Sodium and water
loss during, and replacement after, exercise-induced volume depletion
was investigated in six volunteers volume depleted by 1.89 ± 0.17%
(SD) of body mass by intermittent exercise in a warm, humid
environment. Subjects exercised in a large, open plastic bag, allowing
collection of all sweat secreted during exercise. For over 60 min
beginning 40 min after the end of exercise, subjects ingested drinks
containing 0, 25, 50, or 100 mmol/l sodium (trials 0, 25, 50, and 100) in
a volume (ml) equivalent to 150% of the mass lost (g) by volume
depletion. Body mass loss and sweat electrolyte
(Na+,
K+, and
Cl
) loss were the same on
each trial. The measured sweat sodium concentration was 49.2 ± 18.5 mmol/l, and the total loss (63.9 ± 38.7 mmol) was greater than that
ingested on trials 0 and
25. Urine production over the 6-h
recovery period was inversely related to the amount of sodium ingested.
Subjects were in whole body negative sodium balance on
trials 0 (
104 ± 48 mmol)
and 25 (65 ± 30 mmol) and
essentially in balance on trial 50 (13 ± 29 mmol) but were in positive sodium balance on
trial 100 (75 ± 40 mmol). Only on
trial 100 were subjects in positive
fluid balance at the end of the study. There was a large urinary loss
of potassium over the recovery period on trial
100, despite a negligible intake during volume
repletion. These results confirm the importance of replacement of
sodium as well as water for volume repletion after sweat loss. The
sodium intake on trial 100 was
appropriate for acute fluid balance restoration, but its consequences
for potassium levels must be considered to be undesirable in terms of
whole body electrolyte homeostasis for anything other than the short
term.
fluid balance; electrolyte balance; sweat electrolyte loss; dehydration; rehydration
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INTRODUCTION |
THE NEED TO REPLACE electrolytes after volume depletion
is linked to the loss of electrolytes in sweat; ingestion of large volumes of plain water after exercise-induced volume depletion results
in a rapid fall in plasma osmolality and in the plasma sodium
concentration (12-14), and both of these effects stimulate urine
output. Addition of sodium to ingested fluids maintains the circulating
vasopressin levels, and an increase in urine production is avoided.
There is, however, no clear indication of the amount of sodium that is
necessary to optimize volume repletion after sweat loss: excessive
sodium intake will stimulate a natriuresis, which will be accompanied
by an increased urine flow. It might seem reasonable to replace the
electrolytes in amounts approximately equal to those lost in sweat. The
composition of human sweat appears, however, to be highly variable (8),
and the electrolyte content as well as the total volume of the sweat
loss determines the extent of electrolyte loss.
The importance of the addition of sodium to volume repletion fluids has
been systematically evaluated by Maughan and Leiper (9), who volume
depleted subjects by the equivalent of 2% of body mass by intermittent
exercise in the heat; subjects then ingested one of the test drinks in
a volume equal to 150% of their mass loss. The test drinks contained
~0, 25, 50, or 100 mmol/l sodium. Urine output over the subsequent
few hours was inversely proportional to the sodium content of the
ingested fluid, and only when the sodium content exceeded 50 mmol/l did
subjects remain in positive fluid balance throughout the recovery
period. At the end of the study period the mean difference in net fluid
balance between the trials with 0 and 100 mmol/l sodium was
substantial, amounting to 787 ml. The plasma volume was calculated (2)
to have decreased with volume depletion on all trials and increased after volume repletion on all trials; the mean decrease over all trials
amounted to ~4%. The increase in plasma volume after drinking occurred less rapidly with the 0 mmol/l beverage, so that 1.5 h after
the end of the fluid ingestion period, the increase in plasma volume
was 6.8% compared with increases of 12.4 and 12.0% with the 50 and
100 mmol/l drinks, respectively. There was no difference between trials
in the extent of the change in plasma volume 5.5 h after the end of the
period of fluid consumption, but there was a tendency for it to be
related to the quantity of sodium consumed, with the smallest increase
with the 0 mmol/l beverage and the greatest increase with the 100 mmol/l beverage. These observations were confirmed in a further study
in which the volume of fluid ingested and the sodium content of drinks administered after exercise-induced hypohydration were varied systematically (17). This study showed that even when a volume equal to
twice the sweat loss was ingested, subjects did not remain in a
positive fluid balance when a low-sodium (23 mmol/l) drink was
consumed; when a drink containing 61 mmol/l sodium was consumed, subjects maintained a positive fluid balance when the volume of fluid
ingested was 
1.5 times the loss.
