Post-exercise rehydration in man: effects of volume consumed and drink sodium content

Abstract

The interaction between the volume and composition of fluids ingested was investigated in terms of rehydration effectiveness. Twelve male volunteers, dehydrated by 2.06 ± 0.02% (mean ± SE) of body mass by intermittent cycle exercise, consumed a different drink volume on four separate weeks; six subjects received drink L (23 mmol·l^-1 Na⁺) in each trial and six were given drink H (61 mmol·l^-1 Na⁺). Volumes consumed were equivalent to 50%, 100%, 150%, and 200% of body mass loss (trials A, B, C, and D, respectively). Blood and urine samples were obtained before exercise and for 7.5 h after exercise. Less urine was excreted following rehydration in trial A than in all other trials. Cumulative urine output (median ml) was less in trial B (493, range 181-731) than D (1361, range 1014-1984), which was not different from trial C (867, range 263-1191) in group L. In group H, the volume excreted in trial B (260, range 137-376) was less than trials C (602, range 350-994) and D(1001, range 714-1425), and the volume in trial C was less than in trial D. These results suggest that both sodium concentration and fluid volume consumed interact to affect the rehydration process. A drink volume greater than sweat loss during exercise must be ingested to restore fluid balance, but unless the sodium content of the beverage is sufficiently high this will merely result in an increased urinary output.

Fluid intake during or after exercise is generally intended to replace the water and electrolyte losses incurred as a result of sweat secretion, and also to provide carbohydrate to supplement or replenish the glycogen stores in the liver and the working muscles. The relative importance of providing water or substrate is influenced by many factors, including the type and duration of exercise, the environmental conditions, and the physiology and biochemistry of the individual. Disturbances in body fluid balance and temperature can impair exercise performance and are potentially life-threatening ⁽²⁾. In comparison, the depletion of carbohydrate stores in the liver and working muscles will result in fatigue and a reduction in exercise intensity, but on the whole presents no great risk to health. Therefore, except in situations where depletion of body water is unlikely to occur, the most important aim of ingesting fluid during exercise should be to minimize disturbances in body fluid balance. Replacement of the electrolytes lost in sweat is not normally necessary during the exercise period, except where losses are exceptionally large⁽¹⁷⁾.

The effect of fluid ingestion prior to and during exercise on performance and fluid homeostasis has been extensively studied and comprehensively reviewed ^(5,14,18). Much of this work has implications for rehydration in the post-exercise period, especially where repeated exercise bouts are performed, but there are a number of separate issues related to post-exercise rehydration which must be addressed.

There are relatively few reports of attempts to evaluate fluid replenishment after exercise, but the addition of sodium has been established as a prerequisite for effective rehydration. If large volumes of plain water are ingested following exercise, the plasma sodium concentration and plasma osmolality fall rapidly ⁽²⁸⁾. This has the effect of reducing thirst, thus limiting further intake, and stimulating urine output, both of which will postpone the rehydration process. The results of Nose et al. ⁽²⁸⁾ indicate that when plain water was consumed, it took three times longer (60 min) for the restoration of plasma volume following exercise-induced dehydration compared to consumption of a 77 mmol·l^-1 sodium chloride solution. It was proposed that this delay in rehydration when plain water was consumed was due to a reduction in the stimulus for plasma renin activity and aldosterone release, thus allowing a greater urinary loss of sodium and water ⁽²⁹⁾. The results also indicate that, compared with the saline solution, plain water was less effective in restoring hydration status; not only did plain water reduce the stimulus to drink but the free water clearance was also increased, mainly due to the loss of electrolytes during dehydration. Therefore, it has been suggested that rehydration after exercise can only be complete and rapid if the sodium lost in sweat during exercise is replaced as well as the water. It has been proposed that post-exercise rehydration drinks should have a sodium concentration similar to that of sweat ⁽¹⁸⁾, but as the electrolyte content of sweat itself shows considerable variation between individuals and over time ⁽³¹⁾, it would seem impossible to prescribe a single formulation.

