Updated Fluid Recommendation: Position Statement From the International Marathon Medical Directors Association (IMMDA)

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Updated Fluid Recommendation: Position Statement
From the International Marathon Medical
Directors Association (IMMDA)
Tamara Hew-Butler, DPM,* Joseph G. Verbalis, MD,
w
and Timothy D. Noakes, MBChB, MD, DSc*
(Clin J Sport Med 2006;16:283–292)
Controversy exists regarding optimal fluid guidelines
for athletes engaging in different sports. Most
published recommendations emphasize the detrimental
consequences of dehydration,
1–4
while more recent
reports warn of the morbid consequences of hyperhydra-
tion.
5–7
Accordingly, individualized recommendations
emphasizing a balance between the two extremes have
evolved.
8–10
These revised guidelines, however, continue
to promote static recommendations for what in reality are
very dynamic athletic situations.
Marathon running epitomizes a dynamic situation
that requires a constant adjustment to constantly chan-
ging homeostatic requirements. Real-time assessments of
fluid and sodium homeostasis are physiologically repre-
sented by changes in plasma osmolality (POsm). Pituitary
arginine vasopressin (AVP) secretion is stimulated when
POsm increases by only 1% to 2%, representing the
body’s attempt to prevent dehydration by decreasing
kidney water excretion. After maximal antidiuresis is
achieved, thirst is then stimulated to replace water losses
that are in excess of the ability of AVP-stimulated
antidiuresis to conserve body water. In general, the
osmotic threshold for thirst is 5 to 10 mOsm/g H
2
O
higher than that of AVP secretion. Thus, thirst is
stimulated with decreases in body water of approximately
1.7% to 3.5%.
11–13
Concomitant performance decrements
and cardiovascular strain are also documented when
baseline body fluid losses exceed approximately 2%.
4
The failure of athletes to replace 100% of body-
weight losses from ad libitum fluid intake has been
well-described as ‘‘involuntary’’
14,15
or ‘‘voluntary’’ dehy-
dration;
16–18
this phenomenon has resulted in the devalua-
tion of thirst as a ‘‘poor’’ indicator of body fluid needs.
The combination of laboratory with recent field data (see
Table 1 and subsequent discussion), however, suggests that
the body primarily defends POsm, and not blood or
extracellular fluid volume during prolonged endurance
exercise. Because it is well established that thirst is
stimulated in response to changes in tonicity in most land
mammals,
11,13,19–22
it seems reasonable to conclude that
thirst would be the predominant physiologic and dynamic
regulator governing fluid balance during exercise.
This article will focus on the physiology of normal
fluid balance as the ultimate guide toward the evolution
of optimal fluid recommendations. Six physiologic con-
siderations of fluid and sodium balance will be detailed
followed by 6 practical recommendations. The neuro-
endocrine regulation of homeostasis will be emphasized in
defense of the behavioral drives for thirst and sodium
palatability during dynamic activities such as prolonged
endurance exercise. The behavioral drives to maintain
fluid and sodium balance are evolutionary stable,
essential for the safety and survival of the species, and
deeply rooted within the human genetic makeup.
23
PHYSIOLOGIC CONSIDERATIONS OF FLUID
AND SODIUM BALANCE
1. Bodyweight is not an accurate surrogate for body
fluid volume during prolonged exercise: The field of
exercise science promotes fluid replacement at 100% of
bodyweight losses under the premise that sweat loss is
an accurate surrogate for changes in body fluid
volume.
1,2,4,8,24–28
This recommendation—to externally
defend bodyweight during exercise—remains entrenched
in the literature despite initial observations in 1932 stating
that voluntary water consumption replaced only 56% of
sweat lost during exercise.
29
Research on ad libitum fluid
intakes over the last 74 years have repeatedly verified that
Copyright r2006 by Lippincott Williams & Wilkins
Received for publication May 12, 2006; accepted May 26, 2006.
From the *Department of Human Biology, University of Cape Town,
Cape Town, South Africa; and wDivision of Endocrinology and
Metabolism, Georgetown University Medical Center, Washington,
DC.
This position statement was approved and endorsed by the IMMDA
Body on May 6, 2006 in Barcelona, Spain. The lay statement for
immediate release was editorially prepared by Lewis Maharam, MD,
FACSM (Chair), Arthur Siegel, MD, Marvin Adner, MD, W. Bruce
Adams, MD, Rudra Sabaratnam, MD, and Pedro Pujol, MD,
FACSM.
IMMDA address: Lewis G. Maharam, MD, IMMDA Board of
Governors at 24 West 57th Street, 6th floor, New York, NY 10019
(e-mail: NYSPORTSMD@aol.com).
