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June 2005: (II)S30 –S39
Human Water Needs
Michael N. Sawka, PhD, Samuel N. Cheuvront, PhD, RD, and Robert Carter III, PhD, MPH
Healthy humans regulate daily water balance remark-
ably well across their lifespan despite changes in
biological development and exposure to stressors on
hydration status. Acute or chronic body water deficits
result when intakes are reduced or losses increase,
but day-to-day hydration is generally well maintained
so long as food and fluid are readily available. Total
water intake includes drinking water, water in bever-
ages, and water in food. Daily water needs determined
from fluid balance, water turnover, or consumption
studies provide similar values for a given set of
conditions. A daily water intake of 3.7 L for adult men
and 2.7 L for adult women meets the needs of the vast
majority of persons. However, strenuous physical ex-
ercise and heat stress can greatly increase daily water
needs, and the individual variability between athletes
can be substantial.
Key words: body water needs, fluid balance, fluid
homeostasis, hydration
© 2005 International Life Sciences Institute
doi: 10.1301/nr.2005.jun.S30 –S39
INTRODUCTION
Water is the quintessential nutrient of life. Despite
its well-established importance, however, it is ironically
often ignored as a dietary constituent. During the past
decade, considerable public attention has focused on the
importance of adequate hydration, but the scientific basis
for the often-recommended daily water consumption
rates is unclear.
1
The National Research Council (NRC)
had recommended a daily water intake of approximately
1 mL/kcal of energy expended,
2
but data supporting that
value are sparse.
3
The Food and Nutrition Board of the
Institute of Medicine (IOM) recently convened a panel of
experts to set new Dietary Reference Intakes (DRIs) for
water and electrolytes to expand and replace the Recom-
mended Dietary Allowances (RDAs). The results of this
extensive effort were published in 2004.
4
The purpose of
this review is to briefly summarize the results of that
effort, with a narrow emphasis on reviewing water needs
for healthy people across their lifespan. In addition, this
paper will give additional attention to the effects of
physical exercise and heat exposure on daily water
needs.
REGULATION OF WATER BALANCE
Water (total body water) is the principal chemical
constituent of the human body. For an average young
adult male, total body water represents 50% to 70% of
body weight.
5
Variability in total body water is primarily
due to differences in body composition. Lean body mass
is about 73% water and fat body mass is 10% water.
6
Differences in total body water often attributed to age,
gender, and aerobic fitness are mostly accounted for by
body composition.
Total body water is distributed into intracellular
fluid and extracellular fluid compartments, which contain
about 65% and 35% of total body water, respectively.
The extracellular fluid compartment is further divided
into the interstitial and plasma spaces. An average 70-kg
male has about 42 L of total body water, so the intracel-
lular fluid compartment contains about 28 L of water and
the extracellular compartment about 14 L of water, with
about 3.2 L in plasma and 10.8 L in the interstitium.
These are not static volumes, but represent the net effects
of dynamic exchange.
7
Approximately 5% to 10% of total body water is
turned over daily,
8
being distributed via obligatory (non-
exercise) fluid loss avenues (Table 1). Respiratory water
loss is influenced by the inspired air (temperature and
humidity) and the pulmonary ventilation. Metabolic wa-
ter is formed by oxidation of substrates and is roughly
offset by respiratory water losses. Urine output generally
approximates 1 to 2 L per day, but can be increased by an
order of magnitude when consuming large volumes of
fluid. This large capacity to vary urine output represents
Drs. Sawka, Cheuvront, and Carter are with the
Thermal and Mountain Medicine Division, US Army
Research Institute of Environmental Medicine, Natick,
Massachusetts.
Address for correspondence: Dr. Michael Sawka,
Thermal and Mountain Medicine Division, US Army
Research Institute of Environmental Medicine, Natick,
MA 01760-5007; Phone: 508-233-5320; Fax: 508-
233-5298; E-mail: michael.sawka@us.army.mil.
S30 Nutrition Reviews姞, Vol. 63, No. 6
the primary avenue to regulate net body water balance
across a broad range of fluid intake volumes and losses
from other avenues.
4
Sweat losses vary widely and
depend upon the physical activity level and environmen-
tal conditions.
9
Net body water balance (loss ⫽gain) is regulated
remarkably well day-to-day as a result of thirst and
hunger drives, coupled with ad libitum access to food
and fluids to offset water losses.