The majority of investigations to date concerned with rehydration after exercise have examined the effects of ingesting different solutions in a volume equal to that lost ^(4,12,15,20), although some have allowed ad libitum drinking^(19,28) or consumption of a predetermined volume⁽²⁶⁾. There are clearly rather few systematic studies in this area, and the possible interaction, if any, between the volume of beverage consumed, its electrolyte concentration, and its rehydration effectiveness has not yet been subject to systematic investigation. The aim of the present study was to systematically vary the volume of fluid ingested after exercise-induced dehydration and to look at the effect of two different concentrations of sodium on the rehydration process. The drinks chosen were such that one had a sodium concentration of 23 mmol·l^-1 and one a sodium concentration of 61 mmol·l^-1: one at the lower end and one at the upper end of the range of normal values for the sodium content of sweat ⁽³¹⁾. The volume of fluid consumed ranged from one half of the volume loss during the dehydration process to twice the volume loss.

METHODS

Twelve healthy male subjects volunteered to take part in this study. The experimental procedures were approved by the Joint Ethics Committee of Aberdeen University and Grampian Health Board. Subjects were fully informed of the nature of the investigation and of the procedures involved before their written consent to participate was obtained. The 12 subjects taking part in this study were divided into two separate groups, approximately matched for physical characteristics: group L, age 27 (±3) yr, height 177.6(±2.5) cm, mass 71.5 (±1.3) kg, body fat 14.5 (±1.5)%,˙VO_2max 58.9 (±3.7) ml·kg^-1·min^-1; group H, age 33 (±4) yr, height 175.0 (±2.7) cm, mass 73.2 (±2.0) kg, body fat 12.4(±0.9)%, ˙VO_2max 57.5 (±3.3) ml·kg^-1·min^-1. Each individual received the same drink on each of the four experimental trials in which he took part: group L were given the low-sodium (23 mmol·l^-1) drink and group H drank the high-sodium (61 mmol·l^-1) beverage. Both drinks contained only small amounts of potassium (0.3 mmol·l^-1), and chloride and acetate were the major anions present; the drinks also contained 90 mmol·l^-1 glucose. Maximum oxygen uptake (˙VO_2max) was determined during a discontinuous, incremental exercise test on a cycle ergometer prior to the beginning of the study. Body fat was estimated by means of skinfold measurements made in triplicate using John Bull skinfold calipers(British Indicators Ltd.) according to the method of Durnin and Rahaman⁽⁸⁾.

Each experimental trial consisted of intermittent exercise to induce a sweat loss equal to approximately 2% of body mass, followed by ingestion of one of the test drinks, with fluid and electrolyte balance being monitored for a further period. Four experimental trials, each separated by 1 wk, were undertaken by each subject. These trials were preceded by an initial familiarization trial 1 wk prior to the start of the experiment. All experimental trials commenced in the middle or late afternoon following a 6-h fast, although water intake was permitted until 3 h before the first blood sample. For the 48 h prior to each test subjects were requested to refrain from strenuous exercise and to follow the same dietary and exercise pattern during this time. These restrictions were made in order to standardize as far as possible the initial state of hydration for each trial.

On arrival at the laboratory, subjects sat fully clothed in a room at a moderate temperature (approximately 20-25°C) for 15 min to allow equilibration of body fluids to take place; a 10-ml blood sample was then collected from a superficial antecubital vein. All blood samples were obtained after at least 15 min of seated rest and were collected without interruption of the circulation. Immediately after collection of the initial blood sample, the subject emptied his bladder as completely as possible and the entire volume was collected. The subjects then showered and dried before nude body mass was measured on a beam balance to the nearest 10 g (Marsdens type 150, Marsdens Weighing Machines, London, UK).

Dehydration was initiated by a 10-min period of immersion in a tank of water maintained at a temperature of 41°C. The subject then dried himself and was weighed again to determine the extent of any mass loss that had occurred. This thermal load was intended to stimulate sweating and reduce the exercise time. Exercise in a warm, humid environment (30-32°C, 70-90% relative humidity) immediately followed the warm water immersion. Subjects exercised dressed in shorts and shoes. The pattern of activity in the hot room consisted of 10 min of cycling at a power output corresponding to approximately 60% of maximum oxygen uptake, followed by a 5-min rest period during which the subject removed his clothing, dried himself with a towel, and was weighed to monitor the mass loss. This was done without the subject leaving the warm environment. This pattern of activity was repeated until the individual had lost almost 2% of his body mass loss or until the individual had completed six 10-min exercise bouts, whichever was the sooner. As subjects continued to perspire for a short time after exercise, each subject stopped cycling at a time that allowed the remainder of the desired weight loss to occur as he cooled down. Mean (SE) total exercise time was 37.4 (±1.8) min: exercise time in week one was 36.8 (±4.0) min, in week two 38.2(±3.0), in week three 37.8 (±3.7), and in week four 36.8(±3.7) min. After exercise, the subjects left the hot room, had a cool shower, and remained in a cool environment to stop sweating as soon as possible. The final body mass was measured at this time, which was within 15 min of the end of the exercise, after which subjects dressed and returned to sit in a comfortable environment.

