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Original Research

Effect of Electrolyte Addition to Rehydration Drinks Consumed After Severe Fluid and Energy Restriction

James, Lewis J.1; Shirreffs, Susan M.1,2

Author Information
The Journal of Strength & Conditioning Research: February 2015 - Volume 29 - Issue 2 - p 521-527
doi: 10.1519/JSC.0000000000000657
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A number of sports separate competitors into categories based on their body mass, with the purpose of these weight categories being to allow competitors of various body sizes to compete at a similar level by minimizing differences in size and strength (6). There is typically some time between weigh-in and competition, but this varies between sports from as low as 1–2 hours in judo and lightweight rowing up to 24–30 hours in professional boxing and mixed martial arts. Many of these athletes practice rapid weight loss (RWL) in the days leading up to the weigh-in to allow them to compete in a category below their natural body mass (32).

The main techniques used by weight category athletes to induce RWL involve severe restriction of fluid and energy intake, as well as activities that promote sweat losses (9,17,18,20,30,31) and consequently reduce body water stores. With the prevalent use of such techniques among weight category athletes, rehydration between weigh-in and competition is vital to prevent any potential decrement in performance (10).

Although rehydration after sweat induced dehydration has been extensively examined, rehydration after a period of severe fluid and energy restriction (FER) has not been studied. Sweat that is produced during exercise or heat exposure contains relatively large amounts of sodium (30–80 mmol·L−1) and chloride (30–70 mmol·L−1), but small amounts of potassium (3–7 mmol·L−1) (27). Severe FER also results in a negative electrolyte balance, as electrolyte losses continue despite little or no intake, with 24-hour severe FER producing large negative balances of sodium (∼60 mmol), chloride (∼55 mmol), and potassium (∼40 mmol) (7).

After exercise-induced dehydration, it has been shown that the addition of sodium to rehydration drinks reduces urine output in a dose-dependent manner (9,28) and that sodium balance must be restored for complete restoration of fluid balance (28). As the major intracellular cation, the addition of potassium to a rehydration drink might enhance water retention in the intracellular space (33), but data from human studies are inconsistent (14,24).

Rehydration after a period of severe FER has not been investigated and with the large negative balances of both sodium and potassium that such manipulations in dietary intake induce, replacement of sodium and potassium during rehydration might be required for complete restoration of fluid balance. The aim of this study was therefore to compare the retention of drinks containing sodium chloride and potassium chloride to a low electrolyte placebo drink. It was hypothesized that both rehydration drinks would reduce urine output compared with the placebo drink.


Experimental Approach to the Problem

The focus of this study was to examine the effectiveness of 3 different rehydration drink compositions, ingested after a 24-hour period of severe FER. Subjects completed all 3 conditions in a counterbalanced manner after identical restriction periods. Urine output was measured and used to quantify the proportion of each drink that was retained.


This investigation was approved by the University's Ethical Advisory Committee (Reference No. R07-95) and 12 healthy subjects (6 male and 6 female) volunteered to participate. Subjects signed an institutionally approved informed consent document after having the benefits and risks of the study explained. The subjects' physical characteristics were age, 24 (4) years; body mass, 70.62 (15.08) kg; and height, 1.72 (0.13) m. Each subject completed a familiarization trial followed by 3 experimental trials, with each experimental trial consisting of a 24-hour period of FER (5 ml water per kilogram body mass, 21 kJ·kg−1 body mass) (FER), followed by a 2-hour rehydration period and a 4-hour monitoring period. During the rehydration period, subjects received 1 of 3 drinks (Table 1). Drinks were a sugar-free lemon squash (P) or the P drink with the addition of 50 mmol·L−1 sodium chloride (Na) or 30 mmol·L−1 potassium chloride (K). Trials were separated by a minimum of 6 days and were administered in randomized counterbalanced order. Trials were undertaken during the months of January, February, March, and April in Loughborough, United Kingdom, with mean daily temperatures of 7.2° C (3.6° C).