4
This is accomplished
by an intricate interplay between neuroendocrine and
renal responses to body water volume and tonicity
changes,
10
as well as non-regulatory, socio-behavioral
factors.
11
These homeostatic responses collectively en-
sure that small degrees of over- and under-hydration are
readily compensated for in the short term.
Using water balance studies, Adolph
12,13
concluded
that daily body water varied narrowly between 0.22%
and 0.48% in temperate and warm environments, respec-
tively. Although acute mismatches may occur due to
illness, exposure, or exercise, the fact that intakes are
generally adequate to offset avenues of net loss from day
to day is a reproducible phenomenon
14
and a cornerstone
basis for establishing water intake requirements from
large population surveys.
4
It is recognized, however, that
after significant body water losses such as those associ-
ated with exercise-heat stress, many hours of rehydration
and solute consumption may be needed to reestablish
body water losses.
15
Specific concepts associated with
exercise-heat stress and fluid replacement are elaborated
upon later in this review.
More recently, isotope-labeled water has been used
to measure body water turnover and water needs. Rates
of body water turnover assume a balance between influx
and efflux, and are determined by following the decline
in hydrogen isotope over time. These values are gener-
ally higher than in water balance studies, because sub-
jects are often more active or are exposed to outdoor
environments, but high correlations have been reported
between both direct and water turnover methodologies in
infants.
16,17
Studies in adults also support close agree-
ment between the two methods when similar but inde-
pendent investigations of sedentary and active people are
compared
18-20
(Figure 1).
HUMAN WATER NEEDS
Normal water needs range widely due to numerous
factors (e.g., metabolism, diet, climate, clothing)
4
; thus,
normal hydration is compatible with a wide range of
fluid intakes. Human water requirements should not be
based on a “minimal” intake, as this might eventually
lead to a deficit and possible adverse performance and
health consequences.
4
Instead, the Food and Nutrition
Board of the IOM bases water needs on adequate intake
(AI). The AI is based on experimentally derived intake
levels that are expected to meet nutritional adequacy for
essentially all members of a healthy population. Data
Figure 1. Comparison of water needs estimated using water
balance or water turnover methodologies for sedentary and
active people. Data are from Consolozio et al., 1968
20
Green-
leaf et al., 1977,
19
and Leiper et al., 2001.
18
Table 1. Daily Water Losses and Production
Reference Source
Loss Production
mL/d
Hoyt and Honig, 1996
60
Respiratory loss ⫺250 to ⫺350
Adolph, 1947
61
Urinary loss ⫺500 to ⫺1000
Newburgh et al., 1930
24
Fecal loss ⫺100 to ⫺200
Kuno, 1956
62
Insensible loss ⫺450 to ⫺1900
Hoyt and Honig, 1996
60
Metabolic production ⫹250 to ⫹350*
Total ⴚ1300 to ⴚ3450 ⴙ250 to ⴙ350
Net loss (sedentary) ⴚ1050 to ⴚ3100
Burke, 1997
40
Sweat losses in various sports ⫺455 to ⫺3630
Net loss (athlete) ⴚ1550 to ⴚ6730
Table modified from IOM.
4
Used with permission.
*Metabolic water production based on 2500 –3000 kcal daily energy expenditure. Additional water production with exercise is
assumed offset by parallel respiratory losses (as illustrated above with rest).
S31Nutrition Reviews姞, Vol. 63, No. 6
from water balance, water turnover, and total water
intake surveys were considered.
For infants, water needs are met primarily by the
consumption of human milk or formula.
4
Throughout
development, more and more water is acquired by a
variety of beverages and food. Developmentally, total
body water per kilogram of body mass is highest in
infancy and gradually declines
6
as fluid regulatory mech-
anisms mature. Although it is not until mid- to late
puberty that responses such as sweating become similar
to adults,
21
thirst and hunger are primitive biological
drives that compensate well for obligatory water losses.
Fluid balance studies show that daily water needs
increase with age from early infancy (⬃0.6 L) through
childhood (⬃1.7 L).
22,23
For adults, the daily water needs
of men approach 2.5 L if sedentary,
3,24
and increase to
about 3.2 L if performing modest physical activity,
19,25
while more active adults living in a warm environment
have daily water needs of about 6 L.
26
Examination of water turnover studies indicates that
daily water turnover is 3.3 L and 4.5 L for sedentary and
active men, respectively.
8,18,27-33
For more active popu-
lations, even higher values (⬎6 L) have been reported.