After sitting for a further 15 min (i.e., 30 min after the end of exercise) a second blood sample was taken via a 21-gauge venous cannula placed into a superficial forearm vein. This remained in place for the remainder of the experimental period and was flushed with a small volume of heparinized saline between samples. Again, the subject gave a urine sample immediately following the blood sample collection. Over the next 60 min, the subjects ingested one of the rehydration beverages; the order of treatment was randomized using an incomplete four-way crossover design. The entire beverage volume to be ingested was divided into four equal parts, each of which was consumed over a 15-min period. A different volume of the same drink was consumed on each of the trials: the volume ingested (in ml) was related to the mass loss (in g) such that volumes equivalent to one half of the mass loss (trial A), equal to the mass loss (trial B), one and a half times the mass loss (trial C) and double the mass loss (trial D) were ingested.

As the subject started his first drink, he completed a questionnaire with regard to his subjective feelings and the taste characteristics of the drink. This questionnaire was removed from the subject when he had completed it, and a second questionnaire, identical to the first, was given as he finished his final drink. Throughout the 15-min period over which the subject consumed his final drink, he was asked to remain seated in preparation for collection of a third blood and urine sample. For the remainder of the study period, subjects remained within the laboratory: most of this time was spent sitting except for essential movements. Further blood and urine samples were collected from the subjects at 1, 2, 4, and 6 h after the end of the rehydration period. If a subject required to urinate at any time other than at a scheduled urine collection time, he was free to do so and the urine voided was collected and mixed with that collected at the next scheduled time, so that the volume excreted over the entire period was measured.

Analytical procedures. All analyses were carried out in duplicate, except where otherwise indicated. The coefficients of variation of analyses (CV), expressed as a percentage of the mean value, were calculated based on duplicates of the same sample (N = 25) and are reported in brackets for each method.

Part (2.5 ml) of each blood sample was mixed with anticoagulant (1.5 mg·ml^-1 EDTA) and used for measurements of hematocrit (CV 0.8%)(in triplicate, by microcentrifugation) and hemoglobin concentration (CV 0.4%)(by the cyanmethemoglobin method). These results were used to calculate changes in blood and plasma volumes relative to the value immediately before rehydration ⁽⁷⁾. A further 2.5 ml was added to a refrigerated tube containing anticoagulant (1.5 mg·ml^-1 EDTA); the plasma was removed after centrifugation at 4°C. In trials A and D for both groups, the pre-exercise, post-exercise, and 2-h and 6-h post-rehydration plasma samples were analyzed for vasopressin, aldosterone and angiotensin ll(all singly by radioimmunoassay (Euro-Diagnostica, Cornwall, UK; Serono Diagnostics, Rome, Italy)). The remainder of each blood sample was allowed to clot and the serum separated by centrifugation. The serum was analyzed for osmolality (CV 0.3%) by freezing-point depression (Gonotec Osmomat 030 Cryoscopic Osmometer; YSI Ltd Farnborough, UK), sodium (CV 0.5%) and potassium(CV 1-2%) concentrations by flame photometry (Corning Clinical Flame Photometer 410C; Corning Ltd., Halstead, Essex, UK) and chloride (CV 1.4%) concentration by coulometric titration (Jenway Chloride Meter; Jenway Ltd., Dunmow, Essex, UK). All analyses, except those by radioimmunoassay, were carried out within 96 h of collection.

The total volume of each urine sample was measured and a sample retained for estimation of osmolality (CV 0.1%) and electrolyte concentrations (CV Na⁺ 0.7%, K⁺ 0.3%, Cl^- 1.9%) using the same methods as for serum analysis. The cumulative urine output following rehydration was calculated. Whole body net fluid balance was calculated from the body mass loss, volume of fluid ingested and urinary volume excreted, excluding the urine passed before dehydration.

