bottle and consumption of fluid was closely monitored during the training session. Participants were again weighed immediately afterwards (nude weight, surface sweat removed via towel dry). The time of day, length of training session, temperature, and level of humidity during the session were also recorded. For reference, all sweat assessments took place during the cooler months (November–March) within the New England region of the U.S. Fluid loss was determined from the change in pre-training to post-training body mass and corrected for fluid intake. Sweat rate was expressed in L. h− 1 by taking the total fluid loss and adjusting for the duration of the training session. Relative sweat rate was expressed as ml. kg− 1. hr.− 1. To determine sweat [Na+], sweat was induced and collected from each participant using a Macroduct® Sweat Collection System (ELITech Group, Model #3700 SYS), according to the manufacturer’s instructions [19, 20]. Briefly, sweat induction occurred via the use of Pilogel® Iontophoretic Discs, placed in two electrodes (red and black) that were strapped to the participant’s forearm (Fig. 2). Activation of the sweat inducer served to deliver enough pilocarpine for sweat gland stimulation (equivalent to 5 min of iontophoresis at 1.5 milliamps). Following induction, a macroduct sweat collector was placed over the skin where the red electrode was previously. The collector contained a blue dye that allowed the researchers to observe the collection of sweat by capillary action. Once enough sweat was collected, the [Na+] in each sample was assessed using a Sweat•Chek™ Conductivity Analyzer (ELITech Group, Model #3120) as described previously [21].
Prescription hydration plan (PHP) development
Fluid losses for each athlete (determined previously) were expressed in ounces. 15 min− 1. This time measurement was agreed upon by participants and coaches and represented a feasible fluid consumption plan during training sessions. A range of fluid consumption per 15 min was calculated by determining the minimum consumption rate (enough to prevent mild dehydration or 2% bodyweight loss [5]) and the maximum consumption rate (fluid loss determined earlier, which is equated to maintaining, but not exceeding pre-training bodyweight). For example, if an 82 kg athlete with an absolute sweat rate of 1.4 L. h− 1 engaged in a 90 min training session, maximum fluid consumption was calculated as: 1.4 L × 1.5 h = 2.1 L (71 oz) fluid lost / session. Convert to six 15 min drink intervals in a 90 min session = no more than 11.8 oz. consumed every 15 min. Minimum fluid consumption was calculated as follows: 82 kg × 2% = 1.64 kg (57.6 oz. equivalent) allowable sweat loss. 71 oz. lost / session – 57.6 oz. allowed / session = 13.4 oz. (at minimum) that need to be made up via fluid consumption. 13.4 oz. / six 15 min intervals = 2.2 oz. of fluid consumed every 15 min at minimum. This participant would then be advised to consume between 2 oz. to 12 oz. of fluid every 15 min of activity. The bottles used in this study were individually marked for quantity to delineate how much should be consumed at each 15 min interval. More specifically, there would be two markings for each 15 min interval such as “Min-15”, “Max-15”, “Min-30”, and “Max-30,” beginning from the top of the bottle to the bottom. The exact volumes would vary from athlete to athlete and each participant would be instructed to sip their bottle at each interval such that the fluid line was between the minimum and maximum. For athletes engaging in training sessions that exceeded the fluid capacity of the bottle, multiple similarly marked bottles would be provided. Researchers monitored fluid consumption throughout the training session to gauge whether an athlete was on track with their prescribed volume. The composition of fluid that each participant consumed for the PHP was based upon what he or she regularly consumed (NHP fluid), supplemented with a level of NaCl corresponding to the participant’s sweat sodium loss. This usually involved adding NaCl to 32 oz. of a commercially available sports drink or water depending upon which beverage-type was normally consumed by the individual. For example, if an athlete lost 43.5 mmol Na+. L− 1 (1000 mg Na+. L− 1) of sweat and preferred lemon-lime Gatorade G2™, which contains 480 mg sodium/32 oz [22], the researchers would add 520 mg NaCl to the beverage to create a solution that was isotonic relative to sweat sodium content. Lastly, 30 min prior to engaging in a PHP training session, participants were instructed to consume 8 oz of their prescribed beverage.
Neurotracker
Spatial awareness and attention was assessed by 3-dimensional multiple object tracking (3D-MOT) capacity via the NeuroTracker™ system (CogniSens Athletic Inc., Montreal, Canada) as described previously [23, 24]. All testing was conducted in a quiet, dimly lit room with minimal outside distractions and consisted of three 10 min trials interspersed with five minute rest periods. During these assessments, participants wore 3D glasses and were required to track designated objects on a screen as they moved in variable patterns and at subsequently faster speeds. Each of the assessments began at a preliminary speed of 1.0 m per second. The degree of difficulty associated with the assessment progressively increased with every correct answer provided by the participants. In contrast, the level of difficulty associated with the assessment progressively decreased with every incorrect answer. The mean score of the three trials (expressed as tracking speed in meters/second) was used. Each participant performed the neurotracker assessments before and immediately after the training sessions. Changes in spatial awareness and attention were illustrated by comparing pre-training with post-training scores.
Standing long jump
To gauge lower body anaerobic power [25], three standing long jump tests (SLJs) were performed before and after the NHP or PHP training sessions. The pre-training SLJs immediately followed the neurotracker assessments, while the post-training SLJs preceded the neurotracker. Prior to completing the first of the three maximal SLJs, each participant completed two submaximal trials to become familiarized with the protocol. For the test itself, participants were instructed to stand with their feet should-width apart behind a starting line. On the command “ready, set, jump!” the participants executed the jump. Researchers measured each of the jumps from the participants’ rear-most heel and took the average of the three attempts in inches.
Statistical analysis
Wilcoxon Signed Rank test for paired samples was conducted in order to determine if there was a significant difference in the pre and post athletic performance measurements and when participants followed their normal hydration plans compared to when they followed their individualized prescription hydration plans. All data are presented as means ± SD except where otherwise specified. SPSS 23 for Windows (IBM SPSS, Chicago, IL) was used for all statistical analyses. GraphPad Prism® software (version 6.07) was used for graphical displays. A value of P < 0.05 was regarded as statistically significant. Where statistically significant effects were observed, effect sizes (Cohen’s D) were determined by assessing the differences between the two group means based on > 0.2 SD indicating a small effect, > 0.5 a moderate effect, and > 0.8, a large effect [26].