Oral Rehydration Beverages for Treating Exercise-Associated Dehydration: A Systematic Review, Part I. Carbohydrate-Electrolyte Solutions

This systematic review was carried out according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses checklist. The methods for both parts of the review are described in this section. All steps were performed simultaneously for both parts. A protocol was submitted at PROSPERO (CRD42020153077).

Three databases were searched for studies meeting our criteria: the Cochrane Library, MEDLINE (using the PubMed interface), and Embase (using the Embase.com interface). Search strategies were developed by 2 reviewers (V.B. and N.D.B.), using both index terms and text words. We searched the databases for relevant literature up until June 1, 2022. Search strategies can be found in the Supplemental Appendix, available online at https://dx.doi.org/10.4085/1062-6050-0682.22).

After removing duplicates, 2 reviewers (V.B. and N.D.B.) independently screened the titles and abstracts and then assessed the full texts by applying the selection criteria. Reference lists of included studies were screened to identify additional relevant studies. Any discrepancies between authors were discussed, and if necessary, a third reviewer (E.D.B., E.S., or D.Z.) was consulted. Studies were eligible if they met the inclusion criteria outlined in Table 1.

Outcomes were selected a priori by the ILCOR First Aid Task Force. In selecting these outcomes, the task force considered not only known and commonly reported outcomes in relation to hydration but also whether outcomes were clinically relevant and easily measured in real-life situations.

Cumulative urine output is calculated as the sum of the urine volumes measured during the recovery period. When individuals are consuming a fixed volume of a drink, a reduced urine output indicates a better retention of the consumed drink and, hence, is beneficial by enhancing the rehydration process.¹⁴ 

Net fluid balance is calculated as the sum of the volume of sweat loss during exercise and urine loss during exercise and recovery periods subtracted from the volume of drink consumed.¹⁵  Restoration of the net fluid balance is beneficial, and larger values indicated that the sweat loss has been replaced effectively.

Plasma volume is decreased by exertion-related dehydration in proportion to the level of fluid loss due to sweating. Positive changes in plasma volume are beneficial during rehydration. Fluid replacement should be targeted at plasma volume restoration to ensure that circulation and sweating can progress at optimal levels.

Hemoglobin and hematocrit values are based on whole blood and, therefore, are dependent on plasma volume. If a person is severely dehydrated, the hemoglobin and hematocrit values will be higher, and restoring these measures to their baseline values would suggest effective rehydration.

The combination of exercising, heat stress, and hypohydration contributes to high levels of cardiovascular strain (high heart rate). Lower heart rates are beneficial during the recovery period.¹⁶ 

During rehydration, the restoration and maintenance of a high serum or plasma osmolality and serum electrolyte concentrations are desirable and avoid the stimulation of diuresis.¹⁷ 

Because different rehydration periods were used in the studies, we decided to only include time points after completion of drinking water or the rehydration fluid. Given that this review focused on the out-of-hospital setting after exertion-related dehydration, we limited relevant time points to 1 and 2 hours after completion of drinking; any later time point was excluded (except for cumulative urine volume, for which the endpoint of the study was included). If 1- or 2-hour results were not reported, any other time point within 2 hours after completion of drinking was included. For patient satisfaction outcomes, we included time points starting from immediately after the completion of drinking until 2 hours after the completion of drinking. In addition, studies in which authors only reported mean values without SDs, effect sizes, or P values were excluded.

Two reviewers (V.B. and N.D.B.) independently extracted data on the study population, intervention, outcome measures, and study limitations. Data are presented as mean differences with 95% CIs, where possible. If no sufficient data were available to calculate 95% CIs, only mean differences and P values were reported. Significant P values were <.05. GRADEPro software was used to tabulate the data.¹⁸ 

We assessed the certainty of evidence according to the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach.¹⁹  The certainty of evidence can be downgraded for limitations in study design, inconsistency, indirectness, imprecision, or publication bias. Limitations in study design were assessed using the Risk of Bias 2 tool for randomized studies and the Risk of Bias in Non-Randomized Studies of Interventions tool for nonrandomized studies.^20,21  The initial certainty of evidence of all studies was high. Based on the assessment, the final level of certainty can be high (ie, the authors have high confidence that the true effect is similar to the estimated effect), moderate (ie, the authors believe the that the true effect is probably close to the estimated effect), low (ie, the true effect might be markedly different from the estimated effect), or very low (ie, the true effect is probably markedly different from the estimated effect).¹⁹  The certainty of evidence is reflected in the wording of the conclusions or recommendations, as stated in the GRADE methodology.²² 

