The results of this study both partially agreed and disagreed with our hypotheses. The AE and CE beverages rehydrated about equally; however, they were also equal to the FW beverage. The purpose of this investigation was to compare the rehydration capabilities of AE, CE, and FW beverages. The primary finding of this study was USG for the AE condition at 4 hours returned to baseline following dehydration while CE and FW trials resulted in more diluted urine (lower USG values). Additionally, there were no significant interactions for percent body weight loss, urine volume, drink volume, or fluid retention for any condition. On the basis of these findings, it would appear that all conditions were equally capable of rehydration. However, because an equal fluid retention was observed between all conditions, yet USG did not show the same decremented pattern (dilution at 4 hours) as CE or FW, it appears that the AE beverage increased intracellular hydration more favorably than either CE or FW since the urine was not as diluted at 4 hours in the AE trial with equal urine volume (Table 2). Thus at 4 hours post-dehydration, subjects experienced the same USG as pre-testing, while at the same time-point, CE and FW trials resulted in significantly lower USG values compared to pre-testing which suggests increased diuresis through urine and less cellular retention. Furthermore, based on the USG results, it is hypothesized that if urine volumes were collected beyond 4 hours post-dehydration (3 hours post rehydration) that both CE and FW would have resulted in greater urine volumes compared to AE and USG would eventually return to baseline. Since there was an apparent increase in water in the bladder at 4 hours post-dehydration for CE and FW and USG had not returned to baseline, the body may continue to shed water until USG returned to baseline. This hypothesis can be supported further by the reduced urine volume in the AE treatment compared to CE and FW as there was a trend at 4 hours post-dehydration for AE to retain more water (Table 2).
No significant interactions were observed between conditions for urine volume, drink volume, or fluid retention at any time point. These results are contradictory to those previously reported [16, 19, 28]. In each of these studies, greater electrolyte content produced greater fluid retention. A greater fluid retention was expected for the AE and CE conditions compared to the FW. However, it is possible that the 121.3 mg of potassium in the FW condition was sufficient to produce insignificant results compared to the greater overall sodium and potassium content of the AE and CE conditions. In agreement with the present results, Mitchell and colleagues  examined 4 different relationships of drink volume and sodium concentration, and it was apparent that drink volume had a greater influence on rehydration compared to sodium content. However, it is important to note that the low sodium condition contained half of the sodium of the high sodium conditions, and the beverages were not devoid of electrolytes. From these results as well as our own, there is evidence for a limited amount of electrolyte present in the rehydration beverage to be capable of modest rehydration.
While sodium is the primary electrolyte of the extracellular space and has a pronounced role in rehydration , potassium, the primary electrolyte of the intracellular space, has demonstrated similar rehydration capabilities [26, 27]. Yawata  examined the replacement of body fluids in each fluid compartment for sodium and potassium in dehydrated rats. It was observed that sodium preferentially rehydrated the extracellular space and potassium rehydrated the intracellular space. It was also noted that rehydration of the intracellular space took precedence over rehydration of the extracellular space, potentially indicating the relative importance of rehydrating the intracellular space. In the present study, the absolute amount of potassium consumed was similar between the CE and FW conditions. However, the AE condition consumed nearly twice as much as the CE and FW conditions, 643.6 mg vs 354.5 mg and 379.3 mg, respectively. The current findings indicated less diluted urine 4 hours following dehydration and a USG not significantly different than pre-testing for the AE condition only. Considering this and the absence of differences observed in percent body weight loss, urine volume, drink volume, or fluid retention, it appears that the AE beverage favored rehydration of the intracellular space. This is in agreement with Nielsen et al.  who observed preferential rehydration of the intracellular space with potassium compared to sodium. Additionally, the aforementioned researchers discovered that sodium increases plasma volume to a greater extent than potassium, which may also explain the differences observed for USG in the present study.
In addition to potassium differences, the AE beverage contains BCAAs as well as taurine and coconut water powder. While this may be the first study to investigate BCAAs effects on hydration, taurine has been previously examined. Muscles expel taurine during contraction [38, 39]. Cuisinier et al.  examined plasma taurine levels during exercise in both hydrated and dehydrated states, observing a 32% greater increase in plasma taurine levels in the hydrated condition. This led researchers to conclude taurine is released due to an osmoregulatory process and to be taken up by other cells which have a role in osmotic regulation. Coconut water has demonstrated efficacy over water yet with mixed results. It has been observed as equal to a CE beverage for rehydration [20, 41]. In contrast, Saat et al.  have reported greater rehydration effects with a CE beverage, despite the coconut water condition consuming more electrolytes, yet it has been shown as well that the addition of sodium to coconut water increases rehydration .
From the present results, it is also possible that carbohydrates were not necessary for rehydration, as no differences between the AE, CE, and FW beverages were observed for fluid retention. Few researchers have examined carbohydrates for rehydration in the absence of electrolytes. However, lambert et al.  conducted such comparison and found no significant differences between beverages containing or not containing carbohydrates, concluding that the benefit of carbohydrates is glycogen replenishment and taste, not rehydration. In agreement, varying glucose concentrations had no effect on rehydration when controlling for electrolyte content .
Unique to this study, subjects were rehydrated initially with the test beverages using a practical dose and followed up with water to replace 150% of lost body weight, which may more adequately simulate athletes’ common practice. Wherein, an athlete may drink one bottle (24 fl oz; ~26 fl oz were used in this study) of a recovery drink then drink water the remainder of the day or several hours thereafter. In this practical model, there were no observed differences in rehydration capability between all test beverages, yet the AE and CE beverages were anticipated to rehydrate more adequately than FW, as hypothesized. Therefore, it may be necessary for the dehydrated athlete to drink uncommonly more of an AE or CE beverage than they would expect to achieve the results observed in past literature [16, 19, 28]. However, the practicality of ingesting such large amounts of a CE beverage warrants concern as high doses of simple sugars may not be advantageous to all athletes.
The present study provides evidence for electrolytes being the primary determinant of rehydration when ingesting practical/real world volumes, regardless of calorie or macronutrient content. Future research is needed to examine BCAAs effects on rehydration without electrolytes or other confounding factors compared to carbohydrate and/or electrolytes. It may also be of interest to examine recovery of performance following dehydration using an AE compared to a CE beverage.