- It is common for athletes to show up to training already dehydrated and consume inadequate fluid levels despite the ready availability of water or sports drinks [8, 11, 14, 15, 16]
- People sweat at different rates, and sodium losses are unique to individuals 
- losing 1–2% of body mass through sweat can yield
- Sodium is the main electrolyte lost through sweat
- Relying on thirst signals often result in dehydration during exercise [8, 11, 13]
- Randomized, crossover study
- Fifteen collegiate athletes
- 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
- athletes that were currently engaged in heavy, in-season, sports-specific training sessions
- assessed for hydration habits and knowledge
- 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)
- Mean and peak heart rate were recorded throughout the entire training session
- ADDITIONAL DETAILS HERE
- A range of fluid consumption per 15 min was calculated (Min vs. Max in fl.oz) sufficient to prevent mild dehydration (2%)
- participants were advised to drink between the min and max amount of fluid at the agreed upon intervals from pre-measured bottles
- exact volumes varied from athlete to athlete
- The composition of fluid that each participant consumed for the PHP was supplemented with a level of NaCl corresponding to the participant’s sweat sodium loss
- Neurotracker (Spatial awareness and attention)
- Standing Long Jump (lower body anaerobic power)
- Training Session (monitoring sweat losses and HR Recovery)
- decrease in jumping distance following a NHP
- increase in jumping distance following a PHP
- PHP improved attention and awareness
- NHP did not significantly impact movement tracking, but result was negative
The margin of difference between athletes on a NHP vs. athletes on a PHP is significant
- PHP improved HR Recovery significantly at both 10 & 15 min intervals, post training
So the authors of this study ultimately concluded that a PHP was effective in improving heart rate recovery, attention and awareness, and mitigating the loss in anaerobic power in collegiate athletes from various sports.
Their conclusions are supported previous research investigating the effectiveness of hydration plans against ad libitum fluid consumption.
Results were not confounded by carbohydrate content of fluid replacement drinks used in the study because beverage consumption of each participant was held consistent between NHP and PHP training sessions.
"In all cases, the final [Na+] of the PHP beverages were higher than any of the sports drinks available to our athletes"
For activity lasting 120 min or less, commercial drinks are likely sufficient. However, in this study benefits were seen in athletes who trained 70 min - 2 hours.
The authors listed some modest and understandable limitations of the study. You can view them HERE.