Statistical analysis. Comparisons were made between treatments(trials A-D) within each group and between groups (L and H) to examine the effects of drink composition and volume. All data were subjected to an initial two-way repeated measures analysis of variance. For those parameters shown to be normally distributed, this was followed by one-way analysis of variance and Tukey's multiple range test as appropriate. For those parameters shown not to conform to a normal distribution, the initial analysis of variance was followed by the Kruskal-Wallis k-sample test and the Mann-Whitney test where appropriate. Differences between treatments were accepted as being significant when a P-value of less than 0.05 was obtained. All data in the text, tables and figures are expressed as mean (±SE) if normally distributed and as median (range) if not normally distributed. The statistical power of the analyses, both within and between groups, has been given in the text where appropriate.

The repeatability of the response to ingestion of the same volume of the same beverage has previously been established ⁽²²⁾ and therefore it is with confidence that significant effects, when obtained, can be apportioned to the experimental interventions employed.

RESULTS

The mean body mass loss during the dehydration process was 1.48(±0.02) kg for group L and 1.51 (±0.03) kg for group H: expressed as a percentage of the initial body mass, this corresponds to a loss of 2.06 (±0.02)% for group L and 2.07 (±0.03)% for group H. Based on the assumption that total body water is approximately 72% of an individual's lean body mass ⁽¹⁶⁾, a loss of 3.34(±0.08)% in group L and 3.26 (±0.10)% in group H of total body water occurred. There was no difference in body mass loss between the four trials in either group (P = 0.65). Both groups therefore received the same fluid volume in each of the corresponding trials. The volume of fluid ingested in each trial by subjects in the two groups is shown inTable 1. Both groups exercised for the same duration to achieve this mass loss (group L 38 (±2) min, group H 37 (±3) min; P = 0.70) and no acclimation or training effect, in terms of sweat rate, occurred from week to week over the duration of the study(P = 0.99).

Urine volume and composition. The cumulative volume of urine excreted over time following rehydration is shown in Figure 1. In each group, the total urine output over the whole time span of the post-exercise period was proportional to the drink volume consumed: group L, A= 135 (114-240) ml, B = 493 (181-731) ml, C = 867 (263- 1191) ml, 1361(1014-1984) ml; group H, A = 144 (124-162) ml, B = 260 (137-376) ml, C = 602(350-994) ml, D = 1001 (714-1425) ml. For group L, the cumulative urine output in trial A was less than that in any other trial at each of the time points, and from 1 h after the end of the rehydration period onward, the volume excreted in trial B was less than that in trial D. For group H, from 1 h after the rehydration period onward, a smaller volume was exreted in trial A than in each of the other trials and in trial B the volume was less than in trials C and D over the same period. From 2 h after the end of the rehydration period onward, all trials were significantly different from each other.

Because of the relatively large variation between individuals, differences between groups are difficult to detect; the power of the statistical tests was 95% for a 250-ml difference between trials within a group but was reduced to only approximately 30% between groups. When comparisons are made between the two treatment groups a greater volume was always excreted by group L and the average differences at the end of each trial amounted to 8 ml, 233 ml, 265 ml, and 360 ml in trials A, B, C and D respectively; the difference was significant in trials B and D (P < 0.05).

Despite significant differences in the amount of sodium ingested in each trial (Table 1), in neither group was there a difference between trials in the quantity of sodium excreted over the study period(Table 2; group L, P = 0.27; group H,P = 0.55). Additionally, despite approximately three times as much sodium being ingested in group H compared with the corresponding trials in group L, there was no significant difference between the two groups in the quantities excreted in any of the trials (P = 0.31). A similar pattern was apparent for the urinary chloride excretion following rehydration: there was no difference between trials in either group L or group H(Table 2), and in none of the trials was there a difference between groups in the amount of chloride excreted (P = 0.37). The potassium content of both drinks was low (Table 1), and in neither treatment group was there a difference in the quantities excreted (Table 2). In trials A, B and C there was no difference in urinary potassium excretion between groups (P = 0.63), but in trial D a greater amount of potassium (17.9 mmol) was excreted by subjects in group H than by those in group L (13.3 mmol, P = 0.002).