Table 1
Table 1:
Rehydration drink osmolality (in milliosmoles per kilogram) and electrolyte content (in millimoles per liter).*


The familiarization trial was completed after at least a 3-hour fast and was used to familiarize subjects with the rehydration and urine collection protocols involved in the study. Subjects arrived at the laboratory and provided a urine sample, after which their nude body mass was measured. They then drank a volume of lemon-flavored squash equivalent to 3% of their body mass (1 L fluid per kilogram body mass) in 6 equal aliquots every 20 minutes. Three percent was selected as it was estimated from the study of James and Shirreffs (7) that this would be the approximate amount of fluid subjects would be required to drink during the experimental trials. Additional urine samples were obtained halfway through and at the end of drinking, with a final urine sample collected 1 hour later.

For the 24-hour period before the first experimental trial, subjects recorded their dietary intake and physical activity in a diary and replicated these patterns in the 24-hour preceding subsequent experimental trials. Subjects also refrained from any strenuous physical activity and alcohol ingestion on the day before and during each experimental trial.

For each of the 3 experimental trials, subjects completed a 2-day (30 hours) experimental protocol. Subjects arrived at the laboratory in the morning of day 1 (−24 hours) after a 12-hour overnight fast, except for 500 ml plain water consumed 2 hours before arrival. On arrival at the laboratory, subjects voided their bladder, collecting the entire volume. Subject's nude body mass was then measured, before they were provided with their food (American Hard Gums; Tesco, Cheshunt, United Kingdom) and water for the day. Subjects left the laboratory and consumed only the food and water provided to them ad libitum over the day, but before they went to bed that night. Food and water provided during FER amounted to 1503 (282) kJ, 7 (1) g protein, 83 (16) g carbohydrate, 0 (0) g fat, 1.3 (0.2) mmol sodium, 0.2 (0.0) mmol potassium, 0 (0) mmol chloride, and 365 (76) ml water. Subjects also collected all urine produced over the 24-hour period and refrained from any physical activity for the duration of each trial. Subjects then returned to the laboratory the following morning (0 hours), exactly 24 hours after the initial visit. On arrival, subjects provided another urine sample, and their nude body mass was measured. The change in body mass over the 24-hour FER period was then used to calculate the volume of drink ingested during rehydration. The total volume of drink ingested (in liters) was calculated as 125% of the body mass loss (BML) (in kilograms) and was consumed in 6 equal aliquots every 20 minutes over the 2 hours. After rehydration, subjects remained in the laboratory for further 4 hours.

At hourly intervals throughout the rehydration (1 and 2 hours) and monitoring (3, 4, 5, and 6 hours) periods, total void urine samples were collected. For each urine sample, subjects were instructed to completely empty their bladder and collect the entire volume of the void. If subjects needed to urinate before the designated sample collection time, the urine was collected and pooled with urine produced at the sample time point. The urine produced over the 24-hour FER was weighed to the nearest g, and the volume was calculated from the urine sample's specific gravity, determined from its osmolality using the equation of Armstrong et al. (2).

An aliquot of each urine sample and drink was retained for analysis of osmolality by freezing-point depression (Gonotec Osmomat 030 Cryoscopic Osmometer; Gonotec, Berlin, Germany), sodium and potassium concentration by flame photometry (Corning Clinical Flame Photometry 410C; Corning Ltd., Halstead, Essex, United Kingdom), and chloride concentration by coulometric titration (Jenway Chloride Meter; Jenway Ltd., Dunmow, Essex, United Kingdom). Urine samples were also analyzed for creatinine concentration through a modification of the Jaffe reaction (22).