33
Sparse data are available on women, but they generally
exhibit lower daily water turnover rates (⬃0.5—1.0 L
less) than their male counterparts. Limited data are avail-
able regarding the influence of advancing age on body
water turnover, but reported values are not dissimilar
from younger adults. As age progresses for both genders,
physical activity might decrease and the fluid regulatory
capacity can decline due to reduced renal concentrating
and diluting capacity
34-36
and to a diminished thirst
drive.
37
Despite these facts, hydration status continues to
be normal throughout life.
4
Total daily water intakes from the Third National
Health and Nutrition Examination Survey (NHANES
III)
4
were obtained from a very large population. The
results indicate that for children and adults, about 80% of
total daily water intake is obtained from beverages and
about 20% from food.
4
Both water sources are consid-
ered in the total, since bioavailability of water is similar
for both beverage and food sources of water. Most
importantly, the NHANES data indicate that at all age
groups (12—71⫹years) and at all levels of consump-
tion, the participants were in water balance (i.e., they had
normal plasma osmolality).
The NHANES results were consistent with data
obtained from water balance and water turnover studies.
For adult males, the total daily water intake was about
3.31 L for the fifth decile and increased to over6Lfor
the tenth decile. Because of the close agreement among
experimental methods (water balance, water turnover,
and consumption) and the evidence for normal water
balance, the population survey data (median total intake)
were employed to set the AI levels for children and
adults.
Figure 2 provides a summary of the median total
water intake for each age group for men and women.
4
These data were used to set the AI. For adult men and
women, the AI was 3.7 and 2.7 L per day, respectively,
and required no adjustment for advancing age. For preg-
nant and lactating women, the AI was increased by 0.3 L
and 1.1 L, respectively. Like other AIs for healthy
people, water intakes below or above the AI may not
impose any health risk (or adverse performance conse-
quences) because of the extreme variability in human
water needs. Likewise, the AI should not be interpreted
as a minimal water intake. Approximately 1 to 2 L/d are
required to replace obligatory losses for sedentary adults
residing in temperate climates (Table 1). The IOM
clearly documented that maintaining euhydration is im-
portant, as acute and chronic body water deficits can
adversely impact human health and performance. The
AIs were not intended for use by competitive athletes or
workers performing strenuous activity for extended du-
rations in hot weather. Thus, higher water needs may be
required for those who are more physically active, espe-
cially in hot environments.
WATER NEEDS AND PHYSICAL ACTIVITY
Physical activity results in increased water require-
ments that parallel sweat losses for evaporative heat
exchange.
38
Survey data of individuals reporting five or
more days of leisure time activity per week show higher
median water intakes on the order of 0.5 L/d compared
with their less-active counterparts.
4
Water turnover stud-
ies demonstrate that higher volumes of daily (50 km of
cycling) or weekly (100 km of running) activity in a
Figure 2. Water needs across the lifespan. Columns with
dashed horizontal lines represent requirements for girls or
women in that age category. Data are from the IOM.
4
S32 Nutrition Reviews姞, Vol. 63, No. 6
temperate environment increases water flux by 1.2 to 1.4
L/d, owing primarily to sweat volume loss and replace-
ment.
18,29
The same activities in warmer environments
would exacerbate the outcome.
Figure 3 depicts generalized modeling approxima-
tions for daily sweating rates as a function of daily
metabolic rate (activity level) and air temperature. Ap-
plying this prediction model (the details of which are
elaborated upon elsewhere
4
), it is clear that water re-
quirements can increase 2- to 6-fold from baseline by
simple manipulation of either variable. For example,
daily water requirements for any given energy expendi-
ture in temperate climates (20°C) can triple in very hot
weather (40°C) (Figure 3). In addition to air temperature,
other environmental factors also modify sweat losses;
these include relative humidity, air motion, solar load,
and choice of clothing for protection against environ-
mental elements. Therefore, it is expected that water
losses, and therefore water needs, will vary considerably
among moderately active people based on changing
extraneous influences.
The magnitude of sweat losses incurred during ex-
ercise in a warm environment is dependent primarily on
exercise intensity and duration.
38
Heat gain from metab-
olism is balanced by both dry and evaporative (sweating)
heat loss, but very high metabolic rates coupled with
warm weather demand a larger biophysical requirement
for evaporative cooling,
39
leading to greater sweat losses
and, subsequently, larger water requirements. It is there-
fore expected that athletes will require relatively more
fluid to match their greater sweat losses and maintain
water balance. But, again, sweating rates range widely
between (sports) and within (position) different athletic
arenas.