In none of these previous studies was the sweat composition measured,
with the result that electrolyte balance could not be calculated. The
present study aimed to investigate the effect on fluid balance of
variations in the quantities of sodium consumed with beverages and to
relate the effectiveness of volume repletion to the sweat electrolyte
loss.
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METHODS |
Six subjects (4 men and 2 women) volunteered to take part in this
investigation. All were more-or-less physically active on a
recreational basis, but none were training systematically at the time
of the study. All gave their written informed consent to participate,
and the investigation was approved by the local Ethics Committee. Their
physical characteristics (means ± SD) were as follows: 28 ± 8 yr of age, 168 ± 11 cm height, 69.0 ± 13.8 kg body mass, 53.8 ± 5.9 ml · kg1 · min1
maximum O2 uptake
(
O2 max).
Four experimental trials were undertaken by each subject, and a
different beverage was ingested on each. Each trial had three distinct
phases. The first, the volume depletion phase, consisted of
intermittent exercise in the heat, such that a moderate level of
hypohydration (~2% of body mass) was induced. The second, the volume
repletion phase, consisted of fluid ingestion over a 1-h period
commencing 40 min after the end of exercise. In the third and final
phase, the recovery phase, the subject rested in the laboratory for 6 h. Blood and urine samples were collected at intervals throughout the
study.
Before starting the experimental trials, each subject underwent three
preliminary trials. The first involved measurement of O2 max, which served
to determine the exercise workload to be used during the volume
depletion exercise; this was carried out by means of a discontinuous,
incremental cycle ergometer test. On the second preliminary trial,
subjects underwent only the volume depletion procedures, as described
below, and were then free to leave the laboratory. On the third
preliminary trial, subjects underwent the full experimental procedure,
but the recovery phase was reduced to only 2 h; blood samples were not
collected from those individuals who were accustomed to the procedure
and had previously undertaken studies of a similar nature. The second and third preliminary trials were undertaken to familiarize the subjects with all the procedures involved in the study and to determine
the duration of exercise that was required to volume deplete subjects
by 2% of body mass. In these trials, subjects were weighed every 10 min to monitor their body mass loss; no sweat samples were collected
during the preliminary trials.
All experimental trials commenced in the morning after an overnight
fast, except for the ingestion of ~500 ml of plain water 2 h before
the scheduled start of the experiment. Trials took place at 7-day
intervals, so that, for each subject, trials were conducted on the same
day of the week. For the 2 days preceding each trial, subjects followed
similar diets and activities to standardize the starting conditions for
each trial; they kept a diary on the 2 days before the first trial and
reproduced the same pattern of eating and physical activity on the 2 days preceding each of the subsequent three trials.
On arrival at the laboratory, subjects rested in a seated position in a
room maintained at ~24°C. After 15 min of seated rest, a venous
blood sample was collected without stasis from an antecubital vein.
After the blood sample was collected, subjects were asked to give a
urine sample by emptying their bladders as completely as possible and
collecting the entire volume.
Subjects showered and washed themselves with soap and water and then
rinsed themselves with 4 liters of distilled, deionized water with a
specific resistance >18 M
· cm. Subjects were
instructed to wash themselves thoroughly with copious amounts of soap
in the shower. On leaving the shower, they stood in a stainless steel tray and washed with the deionized water. Subjects then dried themselves carefully with a prepared washed towel and put on a pair of
plastic operating theater overshoes to avoid contact with the floor.