Reprints: Tamara Hew-Butler, DPM, UCT/MRC Research Unit for
Exercise Science and Sports Medicine, Department of Human
Biology, University of Cape Town, P.O. Box 115, Newlands 7725,
South Africa (e-mail: thew@sports.uct.ac.za).
POSITION STATEMENT
Clin J Sport Med Volume 16, Number 4, July 2006 283

humans, when given free access to fluids, replace no more
than 75% of net water losses during physical activ-
ity.
15–17,30–32
Abdominal distress, nausea and vomiting
have been reported in highly trained distance runners who
have tried to match fluid intakes to sweat rates.
17,33
The
ongoing discrepancy between thirst, satiation and the
inability to inherently maintain bodyweight during
prolonged endurance activity suggests that bodyweight
is only a marker of body fluid homeostasis and is not
regulated in itself. The remarkable capacity of the body to
maintain circulating plasma volume despite large deficits
in bodyweight due to ‘‘sweat losses’’ during prolonged
endurance exercise exemplifies the complex regulation of
body fluid volume and distribution
34
that are unrelated to
the maintenance of bodyweight.
2. Blanket fluid guidelines with fixed ranges will not
protect the diverse population currently participating in
athletic events: The American College of Sports Medi-
cine’s (ACSM) current guidelines promote a fluid intake
of 600 to 1200 mL/h; a range largely based on laboratory
studies performed on elite male athletes.
1
The popularity
of marathon running increased markedly at the time these
guidelines were released, with charity running groups
enticing more recreational runners into the sport. Slower,
more inexperienced, female athletes heeded the upper
limit of the ACSM guidelines and developed exercise-
associated hyponatremia (EAH) by ingesting more fluid
than they could excrete over the course of a marathon
run.
35,36
In 2001, the International Marathon Medical
Directors Association (IMMDA) approved guidelines
lowering the ‘‘acceptable’’ range to 400 to 800 mL/h to
adjust the upper limit and protect smaller athletes (mainly
female) from overdrinking.
7
Recently, mathematical models have proposed that
these ranges may not be ideal to cover the current
population of runners participating in marathon events.
37
Close inspection of a marathon field today illustrates the
variety of shapes, sizes and speeds that must be taken into
consideration when formulating fluid intake guidelines.
For example, the average weight of 7299 runners
registering for the 2005 Comrades Marathon was 73 kg.
The lightest competitor weighed 43 kg whereas the
heaviest runner weighed 119 kg. The fastest runner
averaged 16.4 km/h (winning time 5:27) whereas the
slowest runners ran at a 7.4 km/h pace (the official 12-h
cut-off pace). The 11,728 runners who completed the
89.2 km Comrades Marathon ran, on average, 49 minutes
slower during the second half of the race than they ran the
first half of the race with a minimum and maximum range
of 11 (negative split) and 180 minutes (positive split),
respectively. The wide variation between split times
exemplifies the late-race functional deterioration that
has been described elsewhere
38
and highlights the fact that
modern marathon running is a dynamic situation,
including participants with widely diverse physical
attributes and fitness levels. An even wider spread of
running speeds and bodyweights would be expected in a
standard 42.2 km marathon, as the Comrades Marathon
is more than double the distance, requires a qualifying
time for entry, has a defined cut-off and is held over a
grueling terrain; all of which precludes the participation
of unfit and novice runners. Conversely, relatively unfit
and untrained runners can often complete a standard
42 km marathon, amplifying the wide range of participant
profiles.
The 3 main factors governing fluid loss during
exercise are mass (bodyweight), running speed (metabolic
rate), and ambient temperature.
26,27
Race day tempera-
tures in the New York City Marathon have varied
between 1 and 291C and have fluctuated as much as 171C
from start to finish.
39
Therefore, it would seem very
unlikely that ANY single ‘‘range’’ could successfully
encompass this wide spectrum of weather conditions,
bodyweights, and running speeds that characterize
modern day marathon running.
3. Drinking to thirst will preserve POsm within the
normal range and maintain intracellular volume and
homeostasis: Thirst is a subjective sensation characterized
by a deep-seated desire for water
11,20,23
and is generally
associated with oral sensations such as dryness, irritation,
and an ‘‘unpleasant taste’’ in the mouth.
21,22,40
Thirst has
been quantified using geometric and visual analog
scales
11,21,40,41
and documented to increase when POsm
is 5 to 10 mOsm/kg above the threshold for AVP
secretion; well within the normal physiologic range of
B280 to 295 mOsm/kg H
2
O.
11,22,42
POsm is maintained within this narrow range to
protect intracellular volume. Serum sodium concen-
trations [Na
+
] mirror POsm
43
because sodium is
the principal solute of extracellular fluid; thus [Na
+
]
and POsm are viewed as interchangeable in this discus-
sion, although the appropriate calculation is: POsm =
2[Na
+
]+[BUN]+[glucose] (all in mmol/L).