40
Even under standardized conditions in fairly
homogeneous athlete populations, inter-subject sweating
variability is significant.
41
Acute exercise sweat losses commonly fall within a
range of 1 to 2 L/h for team
40
and individual
42
endurance
sports. Table 2 provides sweating rates reported for
athletes participating in a variety of sports.
42
Although
rarely sustainable for more than a few hours due to the
level of exertion required, such losses can markedly
affect water requirements (Table 1). Maximal human
gastric emptying rates are also variable and influenced by
numerous factors, but approximate typical sweat losses
(1–1.5 L/h).
43,44
Despite this fact, only about half of
sweat losses are voluntarily replaced during exer-
cise,
40,45,46
which generally results in moderate acute
water deficits in team (1%–2%)
40
and competitive en-
durance sports (1%– 4%),
46
although individual losses in
excess of 5% have been reported many times for pro-
longed, continuous exercise.
47-51
A modest daily exercise sweat loss of 1.4 L will
increase obligatory daily water requirements by about
45%, all other factors remaining constant (Table 1).
Although this is a relatively small volume, larger losses
do occur (Table 1) and can be difficult to replace and
retain in the short term.
15
Longer recovery periods (⬃24
h) of ad libitum drinking, especially when combined with
food intake and ample electrolyte replacement,
52,53
al-
Figure 3. Water needs estimated from sweat loss predictions due to changes in physical activity and air temperature. Figure is from
the IOM
4
(used with permission).
S33Nutrition Reviews姞, Vol. 63, No. 6
lows a return to normalcy. This is true even when daily
water losses are significant (4% body mass)
54
and daily
water turnover is approximately 40% to 50% above
baseline.
18,29
Although daily strenuous activity in a hot
environment can result in mild water balance deficits,
even with unlimited access to food and fluids,
15,55,56
adherence to recognized water intake guidance
57,58
under
similar conditions prevents accumulating water deficits,
as determined by daily body mass stability.
59
CONCLUSIONS
Healthy humans regulate daily body water balance
with precision despite highly variable water needs and
intakes and exposure to variable stressors on hydration
status. So long as food and fluid are readily available,
this is accomplished by eloquent physiological and be-
havioral adaptations. Among the greatest challenges to
body water homeostasis is exercise and exercise-heat
stress. However, normal hydration can be achieved with
a wide range of water intakes by sedentary and active
people across the lifespan.
PANEL DISCUSSION
Eric Je´quier: I have a question about adaptation to
the heat environment. There have been several studies on
this phenomenon. Do you think that adaptation may have
an effect and may decrease this water requirement that
you have mentioned?
Michael Sawka: No. There have been studies by
Adolph and in our laboratory, and essentially all of these
studies pretty well show the same thing, at least for what
they were studying in terms of the ability to thermoregu-
late and tolerance to exercise and heat stress. There is no
real adaptation in terms of tolerance. There are some
physiological adaptations, though, that occur with heat
acclimatization that influence fluid balance. Now, if you
look at requirements, the first is that as you acclimatize
you increase your ability to produce sweat, so your water
losses are going to be greater. Total body water will also
go up a little bit. How much is controversial, but prob-
ably about 1 to3Latthemost. For a given decrease in
total body water, what you see is that you have that same
increase in core temperature or the same decrement in
the thermoregulatory system. Now, there are also some
other changes. For example, when you start using pa-
rameters like plasma markers as measures of hydration,
one of the things that you see is that for a given level of
dehydration, an unacclimatized person is going to have a
much larger reduction of plasma volume. And as the
person acclimatizes, in addition to increasing sweat, the
sweat becomes more dilute so they retain sodium. This
sets up an osmotic gradient to pull fluids from the
intracellular spaces, so the plasma volume reduction is
about one-half in heat-acclimatized compared with un-
acclimatized persons. So you see subtle changes when
you are acclimatized; a decrease in plasma volume in
proportion to about the level of dehydration, but none of
them really correlate into any abatement of performance
reduction, at least for the ways we’ve studied it.
Friedrich Manz: In the DRIs for water, serum
osmolality was the primary indicator of hydration status.