Nude body mass was measured to the nearest 10 g. Subjects then entered
the exercise room.
Volume depletion was induced by intermittent cycle ergometer exercise
in the heat. Subjects performed a series of 5-min bouts of exercise at
an intensity corresponding to ~60% of
O2 max; each 5-min exercise period was followed by a 5-min rest period. Exercise was performed in a climatic chamber maintained at 34°C and
60-70% relative humidity. The cycle ergometer was contained within a large polythene bag that was stretched over a plastic frame.
The procedure has been fully described previously (16). The bags used
in this study (Anaplast, Ayrshire, UK) were obtained from an
agricultural merchant and were intended to contain silage. Preliminary
trials showed that the bags were free from contamination with
electrolytes. When the subject entered the laboratory, the frame and
ergometer were in place, but the plastic bag was pulled up to a height
of only ~600-800 mm. The subject removed the plastic overshoes
and stepped into the bag; the bag was then pulled up fully and
stretched over the top of the plastic frame. The subject then put on
the prepared washed shorts and was ready to begin exercise. Subjects
were dressed only in shorts: shoes were not worn. The intention was to
volume deplete each subject by 2% of body mass; the duration of
exercise required by each subject to achieve this was determined in the
preliminary trials and ranged from 30 to 70 min (median 43 min). The
total sweat loss in all trials was therefore approximately the same
when expressed relative to each subject's body mass, although sweating
rates were very different.
On completion of the prescribed exercise, the subjects removed their
shorts and washed themselves with 4 liters of deionized water to which
had been added ammonium sulfate (20 mmol/l). This water was presented
in four 1-liter sports drink bottles, and no other washing implements
were provided. Subjects were instructed to wash themselves as
completely as possible and to remove as much surface water as they
could before leaving the bag. An additional 1 liter of deionized water
containing ammonium sulfate was then used to wash down the inside of
the bag and the bicycle, with care being taken to ensure that as much
liquid as possible was flushed toward the bottom of the bag.
The liquid in the bag at this stage contained the sweat that had not
evaporated plus the water used to wash the subject at the end of the
trial. The volume of unevaporated sweat was calculated from the
dilution of the added ammonium and sulfate; preliminary studies had
failed to detect the presence of ammonium or sulfate in sweat.
Duplicate samples of the water in the bag were collected using a
syringe and needle and stored for later analysis.
At the end of the volume depletion phase, subjects dried themselves
completely before nude body mass was again measured. The difference in
mass was used to estimate total sweat loss, with the assumption that
the specific gravity of sweat is 1.0 (7). The respiratory water loss
(48 ± 10 g) and mass loss due to substrate oxidation (43 ± 10 g) were estimated for each subject in the preliminary trials (11);
these were assumed to be the same on each trial and were taken into
account in the estimation of the sweat loss.
Subjects showered and dressed before returning to sit in a comfortable
environment within 25 min of the end of exercise. After ~10 min of
seated rest, a 21-gauge venous cannula was inserted into a superficial
forearm vein and was flushed with isotonic saline. A further 5 min
elapsed before the first postexercise blood sample was withdrawn; this
was collected, therefore, 40 min after the end of exercise and after at
least 15 min in a sitting position. A urine sample was then obtained;
for this, as for all samples, subjects were instructed to empty their
bladders as completely as possible and to collect the entire volume.
The volume repletion period then commenced, and over the following 60 min subjects consumed a drink containing 0, 25, 50, or 100 mmol/l
sodium (Table 1) plus 125 ml/l
low-energy lemon flavoring; for the three sodium-containing drinks, 25 mmol/l sodium was from sodium chloride and the remainder, where
applicable, was from sodium acetate. A different beverage was consumed
on each trial, but in each case the volume consumed (in ml) was
equivalent to 150% of the mass loss (in g), with the assumption that
the entire mass loss was due to water loss. Further blood samples were
collected at the end of the volume repletion period and at 1, 2, 4, and
6 h thereafter, and further urine samples were collected at the end of
the volume repletion period and at 1, 2, 3, 4, 5, and 6 h thereafter.