13
The maintenance of intracellular volume is of vital
importance for cell function and survival. Hypertonicity
causes intracellular dehydration; a bodyweight loss of
5.5% during prolonged severe exercise elicits a reduction
in red cell volume by 3.2%.
44
A reduced intracellular volume can reduce the rates
of glycogen and protein synthesis
45
and hypertonicity
beyond serum sodium concentrations of 160 mmol/L can
cause encephalopathy and death.
46
Conversely, hypoto-
nicity causes intracellular expansion. Although high cell
volume can stimulate glycogen and protein synthesis,
excessive and acute cell swelling from serum sodium levels
below 125 mmol/L can lead to hyponatremic encephalo-
pathy, noncardiogenic pulmonary edema, and death.
47
Thirst and AVP secretion are intimately related:
synergistically lowering elevations in POsm by stimulat-
ing fluid intake and promoting antidiuresis. The threshold
for AVP release (B280 to 285 mOsm/kg H
2
O) is normally
set 5-10 mOsm/kg H
2
O below that for thirst (B290 to
295 mOsm/kg H
2
O), despite significant variation between
individual set points.
11
This evolutionary design liberates
individuals from constantly seeking water, as thirst is only
stimulated when antidiuresis is maximal and hypertoni-
city exceeds the capacity of the kidneys to cope with rising
tonicity. The strength of an intact thirst mechanism is
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284 r2006 Lippincott Williams & Wilkins

exemplified in patients with the disorder of diabetes
insipidus, either due to inadequate AVP secretion or an
impaired kidney response to AVP, where a normal POsm
is maintained by stimulated thirst despite the excretion of
up to 20 L of fluid per day.
11
Oropharyngeal metering
48–50
and stomach fullness
21,22,33
provide inhibitory feedback
to terminate drinking more rapidly than the return of
POsm to normal levels, perhaps as a safety measure to
prevent overdrinking.
The strength and precision of the thirst mechanism
has been demonstrated both at rest and during exercise in
young adults. In one study involving seven men an
increase in POsm, in response to hypertonic saline
infusion, promoted a dipsogenesis proportional to the
increase in POsm; with 82% of the total volume of water
consumed within the first 5 minutes of rehydration with
the immediate cessation of intake due to ‘‘stomach
fullness.’’
22
In a separate fluid deprivation study, 65%
of total water intake was consumed during the first 2.5
minutes of rehydration in 5 males.
21
Subjective ratings of
dry mouth and plasma protein concentrations returned to
baseline levels in 2.5 minutes, plasma volume returned to
baseline levels within 5 minutes and serum sodium
concentrations began to decrease within 5 minutes;
returning to baseline values within 12.5 minutes. This
initial drinking bout was also interrupted by sensations of
‘‘stomach fullness’’. During prolonged endurance exer-
cise, 8 female and 10 male athletes responded to graded
levels of hypohydration with ad libitum fluid intakes
sufficient enough to restore POsm, replace sweat losses,
and attenuate thermal and circulatory strain.
17,30,31
Thus, humans respond to increases in serum [Na
+
]
with AVP secretion, thirst and water intake that occur
when POsm is still well within normal ranges. The
precision and magnitude of the thirst drive is related to
ancestral areas of the brain that are associated with
vegetative functions such as the hunger for air and food,
micturition and pain.
51
To assume that the thirst drive
would be an ‘‘inaccurate index’’ of fluid balance during
exercise (the dynamic homeostatic imbalance alterna-
tively described as the ‘‘flight or fight’’ response) would
seem contradictory to the evolution of our species.
Although there is concern that thirst due to
intracellular dehydration or extracellular volume deple-
tion may be confused with the sensations of a dry mouth
(xerostomia) during exercise, thirst due to xerostomia has
not been shown to lead to severe polydipsia.
41
The
sensation of a dry mouth can promote more frequent
drinking episodes, but total fluid intake is similar, or
counter regulated by increased urinary output, in tested
clinical and experimental settings.
52
Recent studies on
xerostomia, thirst, and interdialytic weight gain have not
factored in osmolality and therefore do not provide
convincing evidence that a dry mouth without changes in
POsm can contribute to excessive fluid intakes or to
hyponatremia from fluid overload without inappropriate
AVP secretion.