Whole body net fluid balance is shown in Figure 2. Over all trials, the dehydration procedures resulted in a fluid loss of 1490(1382-1780) ml in group L and 1555 (1299-1789) ml in group H; there was no difference between groups in any trial. Due to the differences in fluid volume ingested following exercise, net fluid balance at the end of the rehydration period (0 h) was different in all trials within both subject groups. There was no difference between groups in fluid balance at this time or 1 h later. By the end of the study period, subjects in group L were in a more negative state of fluid balance in trial A (-909 (-1011 to -835) ml) than in trials C (-128(-441 to 380) ml) and D (-135 (-503 to 426) ml), but there were no other differences between trials at this time. However, at the same time in group H, subjects were in a greater state of fluid deficit in trial A (-958 (-1018 to-826) ml) than all other trials, and they were in a more positive state of fluid balance in trial D (427 (-139 to 828) ml) than in either trial B (-286(-415 to -159) ml) or trial C (111 (-379 to 394) ml). Relative to their initial hydration state, subjects in both groups were in a state of hypohydration in trials A and B at the end of the study period; subjects in group L were essentially euhydrated in trials C and D. Subjects in group H were euhydrated relative to their initial level of hydration at the end of trial C, but in trial D they were hyperhydrated. Comparing the two groups, there were differences in the state of net fluid balance at the end of the study period in trials B and D, with a more positive fluid balance being indicated with ingestion of the high-sodium drink. These results for whole body net fluid balance are reflected in the fractions of the ingested fluid retained following rehydration (Table 3). The fraction of ingested fluid retained at the end of the study period in trials A(P = 0.69) and C (P = 0.41) did not differ between groups. However, in trials B (P = 0.04) and D (P = 0.04), a significantly smaller fraction was retained in group L than in group H. Despite differences in the fraction of ingested fluid retained between groups not being apparent for all trials, a greater fraction of the ingested sodium was retained in group H than in group L in trials A, B, and C(Table 3).

Blood and plasma changes. Analysis of the pre-exercise blood sample confirmed that all subjects had values for the measured parameters within the normal range obtained in this laboratory for adult males: hematocrit (37-46%), hemoglobin concentration (130-170 g·l^-1), serum osmolality (283-293 mosmol·kg^-1), sodium (133-143 mmol·l^-1), potassium (3.5-4.5 mmol·l^-1) and chloride (98-105 mmol·l^-1) concentrations.

Although post-exercise samples were not collected until 30 min after the end of the exercise period, blood and plasma volumes were still less at this time than before exercise in all trials, with no differences between the different trials or groups (Fig. 3). The mean plasma volume decrease for all trials was 4.8 (±0.7)% for group L and 5.7(±0.5)% for group H. Plasma volume increased over the hour during which fluid was ingested to a value greater than that measured after exercise and remained at this elevated level for the remainder of the study in all trials in group H. The same change occurred in group L for trials C and D, but in trial B the increase at the end of the rehydration period was only transient and this level was not reached again until 4 h after the end of the rehydration period; in trial A there was little fluctuation in plasma volume throughout. In group L, the only difference between trials occurred 1 h after the end of the rehydration period when the plasma volume was greater in trial D than in trials A and B. At the same time in group H, the plasma volumes in trials C and D were both greater than trial A. At the end of the study period, the general pattern for both subject groups was for the increases in plasma volume to be a direct function of the volume of fluid consumed and for the increase to be greater for subjects ingesting the high-sodium drink.

At no time throughout the study, was there a difference between trials in serum osmolality for subjects in group H (P = 0.44). For group L, however, the osmolality was lower in trials C (283 (±1) mosmol·kg^-1) and D (282 (±2) mosmol·kg^-1) than in trial A (289 (±2) mosmol·kg^-1) 1 h after the end of the rehydration period, and 1 h later the osmolality in trial D was still less than that in trial A. There was a tendency for serum osmolality to increase with dehydration in all trials in both groups, but in no instances did this reach significance (Table 4). For both groups, there was no fluctuation in serum osmolality throughout trial A. In trials C and D in group L, the serum osmolality had fallen relative to the first post-exercise value by 1 h after the end of the rehydration period, and it was maintained at this level for the remainder of the study. In this group, the osmolality 1 h after the end of the rehydration period was reduced relative to the initial value in trial C: the same was true in trial D but the reduced level was maintained until 4 h after the end of the rehydration period. In group H, there was less fluctuation in serum osmolality; in trial B, the value 2 h after the end of the rehydration period was reduced relative to the post exercise value, and in trial D, the values 1 and 6 h after the end of the rehydration period were reduced relative to that value. Serum sodium concentration (Table 4) was little affected by any of the treatments in group H: there were no differences between trials at any time and there were no changes in concentration throughout the course of any trial(P = 0.95). In group L, at the end of the rehydration period and 1 h later, the serum sodium concentration in trials C (135 (±1) mmol·l^-1) and D (133 (±1) mmol·l^-1 was less than that in trial A (139 (±1) mmol·l^-1). In trials C and D, the concentration decreased after the rehydration period.