As serum osmolality concentrations were essentially
identical for the first (lowest), fifth, and tenth deciles of
daily total water intake within each age group, euhydra-
tion was assumed even in the decile with the lowest total
water intake. The renal concentrating mechanism is stim-
ulated by vasopressin. Low serum vasopressin levels
result in a high diuresis and high levels in a low diuresis.
Serum osmolality is the most important factor for vaso-
pressin secretion. However, there are a lot of confound-
ing factors influencing the relation of the serum levels of
osmolality and vasopressin (e.g., genetic factors, blood
volume, blood pressure, age, phase of the menstrual
cycle, and drugs). Could it be that confounding factors
equilibrated the difference in serum osmolality between
the subjects in the lowest and those in the highest decile
of water intake? Furthermore, plasma osmolality is an
acute parameter, going up and down with circadian
rhythm, whereas water intake is a 24-hour measurement.
At what time was plasma osmolality determined?
Michael Sawka: There are two sets of osmolality.
You have the first set of osmolalities that were obtained
from the NHANES samples, which were obtained from
many thousands of individuals. Very few of them had
indications that they were consuming large amounts of
water beforehand, so you probably had adequate time to
retain fluid balance. In the second, you do see that
osmolality goes up with age, as you would expect, by
several osmoles, and you see some fluctuation that re-
flects many of the things that you mention. Now, in terms
of the relationship between dehydration and osmolality,
that was taken from a number of studies, most of which
were very careful fluid balance studies. For the majority
Table 2. Sweating Rates for a Variety of Sports
Sport
Mean Range
L/h
Water polo 0.55 0.30 to 0.80
Cycling 0.80 0.29 to 1.25
Cricket 0.87 0.50 to 1.40
Running 1.10 0.54 to 1.83
Basketball 1.11 0.70 to 1.60
Soccer 1.17 0.70 to 2.10
Rugby 2.06 1.60 to 2.60
Data are from Rehrer and Burke, 1996.
42
S34 Nutrition Reviews姞, Vol. 63, No. 6
of those studies, those individuals achieved a certain
hydration level, or were monitored over many days and
essentially were in actual fluid balance or dehydrated,
and were held there for some time. So there was a period
of compensation. Essentially, if you give someone water
or you dehydrate him and you wait long enough, 6 or 8
hours, the osmolality will come back. Either they return
to euhydration or to a certain level reflective of their level
of dehydration. We use it in the lab all the time. It’s a
very nice, precise relationship.
Friedrich Manz: In Muslim countries, the whole
population is dehydrated during Ramadan, at least in the
afternoon. Is dehydration associated with an increased
risk of accidents or emergency cases? There are very few
data in the literature. In a retrospective study of all road
traffic casualties observed in a local hospital in the
United Arab Emirates, a greater number of injuries was
found during Ramadan than in the rest of the year (Bener
A, Absood GH, Achan NV, Sankaran-Kutty M. Road
traffic injuries in Al-Ain City, United Arab Emirates. J R
Soc Health. 1992;112:273–276).
Michael Sawka: I don’t know the specific study you
are addressing, but in many studies that are epidemio-
logical in nature that look at individuals during pilgrim-
ages, there is a variety of factors that are changing: heat
stress, fitness level, nutrition, a whole host of things, so
to be able to go retrospectively back and pull out the
effects of hydration would probably be difficult. But I do
think that, based upon our observations and many exper-
iments over the years, and those of my colleagues, there
are a variety of changes that occur that very much would
logically result in an increased injury rate, and they may
be independent or accentuated by heat. I think it’s a very
fertile area of research.
Antonio Dal Canton: I wonder whether a 12-hour
privation of water can be considered a way to become
dehydrated, referring to Ramadan, for example. I take
food and water only once a day at dinnertime and I feel
well and believe that my plasma osmolality remains
within the limits of normal range. In general, I think that
our body has developed a great ability to resist insults, in
terms of having water or not having water available. We
have bottles of water in the refrigerator and tap water
easily available, but up to 10,000 years ago, it was
normal for our ancestors to go 12 or 24 hours, perhaps,
without drinking. This does not mean that they devel-
oped pathological dehydration. Once again, I would go
back to the definition of normality as a different concept
from being in the ideal condition.
Patrick Ritz: If we focus on elderly people, and if
I understand correctly, you said that is above 50 years of
age in the NHANES study, you measured water intake
and then added a little bit.