A total of 4445 references were identified. After removing duplicates, we screened 3485 articles based on title and abstract and then assessed 107 full-text articles. Of these, 84 studies were excluded. A total of 20 studies were included in this review, with 12 studies examining rehydration by CE solutions and 8 studies examining both CE and alternative solutions.^16,23–41  Authors of the 3 remaining studies examined alternative solutions only, and the results can be found in Part II of this review.¹³ 

We only discuss the results of the 20 studies in which authors examined CE solutions in this paper. The characteristics of these studies, including study design, population, relevant comparisons, and outcome measures, are summarized in Table 2. A total of 15 studies were randomized controlled trials (RCTs), and 5 studies were nonrandomized studies. Ten studies were published between 2011 and 2021, and 10 studies were published between 1992 and 2010.^16,23–41  All were crossover studies with a total of 273 (233 male, 40 female) healthy children (age ≥ 7 years) and young adults performing exercise in a controlled environment until a predefined point of hypohydration was reached.^16,23–41 

The percentage carbohydrate in the solutions ranged between 0% and 8.75%, and the type of carbohydrate used was either isomaltulose, glucose, a combination of glucose with maltodextrin or fructose or sucrose, or unknown. The volume of fluid ingested for rehydration varied between 100% and 200% of body mass lost during exercise (Table 2). In most studies, participants drank 100% of the lost body mass.^{16,23–29,33,34,41}  In 3 studies, participants drank 120% of the body mass lost; in 4 studies, 150%; and in 1 study each, 125% and 200%.^{30,32,35–40} 

The synthesis of findings of all included studies is summarized in Tables 3 through 5, and a narrative overview of the results is given below. Given the large heterogeneity between studies (ie, different protocols for dehydration, large variability in time to rehydrate, and time points measured), it was not possible to perform meta-analyses.

Results for 4.0% to 9.0% CE solution are summarized in Table 3.

Volume/hydration status was measured as cumulative urine output, net fluid balance, plasma volume change, or hematocrit. We identified 9 RCTs and 4 nonrandomized studies, including 200 participants, in which authors assessed the effect of 4.0% to 9.0% CE solution compared with water on volume/hydration status.^{23–25,27–29,31–34,36,40,41}  A decrease in cumulative urine output was shown with 4%, 6%, and 6.6% CE solutions.^{24,25,27,31,36}  However, authors of 5 RCTs and 1 nonrandomized study did not show a difference for cumulative urine output, comparing a 6%, 6.5%, 6.9%, 7%, 8%, or 8.75% CE solution with water.^{23,32–34,40,41}  Authors of 4 RCTs did not show a difference in net fluid balance 1 hour after participants finished drinking a 6%, 6.9%, 7%, or 8% CE solution.^32,34,40,41  In addition, authors of 3 RCTs did not show a difference in net fluid balance 2 hours after participants finished drinking a 6%, 7%, or 8% CE solution.^34,40,41  Authors of 5 RCTs and 2 nonrandomized studies measured plasma volume (change) 35 minutes to 2 hours after participants finished drinking a 6% to 6.9% CE solution, but authors of only 2 studies showed a difference compared with water.^{23,24,27–29,31,32}  Authors of 1 nonrandomized study did not report a difference for hematocrit 30 minutes after participants finished drinking an 8.75% CE solution.³³ 

Authors of 3 RCTs and 1 nonrandomized study, including a total of 53 participants, assessed vital signs (ie, heart rate) and did not show a difference at different time points after drinking a 5.0% to 6.6% CE solution compared with water.^16,23,31,39 

Authors of 3 RCTs and 4 nonrandomized studies, including a total of 86 participants, assessed serum or plasma osmolality or serum sodium concentration.^{23,25,27,31–33,39}  Authors of 2 nonrandomized studies showed an increase in serum osmolality 1 and 1.25 hours after participants finished drinking a 6% CE solution; however, authors of 1 RCT and 2 nonrandomized studies did not report a difference in serum osmolality 30 minutes, 1 hour, or 2 hours after participants finished drinking a 6% to 8.75% CE solution.^25,27,32,33  In addition, authors of 2 RCTs and 1 nonrandomized study did not show a difference in plasma osmolality 1 to 2 hours after participants finished drinking a 5% to 6.6% CE solution compared with water.^23,31,39 

Authors of 1 RCT showed an increase in serum sodium concentration 1 hour after participants finished drinking a 6.9% CE solution compared with water.³²  However, authors of 2 nonrandomized studies did not report a difference in serum sodium concentration 1.25 hours after participants finished drinking a 6% CE solution or 30 minutes after participants finished drinking an 8.75% CE solution.^25,33 