Subjects remained within the laboratory environment at all times and
were seated for at least 15 min before collection of each blood sample
to minimize the effects of postural changes on the redistribution of
water between the major body water compartments. When blood and urine
sample times coincided, the urine sample was collected immediately
after blood sample collection.
Analytic methods.
For each blood sample, 8 ml were collected. Part of this (2.5 ml) was
added to a K2EDTA tube and was
used for measurement of hemoglobin concentration (by the
cyanmethemoglobin method) and of packed cell volume (by
microcentrifugation). The values so obtained were used to calculate
changes in blood, red cell, and plasma volumes using the equations
described by Dill and Costill (2); all calculations were made relative
to the blood sample collected 40 min after exercise. Part (2.5 ml) of
each sample was added to a chilled
K2EDTA tube and centrifuged at
1,500 g for 5 min at 4°C before
the plasma was removed. This was then used for measurement of plasma
vasopressin and aldosterone concentrations by RIA (Euro-Diagnostica,
Cornwall, UK; Serono Diagnostics, Rome, Italy). The remainder of the
sample was allowed to clot before being centrifuged, and the serum was
then removed. The serum was analyzed for sodium and potassium
concentration by flame photometry (model 410C clinical flame
photometer, Corning, Halstead, Essex, UK) and for chloride
concentration by coulometric titration
(Cl meter, Jenway, Dunmow,
Essex, UK). Serum osmolality was assessed by freezing-point depression
(Osmomat 030 cryoscopic osmometer, Gonotec, Berlin, Germany).
For each urine sample the volume was measured and a 5-ml aliquot was
retained. Urine sodium, potassium, and chloride concentrations and
osmolality were measured by the same methods used for serum analysis.
Sweat sample analysis was carried out by ion chromatography using an
ion chromatograph system (model DX-100, Dionex, Sunnyvale, CA). Samples
were first diluted 1:100 in filtered, distilled, deionized water, and
sodium, potassium, chloride, ammonium, and sulfate were analyzed.
All analyses were carried out in duplicate, and the mean value of the
duplicates was taken, except for packed cell volume, which was measured
in triplicate, and the serum vasopressin and aldosterone
concentrations, which were analyzed singly.
Statistical analysis.
Values are means ± SD or medians and ranges where they were found
not to be normally distributed. Statistical comparisons were made after
the normality or otherwise of the distribution of the data points was
established. Analysis was by repeated-measures ANOVA followed by
one-way ANOVA and Tukey's multiple range test or the Kruskal-Wallis
and Mann-Whitney tests where appropriate. Differences between and
within trials were considered significant where
P < 0.05.
| |
RESULTS |
The subjects' preexercise body mass was the same on all four trials:
69.05 ± 13.99, 69.00 ± 13.82, 69.20 ± 13.64, and 69.17 ± 13.97 kg for trials 0, 25, 50,
and 100, respectively
(P = 1.000). Subjects exercised for
the predetermined duration, established from the preliminary
procedures, on each of their trials, and they achieved the same degree
of body mass loss on each trial (P = 0.984; Table 2); over all trials the volume
depletion was 1.89 ± 0.17% of body mass. Over all trials the mean
sweat sodium concentration was 49.2 ± 18.5 mmol/l and the mean
sweat potassium concentration was 5.5 ± 1.4 mmol/l. The quantities
of sodium (P = 0.999), potassium
(P = 0.979), and chloride
(P = 0.939) lost in the sweat did not
differ from week to week (Table 2). Because the total volume of sweat
loss was the same on each trial, the volume of drink consumed also did
not differ between trials. The volume of drink consumed and the
electrolyte intake on each trial are shown in Table 2.
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Table 2.
Sweat electrolyte loss during volume depletion and the volume of drink
consumed on each trial together with the electrolyte intake
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Urine output and fluid balance.