Michael Sawka: Yes, from that age beyond, essen-
tially, the water intakes were a little bit lower than for
younger adults, and knowing that older individuals have
a reduced drive for thirst, knowing that there are changes
in renal function, and also having essentially a smaller
population available to arrive at those numbers, we felt it
advisable to add a couple of hundred milliliters, so we
wouldn’t give the impression that water requirements
decreased with aging. So, yes, those numbers are slightly
inflated.
Denis Barclay: Dr. Shirreffs was describing earlier
the phenomenon of people who perspire a lot. Is this a
well-described phenomenon? Is there any known physi-
ological basis for this, and for a given activity, what sort
of range would there be in terms of sweat loss per hour?
Michael Sawka: First of all, you do see variability
but there are also certain populations, such as individuals
who have certain mutations for cystic fibrosis, who also
have very high losses of sweat and sodium. So there is
variability in sweat loss, just as there is variability for
acclimatization to heat.
Irwin Rosenberg: Perhaps you might want to elab-
orate on this issue of the AI. When dealing with other
nutrient requirements in the whole DRI process, we use
AI as a kind of a fallback position when there isn’t
enough information to derive an estimated average re-
quirement from which you could then calculate an RDA.
It seems to me that when you’re dealing with something
like hydration, which is so well-regulated in the healthy
individual, that there is even more reason to use an AI,
because it does then represent the way in which, in a
well-regulated environment, an intake can be represen-
tative of the actual requirement. Was that part of the
thinking of your committee?
Michael Sawka: Although as a physiologist I was
peripheral to those discussions, it was a fallback position.
Essentially what we are saying with this AI is basically
that this is what the vast majority of people consume and
remain well hydrated. We recognize that there is consid-
erable variability between individuals, but from the best
that we can determine, normal healthy individuals, given
adequate time, will maintain hydration. But we don’t say
that you have to drink 3.7 L a day. Essentially, what we
are saying is that you’re probably doing OK, whatever
you’re doing. That for an average person they consume
about this amount and are well hydrated. You can go
back and look through the beautiful experiments done by
Dr. Adolph and see that if given adequate time, people
will maintain fluid balance. If water is made available
they will rehydrate adequately, and that’s what we seem
to have reconfirmed.
Tyko Persson: This is based on a very large US data
set. Do you think there is any reason that these values
will not be valid for Europe?
Michael Sawka: I really don’t know. I assume the
S35Nutrition Reviews姞, Vol. 63, No. 6
physiology is similar between Americans and Europeans,
but lifestyle is different. People may or may not have
more air-conditioning. People in Europe certainly seem
to be leaner and more active and have different diets.
Certainly we recognize the dietary impact affecting renal
water loss. There probably is some variability and it’s
probably due more to lifestyle and what you’re exposed
to, so it very well could be different. And I’m sure not all
Europeans are the same. The people who live in Lau-
sanne may be different than the people who live in Paris,
but they’re all probably well hydrated. That much I
would say: they’re healthy.
Friedrich Manz: In the DRI for water of 2004,
urine solute excretion on a typical western diet was
estimated to be 650 mOsm/d. In our studies in Germany,
mean urine solute excretion was 942 mOsm/d in men and
752 mOsm/d in women. In the literature, urinary solute
excretion ranged from about 400 mOsm/d*1.73m
2
in
sweet potato eaters in Papua, New Guinea to 1365
mOsm/d in Australian miners working very hard. Con-
sidering the data on urine solute excretion from several
small studies in the United States, and the comparable
levels of protein and sodium intake in American and
German adults, urine solute excretion may be not so
different. The main difference in the hydration status
between the US and German population refers to fluid
intake. In the United States, about 19% of water intake is
from food and fluids; in Germany it is 32% in men and
35% in women. From a nephrological point of view,
what percentage of the US population would be in the
range of risk of hyperhydration?
Michael Sawka: On the question of hyperhydration,
we went very thoroughly through the hyponatremia data
for normal persons eating a normal American diet, and
we just couldn’t find any evidence that these people
suffered from hyponatremia or adverse consequences of
that. We didn’t go back and look at solute load and then
try to calculate water requirements. Basically, we used
three avenues. We looked at water balance studies, we
looked at water turnover studies, and we looked at
consumption, and in all three avenues we came up with
similar numbers and saw that individuals seemed to be
well hydrated. We just could not find any evidence of
any systematic dehydration or hyponatremia from hy-
perhydration. So those are the numbers and those are the
data we looked at, and that’s all I can really comment on.