Patient satisfaction was assessed as thirst perception, nausea, stomach fullness, or stomach upset. Authors of 5 RCTs and 1 nonrandomized study, including a total of 95 participants, assessed patient satisfaction.^{31,32,36,39–41}  Authors of 4 RCTs did not show a difference for thirst immediately after participants drank a 4%, 5% to 6%, 6%, or 8% CE solution compared with water.^36,39–41  Similarly, 3 RCTs did not show a difference for thirst 1 or 2 hours after participants finished drinking a 5% to 8% CE solution.^39–41  Authors of 1 RCT and 1 nonrandomized study did not report a difference for thirst at any time when comparing consumption of a 6.6% or 6.9% CE solution with water.^31,32  Authors of 3 RCTs and 1 nonrandomized study did not show a difference for stomach fullness immediately, 1 hour, or 2 hours after participants finished drinking a 4% to 8% CE solution.^31,36,40,41  Authors of 1 RCT did not show a difference for nausea or stomachache immediately after participants finished drinking a 4% CE solution.³⁶  A difference for stomach upset immediately, 1 hour, or 2 hours after participants finished drinking a 5% to 6% CE solution compared with water could not be demonstrated in 1 RCT.³⁹  In addition, authors of 2 RCTs did not show a difference in bloating immediately, 1 hour, or 2 hours after participants finished drinking a 5% to 6% CE solution.^39,40  Finally, authors of 1 RCT and 1 nonrandomized study did not report a difference in abdominal discomfort immediately or 1 hour after participants finished drinking a 6.6% to 6.9% CE solution.^31,32 

No studies in which authors addressed the need for advanced care were identified.

Results for 0% to 3.9% are summarized in Table 4.

Volume/hydration status was measured as cumulative urine output, net fluid balance, plasma volume change, hematocrit, or hemoglobin.

We identified 5 RCTs and 1 nonrandomized study, including 53 participants, in which authors assessed the effect of 0% to 3.9% CE solutions on volume/hydration status compared with water.^{23,26,30,35,37,38}  A decrease in cumulative urine output was shown after rehydration with a 0% CE solution (saline) and a 3.7% CE solution.^30,35  However, when a 2%, 3.2%, or 3.9% CE solution was compared with water, no difference was shown.^23,37,38  In addition, no difference was shown in net fluid balance 1 hour after rehydration with a 0% (saline) to 3.9% CE solution compared with water.^30,35,37,38  No difference in net fluid balance was shown 2 hours after rehydration with a 0% (saline) or a 3.9% CE solution.^30,38  No difference was shown for plasma volume (change) 1 hour after rehydration with a 2% CE solution.²³  However, in 1 RCT, a difference in plasma volume change was shown 1 hour after rehydration with a 3.7% CE solution.³⁵  In 1 nonrandomized study, no difference was reported in hematocrit or hemoglobin 1 hour after rehydration with a 1.8% CE solution.²⁶ 

Authors of 1 RCT did not demonstrate a difference in heart rate after rehydration with a 2% CE solution compared with water.²³ 

Authors of 1 RCT and 1 nonrandomized study showed increases in both serum osmolality and serum sodium concentration 1 hour after rehydration with 1.8% and 3.7% CE solutions, respectively, compared with water.^26,35  However, authors of 1 RCT did not show a difference in serum osmolality or serum sodium concentration when comparing drinking a 3.2% CE solution with water.³⁷  Furthermore, authors of 2 RCTs did not show a difference in plasma osmolality 1 or 2 hours after rehydration with a 2% or 3.9% CE solution.^23,38 

Patient satisfaction was measured as thirst perception, nausea, gastric fullness, or cramps. Authors of 2 RCTs did not show a difference for thirst, gastric fullness, cramps, or nausea immediately after or 1 hour after rehydration with a 3.2% or 3.7% CE solution compared with water.^35,37  In addition, authors of 1 RCT showed no difference for the perception of thirst at any time when comparing rehydration via a 3.9% CE solution with water.³⁸ 

No evidence was identified to address the need for advanced care.

