Findings from the present investigation indicate that all of the tested beverages are capable of promoting rehydration after one hour of dehydrating exercise. With few exceptions at selected time points, findings for all rehydration variables were essentially the same when comparing the carbohydrate-electrolyte sport drink, coconut water (concentrated and not from concentrate), and bottled water. Moreover, no differences were noted in treadmill performance during the rehydration period. These data are specific to a sample of young, exercise-trained, healthy men.
Maintaining hydration status is vital for athletes and can directly impact exercise performance . As such, many studies have been conducted to determine the optimal rehydration strategies. While water intake is likely an adequate rehydration approach for many individuals, others (e.g., athletes involved in vigorous training) may require intake of water-carbohydrate or carbohydrate-electrolyte mixtures , in addition to other nutrients . Such an approach has been reported to be superior to water alone and is generally considered the ideal recommendation for individuals engaged in long duration, strenuous bouts of acute exercise [2, 4].
Related to the above, the use of coconut water has been considered by many, as this beverage provides a natural source of carbohydrate and electrolytes . Specifically, coconut water has been reported to provide sugar (~1 g ∙ dL-1), potassium (~51 mEq ∙ L-1), sodium (~33 mEq ∙ L-1), and chloride (~52 mEq ∙ L-1) ; however, this may vary depending on species of coconut palm. Coconut water has been reported to provide hydrating effects similar to those of carbohydrate-electrolyte sport drinks [16–18].
Saat and colleagues used a cross-over study to assess the effectiveness of fresh young coconut water and a carbohydrate-electrolyte beverage, compared to water on measures of whole body rehydration and blood volume restoration during a two hour rehydration period following a bout of dehydrating exercise . A sample of eight young men participated and consumed the assigned beverage at a volume equal to 120% of the fluid loss during exercise. No statistically significant differences were noted between conditions for any outcome measure; however, blood volume restoration was noted to be slightly greater for coconut water. This same group reported similar findings in a follow-up study published in 2007 , using the same volume of beverages (120% of fluid loss during exercise). More recently, Idárraga and Aragón-Vargas studied the rehydrating effect of coconut water following exercise . On three different days, six men and five women were dehydrated to approximately 2% body mass by exercising in a climate-controlled laboratory. On each day and in random order, subjects were rehydrated with fresh coconut water, a sport drink, or plain water using a volume equal to 120% of body weight lost during exercise. Subjects were then monitored for three hours, with urine collection every 30 minutes. No differences were noted between coconut water and sport drink for urine volume or fluid retention (both were better than plain water).
These above studies focused exclusively on hydration measures, following a period of dehydrating exercise and consumption of the assigned beverage, while not emphasizing exercise performance during the rehydrating period. The present study, using a similar fluid volume as used previously, extends these findings by noting similar exercise performance results for natural coconut water (concentrated and not from concentrate) and a carbohydrate-electrolyte sport drink. For most athletes and coaches, this finding is likely of most importance. Our data indicate that coconut water can provide similar benefits as compared to a typical sport drink in terms of exercise performance (as measured based on treadmill time to exhaustion), in addition to measures of hydration. That being said, one potential concern is subject tolerance to coconut water in such high volumes. Subjects reported feeling somewhat bloated and experienced mild stomach upset with the two forms of coconut water used in the present investigation (Table 7), which is likely due to the high volume of fluid required to be consumed in such a short period of time. As with most beverages, individual tolerance to coconut water should be determined prior to use.
It should be noted that this study explored many endpoints at many time-points, each being compared between four products. Consequently, many hundreds of separate pairwise comparisons were carried out, each generating a p value, raising the issue of multiplicity and inflated Type-1 error. No multiple-test adjustments (Bonferroni or other) were applied - it would have been unrealistic and unproductive to try to establish a study-wide 0.05 alpha level, which would have required impossibly small p-values on individual tests. So it should be kept in mind that each individual p value has a one-in-twenty chance of being nominally significant (p < 0.05) purely from random fluctuations. Conclusions of relative efficacy among the different products should not be based simply on isolated p values, but rather on a consideration of the complete set of data for each endpoint. Likewise, observed values were not simply put into a repeated-measures ANOVA to test for overall changes over time - most endpoints displayed very significant changes at certain time points (such as from baseline to immediately post-dehydrating exercise). It was much more appropriate to examination the changes (and differences in mean changes between products) only over certain physiologically-meaningful intervals, and individual t-tests were much more directly interpretable, and therefore more useful, in assessing these specific changes.