53–56
4. The body defends POsm and volume over
bodyweight. ‘‘Involuntary dehydration’’ is a homeostatic
mechanism designed to protect POsm and cellular volume
in response to hypotonic sweat losses: The reluctance of
man to voluntarily replace fluids at amounts equivalent to
bodyweight losses during exercise was recognized in 1933,
when Dill
32
hypothesized that humans only drunk as
much fluid to maintain constant osmolarity of the
extracellular fluid. This ‘‘failure’’ of man to replace
100% of body fluid losses, generally using bodyweight
as a surrogate marker of body fluid volume, has been
subsequently recognized as ‘‘voluntary’’ or ‘‘involuntary’’
dehydration; this has had the effect of minimizing the
importance of thirst as an adequate indicator of body
fluid needs.
14–16
Actual field study data from multiple endurance
studies indicates that the maintenance of bodyweight will
actually lead to reductions in serum sodium concentra-
tions from prerace to postrace (Table 1). Most athletes
who lose less than 2% of bodyweight experience a
decrease in serum [Na
+
].
57,58
Athletes who gain weight
progress from normonatremia to hyponatremia.
59
In
contrast, POsm is maintained within normal ranges
(± 3 mmol/L) when percent bodyweight losses are be-
tween 2% and 4%.
60–67
Whereas most athletes who lose
more than 4% of bodyweight demonstrate increases in
serum [Na
+
] above normal ranges.
44,68–71
Thus, the
cumulative data in Table 1 suggest that the body defends
POsm over body fluid volume, as reflected by changes in
bodyweight, during prolonged endurance activity lasting
between 2.7 and 38.2 hours, and can maintain POsm and
therefore intracellular volume, best between 2% and 4%
cumulative bodyweight losses. Acute bodyweight changes
below 2% or above 4% cannot be compensated for
internally, and may lead to dysregulation of serum [Na
+
]
with resultant clinical symptomatology.
Concordant with these field data, 3 laboratory
studies replacing fluids to 100% of bodyweight losses
confirm a reduction in serum sodium concentrations
during prolonged endurance exercise lasting between 2
and 6 hours.
72–74
Although these decreases in serum
sodium concentrations were generally within normal
physiologic ranges, fluid replacement at or above 100%
does not seem to offer any performance benefits.
73,75
Advocates of replacing 100% of bodyweight losses
argue that cardiovascular drift—the downward drift in
central venous pressure and stroke volume with con-
comitant rise in heart rate—commences with a 1%
decrease in bodyweight losses.
76
Subsequent studies
normalizing blood volume while inducing hypohydration
have confirmed, however, that cardiac drift during
exercise can and often does result from factors other
than hypovolemia.
76,77
The corresponding increase in
heart rate with progressive dehydration is accompanied
by a proportional decrease in stroke volume which
serves to maintain cardiac output.
77
These reductions in
stroke volume are more related to elevations in core
temperature, circulating catecholamine concentrations
and heart rate than due to the redistribution of blood
flow or circulating blood volume.
78
Only at degrees of
dehydration beyond B3% is cardiac output significantly
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diminished as a result of a reduction in circulating blood
volume.
78,79
This bodyweight loss exceeds the threshold
level at which AVP secretion and thirst are generally
stimulated.
Equations predicting ad libitum fluid intake from
multiple regression analyses identify POsm as the primary
factor driving voluntary fluid intake during exercise or
under stressful environmental conditions; with plasma
volume and subjective symptoms of secondary impor-
tance.
15,40
Thus, under acute situations involving sweat
sodium losses, the body will defend osmolality and
intracellular volume through fluid shifts from the extra-
cellular (primarily interstitial) fluid compartments
80–82
until B4% bodyweight losses through sweating accrue;
after which intracellular fluid volume is compromised and
an external fluid supply is necessary.
Athletes rehydrating with either water or an
electrolyte-containing beverage restore 68% and 82% of
fluid losses, respectively, after an exercise-induced dehy-
dration of 2.3%.
14
Changes in free water clearance
followed changes in POsm and rehydration with only
water restored POsm to control (isotonic) levels. Con-
versely, extracellular fluid space was fully restored
only through consumption of the electrolyte-containing
beverage. Ad libitum fluid intake did not match 100%
bodyweight losses during 3 hours of rehydration
with either beverage (hence the ‘‘involuntary’’ dehydra-
tion), illustrating the immediate defense of the body to
preserve POsm first and only secondarily intravascular
volume.
Metabolic water formation, from the combustion of
fat and carbohydrate combined with the liberation of
glycogen-bound water, may contribute an internal water
source to offset sweat losses and sustain internal fluid
balance during prolonged or intense physical activity.
83–85
The activation or inactivation of internal sodium stores
has also been hypothesized as a mechanism aiding in the
defense of POsm during prolonged endurance exer-
cise.
86,87
Data from 2135 weighed competitive athletic
performances demonstrate that athletes who overhydrate,
underhydrate, or euhydrate during prolonged endurance
races generally maintain serum sodium concentrations
within normal ranges (Fig. 1 reprinted with permission).