Irwin Rosenberg: Well then, what are we to make
of the differences that we were presented with earlier
from Dr. Manz and Dr. Ferry and now from NHANES?
Is the message here that we need the equivalent of an AI
for each country? Or is the message that we need to look
at and compare these data and try to understand why they
appear to be different, in spite of the fact that there may
be much greater similarities between European and
American populations and Australian populations and so
forth? Can we find some way to harmonize these data
and to understand whether there are in fact numbers that
can be used across numerous populations? Or are these
national phenomena that have to be looked at as individ-
ual research questions?
Susan Shirreffs: The data that were cited from
some of the water balance and water turnover studies
were European data. So those numbers come from a
number of European studies in addition to the United
States.
Patrick Ritz: There are also probably some other
data available. I am thinking about French data from the
SU.VI.MAX study, where there are probably some indi-
cators of water consumption. So if we are to compare
countries, it is probably safer to pool together more than
two.
Irwin Rosenberg: Under average conditions, we
have 2.2 L for female adults and 2.9 L for male adults,
which are close to the fluid intake recommendations in
the United States. If you add the water from food, you
end up with pretty much the same numbers that Dr.
Sawka has been talking about, so I am trying to under-
stand where we have consensus and where we don’t.
Michael Sawka: I don’t think the DRI panel would
argue; maybe it’s not 3.7 but 3.2 L, and that at 3.2 L, the
average person is well hydrated. All we’re saying is that
it seems to be what they consume, and to the best of our
knowledge they are well hydrated. Whether they are
drinking 3.7 L, 2.8 L, or 7.6 L, all of these people seem
to be in fluid balance, as a group. And it could very well
be that as you go down more towards that minimal
requirement that we talked about, that that number could
be lower. It all depends on the context of what you’re
using that value for.
Irwin Rosenberg: Did your committee consider
suggesting a range?
Michael Sawka: Essentially, we did give a range.
We said, this is the number and there is quite a bit of
variability. But essentially the number 3.7 L doesn’t
mean that you need to drink 3.7 L. Dr. Armstrong may
need 2.7 L, Dr. Shirreffs may need 4.6 L, but I am pretty
certain that they are probably both in fluid balance.
Patrick Ritz: Along the same lines, are we about to
get rid of the milliliter of water per kilocalorie? Should
we?
Michael Sawka: Well, we had thought about that,
but decided that it really didn’t add any additional useful
information. We just came up with a rough number
because there are a whole variety of factors that con-
found it. We decided to get away from that. We did not
feel that there was any value added.
Friedrich Manz: In the DRI of 2004 for water, AI
S36 Nutrition Reviews姞, Vol. 63, No. 6
for metabolic water and water in fluids and food is 1.31
mL/kcal in men and 1.22 mL/kcal in women.
Patrick Ritz: Yes, but I think the concept within
that is that if you use a milliliter of water per kilocalorie
spent, you introduce a body size variability component.
If we get rid of that, and I don’t know whether it’s correct
or not, and we just stick to one number for water intake
independent of body size, this number is supposed to be
valid for a 50-kg person and a 100-kg person as well.
Michael Sawka: Again, we are not saying that the
3.7 L is adequate for all individuals. That’s not the
message we are trying to transmit. We’re trying to say
that median intake is 3.7 L. We are saying that for each
individual it could be 3.7, 4.7, or whatever, but what we
are saying is that what they are taking in is adequate and
that there is a lot of variability and the median for that
group is 3.7 L. The message is not that it’s 3.7 L for all.
Antonio Dal Canton: I am wondering what we are
talking about and for what kind of people we are now
defining the right amount of water to be consumed in a
day. If you send soldiers to fight in the desert, you have
to know how much water should be provided for them.
And that’s the reason for having a committee, to under-
stand what is the average need for water and to provide
water in very special conditions. But in day-to-day life,
what is the real meaning of understanding what is the
average amount of water that people drink? Because it
may be normal that I drink 1 L and that he drinks 2 L,
and both of us will be normal. We will not have any
problem with hydration. I would rather address our
attention to special categories: the elderly, or people
who have recurrent urinary infections, and so on, be-
cause in those patients, maybe it is useful to force the
uresis and to increase their water intake daily beyond
their natural intake. But, frankly, this discussion about
the normal water intake in normal people living in
civilized countries, having all the amount of water avail-
able without any restriction, it seems philosophical to
me.