An overview of the risk of bias of the RCTs, as assessed using the Risk of Bias 2 tool, is given in Table 5.²⁰  Only 1 RCT had an adequate randomization process.³²  No other authors of RCTs provided information about the randomization or allocation process. In most studies, blinding of participants, investigators, or outcome assessors was not described. It may be assumed that blinding of participants was not possible due to difference in taste between the intervention and the comparison. For objectively assessed outcomes, such as urine output, plasma volume, and hematocrit, lack of blinding would probably not pose a major problem. However, knowledge about the intervention may influence participants’ subjective outcomes such as feelings of stomach fullness, thirst perception, nausea, and bloatedness. Authors of 1 study performed adequate blinding by creating a placebo drink with the same taste as the intervention drink.³² 

No concerns regarding missing outcome data were present in any of the studies. Most studies in which authors addressed subjective outcomes had high risk of bias for measurement of outcomes because participants were not blinded and, for the subjective outcomes, participants were also the outcome assessors.^35–41  Only the study by Wong et al had low risk of bias for the measurement of subjective outcomes because they used a placebo drink as a comparison.³²  Regarding the selection of reported outcomes, bias was difficult to assess because a preregistered protocol was not available for any of the studies. However, serious concerns existed for 3 studies in which authors collected urine but did not report cumulative urine output, which was an important outcome in all other studies.^28,29,39  Overall, the included studies exhibited some serious limitations in their study designs, mainly due to risk of bias in the measurement of subjective outcomes. An overview of the risk of bias of the nonrandomized studies, as assessed using Risk of Bias in Non-Randomized Studies of Interventions, is given in Table 6.²¹  As the sequence of interventions was not randomly assigned in these studies, some concerns of bias existed due to confounding arising from selection bias. In the studies in which authors measured subjective outcomes (ie, patient satisfaction outcomes), a high risk of bias was present concerning the measurement of these outcomes, as they were self-reported.^25,31  One study was assessed as having high risk of bias in the selection of the reported results, as the authors did not seem to have undertaken a paired or repeated-measures analysis, and some concerns existed regarding the analytical methods used to analyze the paired data.³³ 

The certainty of the body of evidence was assessed using GRADE.¹⁹  Details can be found in Tables 3 and 4. Overall, the certainty was downgraded (−1) for limitations in study design, based on the risk-of-bias assessment described above. Furthermore, the certainty was further downgraded because of imprecision due to limited sample sizes or lack of data. In addition, in some cases, publication bias was strongly suspected because the studies were often funded by the company providing the CE drinks, who therefore had a direct interest in the effectiveness of these rehydration beverages. This led to concerns about the independence of the decision-making in these trials. Overall, the certainty of the body of evidence was low or very low.

Exercise may lead to excessive body fluid loss of up to 1 L/h on average, with higher rates extending to 2 L/h and even 6 L/h.^25,42  To avoid serious hypohydration, with symptoms such as dizziness, decreased sweat rate, confusion, rapid breathing, or fast heartbeat, individuals need to replenish this fluid loss as soon as possible.¹  Guidance about the amount of fluid that should be taken in is available, but no clear guidance on the type of rehydrating fluid is available.¹⁰  Many different types of fluids, such as CE solutions, milk, coconut water, and even beer, have been suggested. The choice of beverage will often be made based on what the dehydrated person is willing to drink and what is accessible at the time of need. Furthermore, first aid providers, who are often recruited to assist at first aid stations at sporting and challenge events, may not be able to determine the exact quantity or percentage of fluid loss or the volume required for adequate rehydration. In Part I of this 2-part systematic review, we searched for the best available evidence on the use of CE solutions to treat exertion-related dehydration in an out-of-hospital setting.

Of 4445 references identified, 20 studies in which authors compared CE solutions of different percentages with water were included in Part I of this systematic review. Authors of 13 studies (9 RCTs and 4 nonrandomized studies) assessed the effect of a 4.0% to 9.0% CE solution compared with water on volume/hydration status.^{23–25,27–29,31–34,36,40,41}  Authors of 5 studies showed a decrease in cumulative urine output that could not be demonstrated in 6 other studies.^{23–25,27,31–34,36,40,41}  In addition, a difference was not demonstrated for net fluid balance, plasma volume change, hematocrit, or vital signs. Authors of 2 of 4 studies showed an increase in serum osmolality 1 and 1.25 hours after rehydration using a 4.0% to 9.0% CE solution.^25,27,32,33  A difference in plasma osmolality was not demonstrated. In 1 of 3 studies, an increase in serum sodium concentration was shown.^25,32,33  No difference was observed for any of the patient satisfaction outcomes.

Authors of 6 studies assessed the effect of 0% to 3.9% CE solutions compared with water.^{23,26,30,35,37,38}  Authors of 2 of 5 studies reported a decrease in cumulative urine output.^{23,30,35,37,38}  In addition, no difference in fluid balance, hematocrit, or hemoglobin was demonstrated. Furthermore, authors of 2 studies showed an increase in serum osmolality and serum sodium concentration.^26,35  However, no difference in serum or plasma osmolality or serum sodium concentration was demonstrated in 2 other studies.^37,38  In addition, no difference was observed for any of the patient satisfaction outcomes.