Statistical Analyses and Calculations

All data were analyzed using SPSS 16.0 (Chicago, IL, USA). All data were checked for normality of distribution using a Shapiro-Wilk test. Data containing 2 factors were then analyzed using 2-way repeated measures analysis of variance (ANOVA) followed by Bonferroni-corrected paired Student's t-tests for normally distributed data or Bonferroni-corrected Wilcoxon signed-rank tests for non-normally distributed data. Data containing 1 factor was analyzed using 1-way repeated measures ANOVA followed by Bonferroni-corrected paired Student's t-tests for normally distributed data or Friedman's ANOVA followed by Bonferroni-corrected Wilcoxon signed-rank tests for non-normally distributed data. For ANOVAs, the Mauchly's test was used and where the assumption of sphericity had been violated the degrees of freedom were corrected using the Greenhouse-Geisser estimate. Differences were accepted as being significant when p ≤ 0.05. Normally distributed data are presented as mean (SD). Non-normally distributed data are presented as median (range).

Electrolyte balance was determined from losses in urine throughout the study and gains through food intake during FER and drink intake during rehydration. It is acknowledged that some avenues of electrolyte loss were not measured (e.g., sweat and fecal losses) and thus electrolyte balance values represent overestimates of true values. However, sweat and fecal electrolyte losses at rest in healthy subjects are likely to be small and will only make a small contribution to daily balance and given the dietary standardization involved are likely to be similar between trials.


With the exception of nude body mass (p < 0.001), there were no differences between males and females (p > 0.300) for any of the other measured variables during FER and given that the menstrual cycle has been shown not to affect rehydration (10), data for both males and females are presented together. Twenty-four–hour urine creatinine excretion was not different between trials (p = 0.960), and over all trials was 0.17 (0.03) mmol·kg−1 body mass per 24 hours, which is indicative of a complete 24-hour urine collection (3).

Pretrial Measurements

Pretrial body mass (p = 0.421) and urine osmolality (p = 0.500) were not different between trials, indicating subjects begin each trial in a similar state of hydration.

Fluid and Energy Restriction Period

Over the 24-hour FER period, there was no difference between trials for BML (p = 0.861), urine output (p = 0.743), or the excretion of sodium (p = 0.465), potassium (p = 0.360), and chloride (p = 0.364) in urine. Over all trials BML was 1.47 (0.34) kg, which equated to 2.13% (0.48%) of the subject's pretrial body mass. Over all trials, total urine output during FER was 888 (225) ml, whereas sodium, potassium, and chloride excretion were 68 (27) mmol, 48 (23) mmol, and 84 (32) mmol, respectively. As BML was not different between trials, neither was the volume of drink ingested during the rehydration period and totaled 1863 (356) ml (P), 1810 (329) ml (K), and 1865 (368) ml (Na).

Urine Output, Drink Retention, and Urine Osmolality

There were main effects of time (p < 0.001) and trial (p ≤ 0.05), as well as an interaction effect (p < 0.01) for cumulative urine output after the onset of drinking (Figure 1), with greater cumulative urine output at 4, 5, and 6 hours during trial P compared with trial Na (p ≤ 0.05). At the end of the trial, the proportion of the rehydration drinks that had been retained was 32% (14–69%) (Na), 22% (0–57%) (K), and 6% (−10 to 64%) (P) (p < 0.01), with drink retention greater for Na compared with P (p < 0.01). There were main effects of time (p < 0.001), trial (p < 0.01), and an interaction effect (p < 0.001) for urine osmolality (Figure 2). Compared with −24 hours, urine osmolality was increased at 0 hours and decreased at 2 hours during all trials (p < 0.001), as well as decreased at 3 and 4 hours during trial P (p < 0.001), decreased at 3 hours during trial K (p < 0.01), and increased at 6 hours during trial Na (p ≤ 0.05). Compared with trial P, urine osmolality was greater during trial Na at 3–6 hours (p ≤ 0.05) and during trial K at 2–5 hours (p ≤ 0.05).

Figure 1
Figure 1:
Cumulative urine output (in milliliters) after the onset of rehydration. †Significantly different from trial P. Points are mean values. Error bars areSD.
Figure 2
Figure 2:
Urine osmolality (in milliosmoles per kilogram). ***All trials significantly different from 0 hours. *Significantly different from 0 hours. †Significantly different from trial P. Points are median values.