Serum chloride concentration also was little affected by any of the treatments, and the pattern of change in serum chloride concentration was generally similar to the results obtained for sodium concentration. In group H there were no differences between trials at any time (P = 0.54) and only in trial C was the concentration altered by the rehydration treatment when the concentration fell significantly from 102 to 98 mmol·l^-1 over the 1-h rehydration period. In group L, at the end of the rehydration period, the concentration in trial A (101 (±0) mmol·l^-1) was greater than that in trials C (98 (±1) mmol·l^-1) and D (97 (±1) mmol·l^-1). Also in trial C, the concentration was altered by the rehydration treatment when the concentration fell over the 1-h rehydration period from 101 to 98 mmol·l^-1, and in trial D, the same reduction (from 101 to 97 mmol·l^-1) occurred and was maintained for the next 2 h.

Serum potassium concentration was unchanged throughout trials A and B in both groups of subjects. In trials C and D in the low-sodium group and trial D in the high-sodium group, the serum potassium concentration altered during the study. In group L, both 1 and 2 h after the end of the rehydration period, the serum potassium concentration was lower in trial D (3.4 (±0.1) mmol·l^-1), than in both trials A(3.8(0.1)mmol·l^-1) and B (3.7 (±0.1) mmol·l^-1). There was no difference between trials at any time point in group H.

Plasma hormones. Hormone measurements were made before and after exercise and at 2 and 6 h after rehydration in trials A and D only. The resting pre-exercise plasma vasopressin concentration was not different between trials A and D in either group, and there was also no difference between groups after exercise immediately before rehydration began(Table 5). Two hours after the end of the rehydration period vasopressin concentration was greater in trial A than trial D in both groups. However, by the end of the study period, 6 h after the end of the rehydration period, this difference between trials was no longer present.

The pre-exercise plasma aldosterone concentration did not differ between trials A and D in either group, and there was no difference between groups after exercise immediately before rehydration began (Table 5). In group L, 2 h after the end of the rehydration period the concentration in trial A was greater than that in trial D, but there was no difference between trials in group H at this time. However, by 6 h after the end of the rehydration period, the concentration was greater in trial A than D in both groups L and H.

The initial plasma angiotensin II concentration did not differ between trials A and D in either group, and neither was there a difference between groups after exercise immediately before rehydration began(Table 5). Two hours after the end of the rehydration period the concentration was greater in trial A than trial D in group H and in group L. By the end of the study period, 6 h after the end of the rehydration period, the concentration was greater in trial A than D in both groups L and H.

Drink palatability. No clear pattern emerged from these results, perhaps due to the small numbers of subjects in each group and the inevitable large variations in subjective responses (Table 6). However, despite the fact that these were not commercially formulated test beverages but were made using a low-calorie, low-electrolyte lemon drink with the addition of glucose and sodium salts, neither of the drinks was considered to be particularly unpleasant in taste. The high-sodium drink had a tendency to be perceived as having a slightly more sticky mouth feel and to be slightly more salty in taste, with a tendency for less fruitiness. However, it must be remembered that each group was composed of different subjects, so comparisons of taste preferences between groups should be treated with caution. Both groups of subjects unsurprisingly reported to be feeling more hungry and thirsty at the end of the rehydration period when they received the smallest volume of drink as opposed to the other trials and were least hungry after consuming the largest volume of drink.

DISCUSSION

The results of this study demonstrate that the rehydration effectiveness of a drink consumed following dehydration induced by exercise in the heat is influenced by both the volume ingested and the sodium concentration of the beverage. Of the relatively few previously published studies investigating post-exercise rehydration, only one ⁽²³⁾ has given different volumes of a rehydration beverage, other than where ad libitum intake was permitted, and no study has systematically evaluated the effect of ingesting different volumes of a beverage or assessed the possible interactions that may exist between the volume of beverage consumed and its electrolyte content. Other studies have compared solutions with different compositions ^(4,15): the test drinks have often been commercial products, containing a wide variety of sugars, minerals, and other components ^(12,19,26). The low-sodium beverage used in this study had a sodium concentration at the lower end of the range for sweat sodium concentration, and the high-sodium drink had a sodium content at the top end of the range for sweat sodium concentration⁽³¹⁾.