Patrick Ritz: When we were with Dr. Rosenberg on
the committee for energy requirements, it was about the
same discussion. When we discussed energy require-
ments, we were very careful to define minimal energy
requirements, and those minimal energy requirements
have nothing to do with optimal requirements. They are
made for politicians in a country to know how much food
should be produced for the country, or in disaster relief,
for example. This was minimal energy requirement. I
agree that there are regulators for energy intake and
expenditure in the body, but we have a very easy marker,
which is body weight. But for water, so far we have no
good indicator of body hydration. Secondly, what we
have been talking about since the beginning of the
symposium is functional status associated with optimal
hydration, and we have to work along these lines. In
terms of energy, there is no functional status. We know
what is excess and what is a shortage, but we have no
data on the relationship between optimal energy intake
and optimal health. So I think it is not irrelevant to
discuss what is the mean intake of water for a population
and to compare it from one to the other.
Friedrich Manz: Before the 1990s, RDAs were
defined as the levels of intake of essential nutrients that
are judged to meet the known nutrient needs of practi-
cally all healthy persons. Prevention of nutritional defi-
ciencies was the primary concern. DRIs represent a new
paradigm. Reference values are defined by a specific
indicator of nutrient adequacy, which may relate to the
reduction of the risk of chronic disease or disorder.
Which are the specific indicators to identify the level of
water intake to prevent dehydration and certain diseases
in practically all persons? Free water reserve may be a
physiologically founded and empirically based indicator
to establish guiding values to prevent dehydration in
almost all persons of a specific population. Epidemiolog-
ical studies are needed on health outcomes given differ-
ent hydration status and water intakes. In the United
States and Germany, there is a trend to increase water
intake. Should the AI values of the United States be
adapted in the next revision of the DRIs for water? Thirty
years ago, as a young pediatric nephrologist, I regularly
collected 24-hour urine samples in children with kidney
stones. I wondered why it was so difficult for certain
children to increase their water intake even in the pres-
ence of clinical symptoms. Today, we know that the
recurrence rate of kidney stones can be substantially
decreased by an increased water intake. If, as in Ger-
many, more than 5% of the population will develop
urolithiasis, why should we not promote an increased
water intake not only on the individual level, but also on
the population level? There are at least three arguments
for different AI levels for water in different countries.
Firstly, the climate may be different. You cannot have
the same recommendations for Sicily and Finland. Sec-
ondly, nutrition depends on culture and climate. Water
content of food, therefore, is different. Thirdly, the os-
molar load of a diet is different. Energy and water
metabolism are combined in many ways. If we compare
small and big animals or toddlers and adults, both energy
and water requirement are related to body surface area.
Breast-fed infants are an exception. With a mean urine
osmolality of 120 mOsm/kg, their hydration status is
very close to hyperhydration. Three factors may be
responsible: a high metabolic efficiency of breast milk, a
low osmolar load, and a high evolutionary pressure to
increase water intake to prevent death from dehydration
during infant diarrhea. Glycogen is an interesting mole-
cule that stores both energy and water. With a relation-
S37Nutrition Reviews姞, Vol. 63, No. 6
ship of1gofglycogen to 3 mL of water, it is a very
ineffective form of energy storage, with only 1 kcal/g
body mass compared with fat, which has a storage of 9
kcal/g. However, at rest, the water losses from lung, skin,
and kidney are balanced by the water liberated by the
simultaneous glycogen consumption. Thus, during the
night the water is liberated and excreted in the same
amount, relieving the body from any need for additional
regulation. Thus, there are several physiologic arguments
to express water requirement in relation to energy re-
quirement in milliliters per kilocalorie.
Irwin Rosenberg: I think it’s very important that
we continue to consider this question that we have heard
now more than once from Dr. Manz. For the require-
ments that Dr. Sawka described here, none of the three
factors that went into setting the numbers were based on
an epidemiological relationship between water and spe-
cific health outcomes. I think we have to come to grips
with that limitation. At some point, I would argue that we
are going to have to find a way to address the question
posed by Dr. Manz, which is, is there within this range
(which is very nicely regulated because we have kid-
neys), perhaps in the 90th percentile versus the 10th
percentile, will we find one day that those populations
that are drinking higher amounts or drinking lower
amounts are either more susceptible or less susceptible to
a number of the kinds of conditions that Dr. Manz
reviewed and perhaps others as well? Somehow, I think
that lack is going to have to come into our discussions
again.
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