Electrolyte Excretion and Balance

Although there was no difference between trials for the total amount sodium (p = 0.254) or chloride (p = 0.082) excreted after the onset of drinking, there was a main effect of trial for total potassium excretion (p < 0.001), with a larger amount of potassium excreted during trial K than trial P (p < 0.01) or trial Na (p < 0.001) (Table 2).

Table 2
Table 2:
Total cumulative urine electrolyte excretion after the onset of rehydration (in millimoles).*

Sodium, potassium, and chloride balances were calculated from losses in urine over the whole trial period and gains from food consumed during FER and drinks ingested during rehydration. There were main effects of time (p < 0.001) and trial (p < 0.001), as well as an interaction effect (p < 0.001) for sodium (Figure 3A), potassium (Figure 3B), and chloride (Figure 3C) balance. Compared with −24 hours, sodium, potassium, and chloride balance was negative at 0 hours during all trials (p < 0.001). Sodium balance remained negative for all time points during rehydration and monitoring during trial P and trial K (p < 0.001), but was not different compared with −24 hours at any other time point during trial Na (p > 0.083). Potassium balance remained negative compared with −24 hours for all time points during rehydration and monitoring for trial P and trial Na (p < 0.001) and at 1, 5, and 6 hours during trial K (p ≤ 0.05). Chloride balance remained negative compared with −24 hours for all time points during rehydration and monitoring during trial P and K (p < 0.001), as well as at 1 hour during trial Na (p ≤ 0.05). Sodium balance was greater during trial Na compared with trials P and K from 1 hour onwards. Potassium balance was greater during trial K compared with trials P and Na from 1 hour onwards. Chloride balance was greater during trials Na and K compared with trial P from 1 hour onwards, as well as greater during trial Na compared with trial K at 1, 2, 3, 4, and 5 hours.

Figure 3
Figure 3:
Sodium (A), potassium (B), and chloride (C), balance (in millimoles). ***All trials significantly different from 0 hours. *Significantly different from 0 hours. †Significantly different from trial P. ‡Significantly different from trial K. Points are mean values. Error bars areSD.


The aim of this study was to examine the rehydration effects of manipulating electrolyte contents of drinks ingested after 24-hour combined FER. The results of this study suggest that after a 24-hour period of severe FER, ingestion of a high sodium drink (57 [3] mmol·L−1 Na and 1 [1] mmol·L−1 K) reduced urine output and consequently increased the retention of the drink compared with a placebo drink (5 [1] mmol·L−1 Na and 1 [1] mmol·L−1 K), whereas a high potassium drink (5 [1] mmol·L−1 Na and 32 [1] mmol·L−1 K) was not different from either the sodium-containing drink or placebo. These results suggest that in situations where weight category athletes have undergone a period of severe FER to acutely reduce body mass, a high sodium drink might provide the best option to rehydrate the athlete.

Rehydration after exercise-induced dehydration has been well studied (26), but there are differences in the physiological environment when hypohydration is induced by exercise vs. severe FER that might limit the applicability of this postexercise rehydration data. Exercise (particularly in a warm environment) induces a sweat response to help dissipate the increased metabolic heat produced. This sweat contains high concentrations of sodium and chloride, but only small concentrations of potassium (27). In contrast, during 24-hour FER, electrolyte (sodium, potassium, and chloride) losses in urine continue, despite virtually no intake, which results in a large negative balance of all 3 electrolytes (7). The major difference between these 2 methods of water loss is the greater loss of potassium that occurs during severe FER.