It is evident from the results of this study that both volume and composition are important factors influencing the efficacy of rehydration drinks. The importance of including sodium in a beverage designed to be effective at promoting rehydration is well established^(4,21,24,28). If no electrolyte-containing solid food is consumed, drinks ingested at this time should contain enough sodium to replace that lost in sweat during the dehydration process. As body water distribution largely follows solute distribution, replacement of sodium must occur if water lost from the extracellular space is to be replaced. Sodium is the major electrolyte of both the extracellular fluid and sweat, so it is not surprising that several studies have shown that the major part of the water lost in sweat is derived from the extracellular space ⁽³⁾. This then affects the plasma volume, which follows extracellular space alterations. Costill and Sparks ⁽⁴⁾ reported a more effective, yet still incomplete, restoration of plasma volume following ingestion of a carbohydrate electrolyte beverage as opposed to demineralized water: 1 h after the completion of fluid ingestion there was a plasma volume deficit of approximately 4% and 7.5%, respectively. In that study, drinks were consumed over a 3-h period following dehydration in a volume equal to the mass loss by dehydration: a greater volume of urine (approximately 225 ml) was excreted when the water was ingested.

Nadel et al. ⁽²⁴⁾ suggested that beverages with a sodium concentration in the region of 40-60 mmol·l^-1 would have an unpleasant taste, but the questionnaire results from the present study do not support this (Table 6). Where ad libitum intake is permitted, taste is an important factor, and in such a situation the volume consumed may be a better index of taste preference than questionnaire data. However, in the present study, even when the largest volume of the 61 mmol·l^-1 sodium beverage was consumed, no individuals reported finding the drinks unpalatable, and none had difficulty in consuming the drink because of its taste. Also, it should be remembered that these were not commercially formulated test beverages but were concocted in the laboratory using a low-calorie, low-electrolyte lemon drink with the addition of glucose and sodium salts, and so it may be possible to manipulate the taste of these beverages if required.

The interaction between the beverage volume consumed and its sodium content is not apparent with all trials in both groups in this study. There was no difference between the groups in the volume of urine excreted or in whole body net fluid balance in trial A, where the volume of drink consumed was equal to only one half of the sweat loss. However, subjects were in negative fluid balance throughout the post-exercise period: the volume of urine excreted was close to the basal level of about 50 ml·h^-1(13), and there was little scope for the addition of sodium to further reduce output. At the other extreme, by merely increasing the volume of drink consumed, it does not immediately follow that rehydration will be more effective. When the low-sodium drink was consumed, increasing the volume ingested by 672 ml from 2255 (±83) ml (trial C) to 2927(±81) ml (trial D) resulted in an increase in urine output of 494 ml from 867 (263-1191) ml to 1361 (1014-1984) ml: net fluid balance at the end of the study was the same. The subjects who received the high-sodium drink, however, were significantly better hydrated after trial D than after trial C. When the high-sodium drink was given, the influence of the volume ingested on rehydration effectiveness held true for all the volumes consumed in this study. For the subjects who received the drink with the lower sodium concentration, they were significantly better hydrated when the volume ingested was 2255 or 2927 ml corresponding to 150% or 200% of their mass loss by dehydration compared with the trial where they ingested 746 ml, equivalent to half their loss. There was, however, no significant difference in whole body hydration status between trials B, C, and D in spite of the twofold difference in the volume of fluid ingested although there was a tendency for subjects to have a greater degree of hypohydration at the end of trial B (-528 ml) than at the end of trials C (-128 ml) or D (-135 ml).