86
Despite this wide variation in bodyweight changes, 80%
of the athletes in this large and diverse pool maintained
POsm and intracellular volume during prolonged
continuous athletic activity.
Thus, the combination of laboratory work with field
data requires a re-examination of the phrase ‘‘voluntary
dehydration’’ to more accurately reflect the preservation
of POsm (and therefore intracellular volume) over
extracellular fluid (ECF) and plasma volume, secondary
to sweat NaCl losses during endurance exercise. The body
does not defend ECF and plasma volume over POsm
during an acute exercise bout, and the immediate
replacement of body fluids at 100% of bodyweight losses
is not beneficial to performance. AVP secretion and thirst
are stimulated when POsm increases 1% to 4%; well
within the physiologic ranges of homeostatic compensa-
tion. Performance and cardiovascular decrements are
clearly documented when bodyweight losses exceed 4%;
however, this range is well beyond the protective capacity
of the synergistic AVP and thirst mechanisms, whose
primary physiologic interaction is to preserve osmolality
within relatively narrow physiologic ranges. Only after
POsm is stabilized will the body then activate mechanisms
to gradually restore plasma volume by increasing
sodium consumption and fluid intake over the next 24
hours.
20,88
5. Plasma volume contraction, resulting from the
preservation of POsm during exercise,will be restored over
a period of 24 hours by hormonal mechanisms which will
increase the palatability for sodium-containing foods and
beverages. When sodium is ingested,plasma volume will
gradually return to baseline levels: The primary rationale
for the inclusion of sodium into rehydration beverages is
TABLE 1. Field Study Data: Serum Sodium and Bodyweight Change
Study
Race Distance
(km)
[Na
+
] Prerace
(mmol/L)
[Na
+
] Postrace
(mmol/L)
[Na
+
] Change
(Prepost Race)
Bodyweight
Loss (%)
Finish
Time (h)
Twerenbold et al
59
(N = 13) B41 run 137 ± 1 133 ± 2 4 2 ± 1 weight gain 4
Glace et al
57
(N = 13) 160 run 144 140 4126
Gerth et al
58
(N = 51) 100 run 137 ± 5 131 ± 2 6114
Hew-Butler (unpublished) (N = 33) 109 cycle 139 ± 3 138 ± 3 12±15±1
Stuempfle et al
61
(N = 20) 161 snow race 141 ± 1 138 ± 2 3238±7
Nelson et al
62
(N = 45) 42 run 139 ± 0 142 ± 0 3 3 4
Refsum et al
63
(N = 41) 90 ski 142 141 138
Cohen and Zimmerman
64
42 run 139 ± 2 142 ± 2 3 3 3
Noakes and Carter
65
(N = 13) 160 run 144 ± 1 140 ± 3 4316±6
Kavanagh and Shephard
66
(N = 9) 42 run 146 ± 2 148 ± 2 2 3 4± 1
Hew-Butler (unpublished) (N = 82) 56 run 139 ± 3 138 ± 3 14±16±1
Hew-Butler et al
60
(N = 413) 226 triathlon 141 ± 2 141 ± 3 0 4 ± 2 12 ± 2
Maron et al
67
(N = 6) 42 run 141 ± 1 141 ± 1 0 4 ± 1 3 ± 0
Gastmann et al
68
(N = 9) 453 triathlon 138 ± 4 133 ± 4 5526
Rocker et al
69
42 run 144 149 5 5 3
Beckner and Winsor
70
42 run 141 156 7 5 No time
Riley et al
71
32–42 run 143 ± 1 148 ± 1 5 5 4
Astrand and Saltin
44
(N = 6) 85 ski 144 152 8 6 9
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286 r2006 Lippincott Williams & Wilkins

to aid in the restoration of plasma volume
45
rather than
prevent the development of EAH.
5
The presence of a
sodium appetite is equivocal in humans,
89–91
but studies
on sodium palatability and preference have revealed
significant associations with sodium loss and defi-
ciency.
88,90,92–97
A peculiar theme echoing throughout
these studies is the delayed expression of the attractive-
ness of sodium-containing items after cumulative sodium
losses, particularly in studies involving sweat sodium
losses after exercise.
88–95
A study by Takamata et al
88
confirms that the body
protects osmolality over extracellular volume after 7
hours of intermittent exercise at 351C. Elevated serum
sodium concentrations after exercise increased the palat-
ability for water and decreased the pleasantness of
concentrated sodium beverages during the first hour of
rehydration. POsm returned to baseline levels 3 hours
after rehydration had begun, at which time the palat-
ability rating for sodium-containing beverages signifi-
cantly increased over baseline levels. The palatability of
highly concentrated sodium beverages continued to
increase at 6, 17, and 23 hours after the cessation of
exercise which correlated with significant increases in
aldosterone secretion, but not AVP or POsm. The
authors concluded that ‘‘increased H
2
O palatability is
only associated with osmotically induced thirst and thus
contributes to body fluid osmoregulation. In contrast,
ECF thirst accompanied by an increased Na
+
preference
appears after a delay of many hours after osmoregulatory
responses occur and may contribute to ECF volume
regulation’’.