For rehydration after exercise-induced dehydration, it has been shown that the volume of fluid ingested must be in excess of that lost to account for obligatory urine losses in the postdrinking period, as well the diuresis associated with drinking a large volume of fluid over a short time period (29). Shirreffs et al. (29) demonstrated that a volume of drink equivalent to 1.5–2 times the fluid loss should be consumed to account for these postdrinking losses. Although no such study has been performed after severe FER, it is logical to assume that the volume of drink required for rehydration will be similar. With FER, BML over the restriction period is not only due to water loss. This is in contrast to BML induced by an acute bout of exercise, where with the exception of the loss of expired CO2 and feces (if produced) all loss of body mass can be accounted for as water loss. James and Shirreffs (7) reported BMLs of 1.26and 1.64 kg during 24-hour fluid restriction and 24-hour FER, respectively, with trials matched for total water intake (277 [32] ml). As energy intake was maintained at habitual levels during the fluid restriction trial, it is likely that the most BML in this trial can be accounted for by water losses and that because water intake was identical that this water loss was similar in the FER trial. It can therefore be estimated that during 24-hour severe FER, approximately 75–80% of the BML is caused by water loss. Therefore, in this study, we used a drinking volume in liters equivalent of 1.25 times BML in kilograms, which would equate to approximately 1.55–1.65 times the estimated water loss.

As long as a sufficient volume of fluid is ingested, the main factors that determine the success of a rehydration drink are its composition (26) and the rate at which it is consumed (8). During trial Na, 102 (17) mmol sodium was consumed during rehydration compared with the 10 (2) mmol and 10 (3) mmol consumed during trials P and K, respectively. The amount of sodium ingested during rehydration was approximately 1.4 times that lost during the FER of trial Na (74 [33] mmol), which meant that subjects attained a positive sodium balance at the end of rehydration (2 hours). Despite this large difference in sodium ingestion, cumulative sodium excretion was not different between trials, and therefore sodium balance was greater during trial Na and was net neutral from 1 hour onwards.

It has been shown that addition of sodium to a rehydration drink ingested after exercise increases drink retention in a dose-dependent manner (11,16,28). As the major electrolyte in the extracellular space, the inclusion of sodium in a rehydration drink attenuates the hemodilution that occurs after ingesting a large volume of a sodium-free hypotonic drink (19). This study agrees with these previous studies that have manipulated the sodium content of drinks ingested after exercise and demonstrates that after severe FER, the addition of sodium to rehydration drinks might provide the best drinking strategy to restore lost body water. The greater whole-body sodium balance during trial Na likely resulted in an increase in water retained with this sodium and thus explains the reduction in urine output.

During trial K, 59 (13) mmol potassium was consumed during rehydration, compared with the 1 (1) mmol and 1 (1) mmol consumed during trials P and Na, respectively. As with sodium intake during trial Na, the amount of potassium ingested during rehydration on trial K was approximately 1.4 times that lost during the FER of trial K (43 [21] mmol). This meant that subjects reached positive potassium balance at the end of rehydration (2 hours). In contrast to sodium balance during trial Na, this greater ingestion of potassium during rehydration resulted in an increase in potassium excreted in urine after drinking and meant that while potassium balance was greater during trial K compared with trials P and Na, it was still net negative at the end of the monitoring period.

The addition of potassium to rehydration drinks has been suggested to increase rehydration in the intracellular space (33), but studies that have examined postexercise rehydration have reported inconsistent findings (13,23). Maughan et al. (14) reported a reduction in urine output after ingestion of a 25 mmol·L−1 KCl drink compared with an electrolyte-free 90 mmol·L−1 glucose drink, but no difference between the KCl drink and a 60 mmol·L−1 NaCl drink or a drink containing all 3 ingredients (90 mmol·L−1 glucose, 60 mmol·L−1 NaCl, and 25 mmol·L−1 KCl). In contrast, Shirreffs et al. (25) reported no difference in urine output between a commercially available drink containing 30 mmol·L−1 K (Apfelschorle) and bottled mineral water or a carbohydrate-electrolyte sports drink. The difference in findings between these 2 studies might be explained by the use of potassium chloride in the study of Maughan et al. (14), but not in the study of Shirreffs et al. (25). Chloride concentrations in the extracellular fluid are second only to sodium (∼100 vs. ∼140 mmol·L−1) and the addition of chloride to drinks, as with sodium might enhance fluid retention in the extracellular fluid. This study suggests that addition of potassium chloride to drinks ingested after FER does not affect urine output compared with a low electrolyte placebo drink, although mean urine output was lower after the potassium chloride containing drink.