The volume of urine produced at each collection point over the duration
of the study is shown in Fig. 1. On each
trial the greatest urine output occurred over the 1-h period ending 1 h after the end of the volume repletion period. There was no statistical difference in the volume produced at this time between the trials (P = 0.457), but there was a tendency
for the greatest volume on trial 0 and
the least on trial 100. Over the 1-h
period ending 2 h after the end of the volume repletion period, the
volume of urine produced on trial 100 was significantly less than that produced on trial
0 or 25 (P = 0.006). One hour later, 3 h after
the end of the volume repletion period, the same volume of urine was
produced on trials 50 and
100; on both trials the volume was
less than the volume produced on trials
0 and 25 (P = 0.016).
Over the entire time after volume depletion (i.e., the volume repletion
and recovery periods), the total volume of urine produced was
inversely proportional to the quantity of sodium ingested and amounted
to 1,182 ± 438, 970 ± 340, 800 ± 353, and 578 ± 280 ml on trials 0, 25, 50, and
100, respectively
(P = 0.033).
From the volume of drink consumed and the volume of urine produced
together with the volume of sweat lost, whole body net fluid balance
relative to the starting euhydrated position can be calculated (Fig.
2). On losing water, in the form of sweat or urine, subjects move in the direction of negative fluid balance, and
on gaining water by drinking, subjects move in the direction of
positive fluid balance. When the sodium-free drink was consumed, subjects were significantly volume depleted at the end of the study
period (642 ± 342 ml) and, indeed, had been in a state of
negative fluid balance from 2 h after the end of the volume repletion
period. The same was true on trials 25 and 50, although the extent of whole
body negative fluid balance was less on trial 25 (403 ± 394 ml) than on
trial 0 and less on
trial 50 (266 ± 366 ml)
than on trial 25. After consuming the
100 mmol/l drink the subjects remained in positive fluid balance for
the entire recovery period, and at the end of the study period, 6 h
after the end of the volume repletion period, subjects were essentially euhydrated (16 ± 322 ml).
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Fig. 2.
Whole body net fluid balance calculated from estimated sweat loss,
volume of fluid ingested, and urine output over course of experiment.
Preexercise urine sample is not included in these calculations. Points
are mean values.
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Urine composition and electrolyte balance.
Despite large differences in the quantities of sodium ingested on each
trial, the quantities excreted in the urine after volume repletion on
trials 0, 25, and
50 did not differ from each other (P = 0.272), but there was a tendency
(P = 0.093) for a larger excretion of
sodium on trial 100 (Table
3).
On trial 25, only 9% (60 to
60%) of the ingested sodium had been retained by the end of the study
period, 6 h after the end of the volume repletion period; this
represents a retention of ~4 mmol. On trial
50, 53% (2-83%) or ~53 mmol were retained, and on trial 100, 72% (56-80%) or
~148 mmol were retained. The fraction retained was significantly less
on trial 25 than on
trial 100 (P = 0.013).
Whole body net sodium balance was calculated as for water balance, with
the sweat sodium losses taken into account (Fig.
3). On trials
0 and 25, insufficient
sodium was ingested with the drink to replace the losses incurred with
volume depletion, and subjects were therefore in sodium deficit
throughout the recovery period. On trial
50, sufficient sodium was consumed with the beverage to
replace the sodium losses, and with the urinary excretion over the
recovery period, subjects were in slight net negative sodium balance
(13 ± 29 mmol) at the end of the study period. On
trial 100 the amount of sodium
consumed with the beverage (206 ± 55 mmol) was far in excess of the
quantities lost in the sweat during volume depletion (65 ± 46 mmol), and subjects were in a state of positive sodium balance at the
end of the volume repletion period. There was some urinary sodium loss
during the recovery period (Table 3), but at the end of the study, 6 h
after the end of the volume repletion period, subjects were still in a
significant positive sodium balance (75 ± 40 mmol).
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Fig. 3.
Whole body net sodium balance calculated from sweat sodium loss, amount
of sodium ingested, and urine excretion of sodium over course of
experiment. Preexercise urine sodium loss is not included in these
calculations. Points are mean values.