Thus, the palatability for sodium is delayed after
exercise until POsm is restored to normal levels from the
ingestion of, and preference for, plain water. Only after
POsm returns to baseline levels—through the ingestion of
plain water to dilute elevated serum sodium concentra-
tions—will the body seek sodium (by a palatability
increase) to return plasma volume to baseline levels in
the succeeding hours after the exercise bout. This is
presumably facilitated by the natriorexogenic and dipso-
genic actions of aldosterone and angiotensin II, which are
known to stimulate these actions.
23
This phenomenon has
also been supported by studies of exercising students,
with low sweat rates, who expressed a decreased avidity
for sodium when given high doses of salt before
exercise,
94
and has also been clearly demonstrated in rats
made hypovolemic by polyethylene glycol injection.
20
Figure 2 illustrates that hypovolemic rats, on a normal
sodium diet, will respond to isotonic hypovolemia by first
ingesting only water until the decline in POsm inhibits
water intake. Thereafter, the rats will drink saline until
POsm increases, which will again stimulate water intake.
The rat will then alternate between saline and water
consumption until plasma volume is returned to baseline
levels; maintaining a stable POsm throughout the gradual
return to euvolemia. Thus, it has been shown in both
animals and humans that sodium ingestion is necessary to
restore plasma volume once osmolality has been
stabilized. This restoration normally progresses over
the course of 24 hours after the hypovolemic insult.
Because sodium is clearly necessary to restore and
maintain plasma volume after sweat sodium losses from
FIGURE 1. Distributions of postrace [Na
+
] in 2135 overhydrated, euhydrated, and dehydrated athletes. Reprinted with
permission from T. D. Noakes.
86
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exercise, the beneficial effect of sodium ingestion during
exercise is a topic of substantial debate. Studies
have shown that consumption of hypotonic sodium-
containing beverages do not prevent the development of
hyponatremia in athletes replacing 100% fluid loses or
less
72,74,98
or in polydipsic psychiatric patients,
99,100
because most of the sodium ingested is rapidly lost
through the urine.
74,101
In conditions of fluid overload,
however, a mild blunting of serum sodium decline occurs
with consumption of sodium-containing beverages. This
blunting does not eliminate the development of hypona-
tremia in athletes who continue to overdrink, however.
59
There are no studies documenting that sodium
ingestion during exercise provides a clinical, physiological
or performance benefit.
60,102
On the contrary, there
are several studies linking sodium ingestion with negative
physiologic and performance effects.
103–107
Konikoff
et al
105
supplemented normal dietary intakes with 10.2 g
(173 mmol) of sodium 3 days before a 2-hour exercise
bout (60% VO2 Max on a cycle ergometer) with fluid
intake matching sweat rate. The authors concluded that
the ‘‘salt loading’’ did not have a beneficial effect on
temperature and fluid balance and in fact, elicited
undesirable effects including significant increases in
bodyweight, heart rate, and rectal temperature.
105
Furthermore, the extra sodium was rapidly excreted in
the urine, suggesting that the body had no need for the
extra NaCl. Similarly, Pitts and Consolazio
103
adminis-
tered 9 g of NaCl to 11 subjects just before a 10-mile
march. Water was replaced at equal volumes to sweat loss
every 2 miles. The supplemental sodium ingestion also led
to elevated heart rates and rectal temperatures, along with
complaints of ‘‘gastrointestinal uneasiness.’’
103
Perfor-
mance in a VO2 Max test was impaired after rapid
infusion of 30 mL/kg of isotonic saline 30 minutes
before the exercise bout.
107
There was a consistent
increase in ventilation after every exercise work level
after rapid saline infusion, as well as significant reductions
in forced vital capacity and forced expiratory volume. All
of these subjects felt more fatigued at maximal effort and
reported sensations of ‘‘increased leg tightness’’ These
authors hypothesized that the symptoms were due to
increased intramuscular tissue pressure and resultant
edema from the saline infusion.
Thus, sodium supplementation has no documented
advantages when consumed during exercise, with over-
replacement exhibiting detrimental cardiorespiratory and
thermoregulatory effects during exercise. Because sodium
is necessary for plasma volume restoration in the 24 hours
after exercise, once POsm has been normalized,
electrolyte-containing food and beverages should be
freely available after exercise and ingested according to
palatability and tolerability.