Although variability was present among the identified studies, a potential beneficial effect of using CE drinks compared with water was seen for many of the reviewed outcomes. Differences noted in urine production between the various drinks used for rehydration could be a result of the composition of the drinks. Ingested drinks with high energy content (ie, from higher carbohydrate concentration) empty from the stomach more slowly than drinks containing less or no energy products. Therefore, they will potentially reduce or delay diuresis when compared with water. In other words, when large volumes of dilute drinks such as water are consumed, a fall in serum electrolyte concentrations and osmolality may occur, and urine production and excretion are stimulated. However, a high electrolyte concentration in a rehydration drink will maintain high serum or plasma electrolyte concentration and osmolality and thereby reduce the excretion of dilute urine. Consequently, low cumulative urine outputs and, hence, high net fluid balances can be associated with improved fluid retention and thus effective rehydration.^41,43 

In this systematic review, we focused on the effectiveness of ingesting fluids for replacing fluid loss after exercise. Although this strategy may be effective in a first aid setting (ie, over a short recovery period), foods are also commonly consumed after exercise to maximize total recovery.⁴³ 

A strength of this review is that rigorous and transparent methods using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses and GRADE methodology were used to identify the best available evidence on this topic. Moreover, this is the first systematic review in which the effectiveness of CE solutions for rehydrating individuals after exercising compared with the criterion standard of water is addressed. Finally, the findings from this review were used as a direct scientific basis by the ILCOR First Aid Task Force to develop the Consensus on Science with Treatment Recommendations.^44,45  This consensus document allows international first aid guideline organizations to provide recommendations for professional and recreational athletes, their families and coaches, and first aid responders.

This study had several limitations. We identified only low to very low certainty evidence because of limitations in study design, imprecise results (due to limited sample sizes and lack of data), and publication bias. Furthermore, all studies were conducted in a laboratory environment, with exertion-related dehydration induced by asking participants to strictly follow standardized exercising protocols, with or without a thermal vest. Studies of rehydration after extreme events such as ultramarathons were not identified.

The rehydration protocols varied greatly between studies. Differences existed not only in the percentage of carbohydrates in the study solutions but also in the timing of ingestion of the fluid during or after exercise and in the volume ingested (ranging from 100% to 200% of the body weight loss during exercise). In addition, different rehydration intervals were used, and outcomes were measured at different times during or after completion of drinking. Therefore, we decided a priori to only include times after completion of drinking rehydration solutions. Given that this review focused on the out-of-hospital setting after exertion-related dehydration, relevant times were limited to 1 and 2 hours after participants completed drinking rehydration solutions; we excluded any later time points except for cumulative urine volume, for which the endpoint of the study was included and varied between 2 and 5 hours, during which participants remained in the laboratory and refrained from consuming any foods or drinks other than what was included in the study. For patient satisfaction outcomes, we included times beginning immediately after completion of drinking until 2 hours after completion of drinking rehydration solutions. We also specifically looked at sodium levels reported after rehydration in the included studies.

Drinking 0% to 3.9% and especially 4% to 9% CE solutions may be effective for restoring volume/hydration status after exercising when whole foods are not available. Limited evidence suggests that rehydration for exertion-related dehydration or hypohydration using commercial 4% to 9% CE drinks or, if these are not available, 0% to 3.9% CE drinks does occur and is acceptable. However, if clean, drinkable water is available, its cost, relative to CE drinks, makes it an acceptable alternative.

Drs Borra, De Brier, and De Buck are employees at the Belgian Red Cross and receive no other funding. One of the activities of the Belgian Red Cross is providing first aid training to laypeople. This work was made possible through funding from the Foundation for Scientific Research of the Belgian Red Cross (Mechelen, Belgium). The ILCOR and the American Heart Association provided the resources to assemble the task force and manage records and data. In addition to Drs Borra, Berry, Zideman, and Singletary, members of the ILCOR First Aid Task Force include Jestin N. Carlson, Pascal Cassan, Wei-Tien Chang, Nathan P. Charlton, Therese Djarv, Matthew Douma, Jonathan L. Epstein, Craig Goolsby, David S. Markenson, Daniel Meyran, Peter Morley, Michael Nemeth, Aaron Orkin, and Tetsuya Sakamoto.

Supplemental Appendix. Search Strategy.

Found at DOI: https://dx.doi.org/10.4085/1062-6050-0682.22