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The potassium content of the drinks was low (0.6 ± 0.1 mmol/l)
relative to that of sweat (5.5 ± 1.4 mmol/l) and was not sufficient to replace the sweat losses (Table 2). However, although this was small
and did not differ between trials, the amount of potassium excreted in
the urine after volume repletion was greater
(P = 0.006) after consumption of the
100 mmol/l drink than on any of the other trials (Table 3); the
greatest excretion occurred over the 1-h periods ending (0 h) and 1 and
2 h after the end of the volume repletion period. The intake of
chloride in the beverages was the same on trials 25, 50, and 100, but
chloride was virtually absent from the sodium-free drink (Table 2).
However, even on trials 25, 50, and
100, insufficient chloride was
consumed to replace the sweat losses. A smaller quantity of chloride
tended to be excreted over the 1-h periods ending 3 and 4 h after the end of the volume repletion period (both
P = 0.058), and as a result
the cumulative quantity excreted over the entire recovery period tended
to be less on trial 100 than on the
other trials (Table 3).
Urine osmolality values generally mirrored the urine volume. The
osmolality was higher on trial 100 than on trial 0 at 2, 3, 4, and 5 h
after the end of the volume repletion period, was greater than that on
trial 25 at 2, 3, and 4 h after the
end of the volume repletion period, and was also greater than that on trial 50 at 2 h after the end of the
volume repletion period. On each trial the lowest osmolality was
recorded 1 h after the end of the volume repletion period, which
corresponded with the time of greatest urine volume production (Fig.
1).
Blood and serum measurements.
Blood volume was calculated to have decreased with volume depletion on
all trials, and over all trials the decline amounted to 3.6 ± 2.5% (Table 4;
P = 0.782). With beverage consumption there was a restoration to preexercise levels on all trials, and the
increase was generally influenced by the amount of sodium ingested.
Plasma volume generally changed in the same manner as the blood volume,
but the magnitude of the changes was larger (Fig.
4); the decrease with volume depletion
amounted to 5.5 ± 4.7% over all trials
(P = 0.556). Red cell volume decreased
with volume depletion on all trials, and over all the trials the
decline amounted to 1.3 ± 1.6% (Table 4). With volume repletion
the red cell volume increased on trial
0 (P = 0.000), whereas
on trial 100 there was no change from
the volume-depleted volume (P = 0.637); these volume changes were significantly different between the trials at the end of the volume repletion period (0 h) and 1 h later.
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Fig. 4.
Calculated change in plasma volume over duration of study. Calculations
were made relative to values obtained 40 min after end of exercise
(postexercise). Points are mean values.
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On all trials there was a tendency for the serum sodium concentration
to increase with volume depletion (Table
5). On trial 0 the serum sodium concentration at the end of the
volume repletion period (0 h) and at 1 and 2 h later was lower than the
preexercise level, but on none of the other trials did the serum sodium
concentration fall below the preexercise level. The serum potassium
concentration was little affected by any of the treatments (Table 5);
on trial 100 the serum potassium
concentration 1, 2, 4, and 6 h after the end of the volume repletion
period was lower than the preexercise level. The serum chloride
concentration tended to increase with volume depletion and was restored
to preexercise levels with volume repletion (Table 5). There were no
significant differences between trials at any time, nor did the serum
chloride concentration change significantly over the course of any
trial.
Serum osmolality was little affected by any of the treatments over the
duration of the study on any trial (Table
6). Although not significantly different
between trials, serum osmolality tended to increase with volume
depletion (over all trials the increase amounted to ~4 mosmol/kg) and
to decline to values slightly below the preexercise value on all trials
over the recovery period.