6. Conditions where changes in plasma volume alter
the set-point between thirst and tonicity: advanced age,the
syndrome of inappropriate antidiuretic hormone secretion
(SIADH) and exercise in extreme heat or cold: It is well
documented that individuals over the age of 65 years
demonstrate a reduced thirst drive—with corresponding
decreases in fluid intake—in response to rising tonicity,
when compared with younger adults.
108–112
Evidence
suggests that this ‘‘reduced thirst drive’’ alternatively
represents an upward shift in the operating set point ra-
ther than an age-related defect in osmoregulation.
109,113
Studies confirm that healthy older men demonstrate
appropriate, although delayed, reflex adjustments
serving to maintain osmotic homeostasis during condi-
tions of hypertonic hypovolemia
109
and hypervolemia,
114
when the rehydration period is sufficiently long (3 h).
Osmotic stimulation of AVP has been shown to elicit
a 3-fold greater increase per unit rise in POsm
in older men
112
but follows the same pattern as
that of younger men when exposed to similar
osmotic stimuli.
111,112,114
Thus, evidence suggests that it
is not a reduction in the osmotic drive that is primarily
responsible for the attenuation of thirst during aging, but
rather a reduction in the sensitivity of cardiopulmonary
baroreceptors to plasma volume expansion that may
contribute to an upward shift in the operating set
point.
108,110
The diminished inhibitory effect of central
volume expansion on thirst may result from an age-
related impairment of plasma volume maintenance and
expansion; perhaps from the inability to mobilize or
maintain proteins in the vascular space.
113,115
This
impairment in older males to expand or maintain plasma
volume has been demonstrated in situations of hypertonic
saline infusion,
111,114
thermal dehydration,
113
and during
repeated exercise/heat stress.
108,115
The inability of most
elderly people to expand plasma volume may necessitate
elevation of the osmotic thirst threshold because age-
related declines in renal function could concomitantly
facilitate the development of fluid overload hyponatremia
if lower, more ‘‘physiologic,’’ osmotic thresholds con-
tinued to exist.
Exercise in cold conditions may also elevate the
osmotic threshold for which thirst and AVP secretion are
FIGURE 2. Changes in [Na
+
] and cumulative fluid intake in
hypovolemic rats (after subcutaneous injection of 30%
polyethylene glycol solution) with free access to water and
concentrated saline solutions over a 24-hour period. Redrawn
from American Scientist. 1988;76:261–267 with permission
from J. G. Verbalis.
Hew-Butler et al Clin J Sport Med Volume 16, Number 4, July 2006
288 r2006 Lippincott Williams & Wilkins

stimulated. Subjects hypohydrated by B3% to 4%
experience a decline in both AVP secretion and perceived
thirst sensations when entering a cold (41C) chamber,
despite resting plasma osmolalities above 295 mosmol/kg
H
2
O.
116
These subjects relate significantly lower thirst
ratings at 40 and 60 minutes of treadmill exercise—when
euhydrated and hypohydrated—in cold compared with
temperate environments (271C), despite similar plasma
osmolalities. Thus, the authors of this study hypothesize
that thirst and AVP secretion are ‘‘attenuated’’ from
central volume expansion, secondary to peripheral
vasoconstriction in the cold. These findings suggest that
the osmotic operating set point in cold conditions could
be elevated due to increased central blood volume.
Conversely, nonosmotic pituitary AVP secretion,
leading to dilutional hyponatremia and SIADH, may
cause a downward resetting of the osmotic threshold
for thirst.
117
The presence of thirst at low osmolalities
has been hypothesized to pathologically contribute to the
development of hyponatremia,
118
as patients with chronic
SIADH relate the sensation of thirst and report daily fluid
intakes similar to healthy individuals. The study by Smith
et al
117
demonstrates, however, that SIADH patients with
chronic dilutional hyponatremia (average serum sodium
concentration: 125.8 ± 2.7 mmol/L) respond appropri-
ately to osmotic challenge by hypertonic saline infusion.
Baseline plasma osmolalities and the osmotic thresholds
for both thirst and AVP release were 20 mosmol/kg H
2
O
lower in the SIADH group when compared with a group
of healthy matched controls. Thirst ratings, elevations in
AVP, urine osmolalities, and fluid intakes were similar
between the 2 groups with parallel elevations in POsm.
Perceived ratings of thirst returned to baseline levels in
both groups after 30 minutes of drinking and fluid
consumption led to an immediate fall in thirst ratings
within 5 minutes in both cohorts. Thus, the suppression
of thirst in the SIADH group was similar to that of the
control group, suggesting that the neural inhibition of
thirst and drinking is preserved in patients with SIADH.