These contrasting responses may be explained by the different responses of serum osmolality and sodium concentration in the two groups after ingestion of the rehydration drinks. In the subjects receiving the high-sodium drink, there was no change in the serum sodium concentration throughout the study in any of the trials, and in none of the trials did the serum osmolality deviate from the pre-exercise levels. In group L, however, both serum osmolality and serum sodium concentration decreased after the rehydration period in trials C and D. Reductions in serum sodium concentration and osmolality have both been shown to delay the rehydration process in previous studies⁽²⁹⁾. This is due in part to a reduction in the drive to drink, which does not apply in the present study because a fixed volume was consumed, irrespective of the subjects' preferences. Decreases in the plasma sodium concentration and in osmolality also cause a delay in rehydration because these changes stimulate urine production and excretion by virtue of their effects on the release of the antidiuretic hormone vasopressin from the posterior pituitary and the mineralocorticoid hormone aldosterone from the adrenal cortex. Large volumes of dilute urine are produced when vasopressin and aldosterone release is inhibited. These hormones act to increase the permeability of the distal tubule and collecting duct of the nephron to water increasing passive reabsorption, and enhancing the active uptake of sodium ions by increasing sodium permeability in the apical membrane. A reduction in the dehydration-stimulated release of these hormones removes the brake on the inhibition of urine formation. The effects on urinary output of changes in plasma sodium concentration and osmolality will, however, be delayed slightly because of the time to inactivate circulating vasopressin and aldosterone: vasopressin has a half-life in plasma of approximately 10-20 min^(5,9) and aldosterone acts via the synthesis of new proteins, including Na⁺-K⁺-ATPase, so a time lag of approximately 30-90 min occurs before its effects occur⁽¹⁰⁾.

If the cation concentration of the rehydration beverage is sufficiently high, the plasma osmolality remains elevated and vasopressin release continues. As well as maintaining the drive to drink, this prevents the increased formation of dilute urine. However, it has been reported⁽³⁰⁾ that the set point of the osmoregulatory system can be altered by a change in blood volume or pressure, and this will therefore influence vasopressin secretion: differences in blood volume were observed between treatments in the present study such that the general pattern for both subject groups was that the increases in plasma volume were a direct function of the volume of fluid consumed and the increase was greater for subjects ingesting the high-sodium drink (Fig. 3). Also, vasopressin secretion can be maximally suppressed by water loading, even in the presence of a significant hypovolemia. Geelen et al.⁽¹¹⁾ found a decrease in vasopressin secretion after rehydration in the absence of a decrease in osmolality, plasma volume or blood pressure. This therefore suggests a role for oropharyngeal factors working alone or with gastric stimuli in response to local changes in tonicity or mechanical stimulation, and the decrease in vasopressin concentration may be as much a signal to stop drinking as it is an important regulator of urine production.

The differences in fluid balance status at the end of the study period are relatively small in relation to total body water content: the difference due to consumption of different volumes of the same drink amounted to approximately 1385 ml (1.9% of body mass) when the high-sodium drink was consumed. The difference due to variations in the sodium concentration amounted to approximately 562 ml (0.8% of body mass) when the largest volume was consumed. These would appear to be meaningful differences when compared with the volumes of fluid that are typically consumed during exercise: Noakes⁽²⁷⁾ reported that marathon runners will voluntarily consume anything from 100 to 1000 ml of fluid per hour. Additionally, even low levels of hypohydration, equivalent to less than 2% of body mass, have been shown to have an adverse effect on exercise performance^(1,25).

The results of this study show that merely drinking a large volume of beverage after exercise-induced dehydration is not sufficient to achieve complete rehydration if the sodium concentration is not sufficiently high: ingesting a beverage with a high-sodium concentration is not sufficient to achieve complete rehydration if the volume consumed is not appropriate. Both the sodium concentration of the beverage and the volume to be consumed should be considered if optimal rehydration following exercise-induced dehydration is the aim.

Figure 1-Cumulative urine output over time. The pre-exercise sample and the sample obtained immediately after exercise are not included in the total. The top panel shows the results for group L and the lower the results for group H. The median values at each point are plotted. Where significant differences are present, these are identified in the text.

Figure 2-Net fluid balance calculated from the volumes of sweat loss, fluid ingested, and urine output over the course of the experiment. The pre-exercise urine sample is not included in these calculations. The top panel shows the results for group L and the lower the results for group H. The median values at each time point are plotted. Where significant differences are present, these are identified in the text.

Figure 3-Percent change in plasma volume calculated from hematocrit and hemoglobin measurements. Changes are calculated relative to the blood sample taken 30 min after the end of exercise. The top panel shows the results for group L and the lower the results for group H. The mean values at each time point are plotted and the group SE of the values for all trials is shown at each time point. Where significant differences are present, these are identified in the text.