The plasma aldosterone and vasopressin concentrations did not differ
between trials at any time, although both tended to fluctuate over the
course of each trial. With volume depletion there tended to be an
increase in concentrations from 34.1 (range 24.8-77.8) to 43.4 (range 19.6-187.3) pg/ml for aldosterone and from 9.4 (range
4.0-21.9) to 10.8 (range 4.7-18.9) pg/ml for vasopressin (over all trials), followed by a return to the preexercise levels over
the 6-h recovery period. On trials 25, 50, and 100, when sodium was consumed in the beverages, the peak aldosterone
concentration was recorded at the end of the volume repletion period,
but on trial 0, when no sodium was
consumed, the peak plasma aldosterone concentration was recorded 1 h
after the end of the volume repletion period.
| |
DISCUSSION |
The results of this study confirm that the addition of sodium to fluids
ingested after exercise-induced volume depletion has a significant
effect on the restoration and maintenance of whole body fluid balance.
After volume depletion by 1.89% of body mass and fluid ingestion
equivalent to 150% of this volume, subjects retained a mean of 71% of
the ingested volume when the beverage had a high (102 mmol/l) sodium
concentration but a mean of only 37% when virtually no sodium was
present (1 mmol/l).
In the present study the volume of drink consumed was 150% of the
water loss with volume depletion; this ensured a positive fluid balance
at the end of the volume repletion period (0 h). However, on only two
of the trials in this study was sufficient sodium ingested to replace
the losses in the sweat, despite the relatively large volume of fluid
consumed. In previous studies of a similar nature (9, 10), water loss
could be reasonably reliably quantified, but electrolyte losses could
only be estimated. The development of a simple and reliable method for
whole body sweat collection (16) made possible the measurements in the present study and allowed calculation of electrolyte as well as water
balance. The sweat had a mean sodium concentration of 49.2 ± 18.5 mmol/l, and the sweat sodium loss amounted to 63.9 ± 38.7 mmol over
all trials (Table 1).
On trial 50 the quantity of sodium
ingested was >50% as much as the sweat loss (99 mmol intake vs. 64 mmol loss; Table 2), and despite the urinary losses of sodium over the
6 h recovery period, subjects remained in the region of whole body net
sodium balance for the 6 h of the recovery period (Fig. 3). However, subjects were in a state of whole body fluid deficit for the majority of this time (Fig. 2). On trial 100 the sodium intake was ~300% of the sweat loss (206 mmol intake vs.
65 mmol loss; Table 2), and even with the tendency for a greater
urinary excretion of sodium during the 6-h recovery period on this
trial relative to the other trials (Table 3), subjects were in a state
of positive sodium balance for the entire 6-h recovery period (Fig. 3).
Over this time, subjects also remained in a state of positive fluid balance, and at the end of the 6-h recovery period they were
essentially euhydrated relative to their preexercise condition (Fig.
2). The relationship between whole body net fluid balance and whole
body net sodium balance for the last 2 h of the recovery period is shown in Fig. 5; there is a close
relationship between the whole body sodium balance and the whole body
fluid balance (correlation coefficient = 0.521).
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Fig. 5.
Whole body net sodium balance ( ,  ) and whole body net cation
balance ( ,  ) vs. whole body net fluid balance 5 (, ) and 6 (, ) h after end of volume repletion period. For net sodium
balance, y = 59.75 + 0.28 x;
R2 = 0.521. For net cation balance, y = 1.68 + 0.23x; R2 = 0.577. Points
are mean values. Fitted lines are calculated from 6-h values only.
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It is generally agreed that, provided a sufficient volume of water is
consumed, sodium replacement is the most important factor in achieving
effective restoration of fluid balance (4, 9, 14). The requirement for
sodium replacement stems from its role as the major ion in the
extracellular fluid and the major electrolyte lost in sweat. If
sufficient sodium and water are ingested, some of the sodium remains in
the vascular space, with the result that plasma osmolality and sodium
concentration do not markedly decline as may occur if plain water is
ingested (13). As a result, the plasma levels of vasopressin and
aldosterone are maintained, and a diuresis that would result in
creating a whole body net negative fluid balance is prevented. There
was little difference between trials in the plasma concentrations of
vasopressin and aldosterone at any time in the present study; the same
was true of serum osmolality (Table 6), although serum sodium
concentration (Table 5) did tend to decline on trial
0, when the sodium-free drink was consumed. The effects
on urinary output of any changes in plasma sodium concentration and
osmolality will, however, be delayed slightly because of the time to
inactivate circulating vasopressin and aldosterone or stimulate their
release; vasopressin has a half-life in plasma of ~10-20 min (1,
3), and aldosterone acts via the synthesis of new proteins, including
Na+-K+-ATPase,
so a time lag of ~30-90 min occurs before its effects take place
(6). 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 (15). Differences in blood volume were observed
between treatments in the present study, such that the general pattern
was for the increases in plasma volume to be a direct function of the
quantity of sodium ingested (Fig. 4); this would, however, cause
changes in the opposite direction to the effect seen. Therefore, it
seems most likely that the renal response that resulted in each trial
was appropriate, in terms of water and sodium excretion, thus
maintaining the relatively constant vasopressin, aldosterone, and
sodium concentrations.