These laboratory-based conclusions have been verified in
the field setting, where marathon runners diagnosed with
EAH reportedly consumed fluids in excess of excretion
rates not because they were thirsty, but because they were
fearful of becoming dehydrated.
119
The sensation of thirst
should not be confused with the act of drinking. The act
of drinking occurs from regulated (drinking to physiolo-
gic thirst) and unregulated (social, habitual, palatability)
fluid intake which may be governed by factors (market-
ing, commercialism) unrelated to the osmotic set point.
13
Work performance at the initial stages of acclima-
tization during exercise in extreme heat may improve with
‘‘forced’’ fluid intakes—beyond thirst—at volumes
equivalent to sweat loss.
103,120,121
The environmental
temperatures in these trials were extreme, ranging from
38
103
to 461C
120,121
and involved a limited number of
subjects. Subjects in the Bean and Eichna trial
121
ingested
water equivalent to weight lost (1200 mL) and were able
to complete 5 work cycles ‘‘without great difficulty’’ on
the first day of exercise compared with subjects receiving
only enough fluid to quench thirst (600 mL). On the
fourth day of exercise, however, the degree of acclima-
tization—as measured by heart rate, rectal temperature
and blood pressure—was similar between the two groups.
The one subject in the Pitts et al
103
trial also reported less
fatigue and had lower rectal temperatures when ‘‘forced’’
to drink water equal to sweat losses. The fact that the
rectal temperatures between ad libitum fluid replacement
and ‘‘full’’ replacement varied greatly between trials
(B0.3 to 11C) suggests that initial acclimatization
requires fluid intakes beyond thirst to accommodate
plasma volume expansion. After plasma volume is
appropriately expanded, the need to ingest fluid beyond
ad libitum intakes is not necessary to maintain similar
rectal temperatures, heart rates and blood pressures.
Alternatively, Moroff and Bass
120
administered a water
preload of 2000 mL to nonacclimatized subjects and
reported that when these subjects were water loaded, they
had lower rectal temperatures and pulse rates compared
with when not preloaded. Acclimatized subjects given the
same preload, however, displayed lower heart rates with
similar rectal temperatures for reasons that are unclear.
Thus, initial exercise in extreme heat may require transient
fluid intakes beyond thirst to facilitate the expansion of
plasma volume—and heat acclimatization—irrespective
of the current operating osmotic set point.
PRACTICAL RECOMMENDATIONS
1. Drinking to thirst is the body’s dynamic physiologic
fluid calculator and in most cases will protect athletes
from the hazards of both over and underdrinking by
providing real-time feedback on tonicity.
2. A static fluid calculator can provide an estimate of
body fluid losses thereby providing a numeric range as
ageneralized strategy for fluid replacement during
racing and training.
3. Athletes are advised to understand their individualized
fluid needs through use of a static fluid calculator but
ALWAYS defer to physiologic cues to increase fluid
intake (thirst) or decrease fluid consumption (increased
urination, bloating, weight gain) while running.
4. Water, sodium and glucose (in foods and or beverages)
should be freely available at fluid replacement stations,
spaced 1.6 km (minimum) to 5 km (maximum) apart.
The quantity and amount of food and fluid consumed
should be guided by individual palatability and
tolerance for such items.
5. Calibrated scales along a marathon course should be at
the discretion of the medical team; a weight loss of
>4% or any weight gain constitutes justification for
medical consultation.
6. Exercise in extreme heat (>381C) may require
hydration beyond thirst during the initial days of
heat acclimatization whereas advanced age (>65 y)
and cooler environmental temperatures ( < 51C) may
elevate the operating set point for the stimulation
of thirst.
Clin J Sport Med Volume 16, Number 4, July 2006 Updated Fluid Recommendation
r2006 Lippincott Williams & Wilkins 289

FINAL WORD
Marathon running is a serious endeavor that
requires a personal commitment to sustained physical
training. The hazards of marathon running have been
highlighted by the recent deaths of 4 novice female
runners from EAH. Understanding individual fluid
requirements before tackling a 42.2-km race is pivotal
toward ensuring the successful and safe completion of
such an event.
Although our society revolves around rules and
algorithms to guide us through different situations,
individuals should not be confined to these rules in a
dynamic setting. There are no shortcuts toward great
achievement, and marathon running is no exception.
Clinicians and scientists must resist handing out unrea-
listic ‘‘blanket advice’’ to individuals seeking simple
answers, but rather should encourage athletes to explore,
understand and be flexible toward their own needs. By
providing guidelines and advice on how to appropriately
understand individual fluid replacement needs, we can
eliminate future fluid balance problems by avoiding the
temptation to generalize one rule for every situation and
every athlete.
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Hew-Butler et al Clin J Sport Med Volume 16, Number 4, July 2006
292 r2006 Lippincott Williams & Wilkins