In this study, the Na drink was best retained, but its retention (32% [14–69%]) was much lower than that reported for a drink of similar sodium concentration ingested in a volume equal to 150% BML after exercise-induced dehydration (11). Maughan and Leiper (11) observed that 69% (4%) (mean [SEM]) of a 52 mmol·L−1 NaCl drink was retained after a 0.5-hour rehydration period and 5.5-hour monitoring period. Although this study performed a 2-hour rehydration period in comparison to the 0.5 hours used by Maughan and Leiper (11), this longer rehydration period would be expected to increase drink retention (8). This difference in drink retention after a period of FER compared with after exercise-induced dehydration might be explained by the difference in serum osmolality at the onset of rehydration. As urine production is dramatically reduced to basal levels during FER (7), it seems likely that circulating concentrations of arginine vasopressin would have been increased compared with normal resting levels. The isosmotic state observed after FER (22) compared with the hyperosmotic state observed after exercise-induced dehydration (11) might result in a more rapid reduction in arginine vasopressin, as well as other hormones associated with fluid balance (17) once drinking has begun. This will result in a larger diuresis after FER than after exercise-induced dehydration and consequently reducing drink retention after a period of FER.

The aim of this study was to isolate the effect of FER on water and electrolyte balance, and therefore, only the electrolyte content of the rehydration drinks was manipulated. Given that ratings of hunger increased over the 24-hour FER (data not shown), it is likely that after weigh-in, at least some food would be consumed by a weight category athlete that has used severe FER to manipulate body mass (24,31). Furthermore, although a small amount of carbohydrate (83 [18] g) was consumed during the 24-hour FER period, this amount was unlikely to be sufficient to maintain glycogen stores, particularly liver glycogen (15). It is well-documented that 24-hour severe or complete energy restriction impairs exercise performance (1,21), but the weight category athlete has some time between weigh-in and competition, which can be used to minimize the decrement in exercise performance. Where combined FER has been used to make weight, recovery of fluid balance and glycogen stores are going to be the most important considerations for the athlete, but the amount of fluid and carbohydrate that can be consumed will likely be related to the amount of time between weigh-in and competition (24). Adding carbohydrate to drinks (4,5) and consuming food with drinks (11) have been shown to enhance postexercise rehydration, and future studies should aim to investigate these strategies after severe FER or RWL that includes severe FER.

In conclusion, a 24-hour period of FER resulted in a significant reduction in body mass that is accompanied by large losses of sodium, potassium, and chloride in urine. The addition of sodium chloride to a rehydration drink ingested after severe FER resulted in an increase in the retention of the ingested drink, but the addition of potassium chloride resulted in no significant benefit in the recovery of fluid balance.

Practical Applications

For the weight category athlete, this study provides novel data examining the effect of rehydration after severe FER, which is a technique commonly used to induce RWL before a weigh-in (9,17,18,20,30,31). Recovery after exercise-induced dehydration has been well studied, but appropriate recovery and rehydration strategies after severe FER have not. This is the first study to examine different rehydration strategies under such conditions of FER, and the finding that drink retention was much lower than after exercise-induced dehydration warrants further investigation, but the finding that sodium chloride addition to rehydration drinks in this situation is useful for the weight category athlete. The consumption of food alongside an appropriate drink volume has been shown to enhance drink retention compared with drinking alone (12). It is likely that weight category athletes who have restricted their energy intake to make weight will consume some food immediately after weigh-in, which might enhance drink retention to levels sufficient to fully replace fluid balance before competition. However, the amount of food and, indeed, drink that can be consumed between weigh-in and competition will mainly depend on the amount of time available and must be considered by the applied practitioner.


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dehydration; hydration; fluid balance; weight category; rapid weight loss

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