The measurement of water, sodium, potassium, and chloride losses in
sweat and urine and intake via the drinks allowed calculations to be
made of the whole body water and electrolyte
(Na+,
K+, and
Cl) balance over the
duration of the study. In the same way as an excess of water must be
ingested to have a chance of restoring fluid balance, an electrolyte
intake in excess of the losses must also occur to compensate for
ongoing urinary losses. In terms of the cations investigated
(Na+ and
K+), on trial
100, when the large amount of sodium was ingested, a
significant fraction of it was retained [72% (range
56-80%)], although it was not readily apparent that this
excess was being retained in the extracellular space; there was no
increase in serum sodium concentration (Table 5), but the greater
expansion of plasma volume on this trial would accommodate some of this sodium. Because of the rapid exchange of water between the plasma and
the other fractions of the extracellular fluid, it is reasonable to
assume that the changes in the plasma volume will reflect those of the
extracellular fluid as a whole. From the height and body mass of the
subjects in the present study, their plasma volume was estimated to be
2,309 ml (7) and their extracellular fluid volume as a whole to be
9,236 ml (5). Relative to the preexercise situation, the plasma volume
expansion was by ~8.2% on trial
100. This represents an increase in plasma volume of
189 ml and in extracellular fluid volume of 757 ml; the plasma volume
is approximately one-fourth of the extracellular volume (5). Given the
minimal difference in plasma sodium concentration on
trial 100 between the start and end of
the study (Table 5), the increased volume could accommodate an extra
117 mmol of sodium. At the end of the study, subjects were in positive
sodium balance by 75 ± 40 mmol (Fig. 3), and this all could be
accommodated in the increased fluid volume.
However, even though the losses of potassium in sweat are small
relative to those of sodium (7.1 ± 3.0 vs. 63.9 ± 38.7 mmol over all trials; Table 2) and virtually no potassium was consumed with
the beverage on any trial (1.1 ± 0.4 mmol over all trials; Table
2), on trial 100 a large amount of
potassium was excreted over the 6-h recovery period relative to the
other trials. This electrolyte excretion (high
K+, low
Cl) is most likely to be
due to a metabolic alkalosis as a result of the metabolism of the
acetate. Bicarbonate is likely to have been secreted in greater amounts
on this trial than the others, but this was not measured in our study.
This has the effect of giving the whole body net cation balance status
shown in Fig. 5. However, in terms of potassium balance, subjects were
in deficit throughout the recovery period due mainly not to the loss of
potassium in sweat during exercise but, rather, to the loss in urine
during the recovery period. Calculations suggest that only
a small fraction of this (~0.5 mmol) came from the extracellular
space and that the majority, as expected, came from the intracellular
fluid.
The quantification of sweat electrolyte losses during exercise and
replacement in the postexercise recovery period has allowed calculation
of whole body water and electrolyte balance. In the present study this
has enabled the monitoring of the recovery of water and sodium balance.
The 102 mmol/l sodium drink in this study appears to maximize the acute
restoration of fluid balance, but its consequences on potassium levels
must be considered undesirable in terms of whole body electrolyte
balance for anything other than the short-term situation.
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Abstract
Introduction
