Just How Much Water Do We Really
Need?
The Answer May Depend On Our Age
(February 24, 2004) – BETHESDA, MD
– Just how much water does each of us
really need? Not to swim in, or diet with. Not to respond to marketing
claims, or counter salty foods or to cope with dry environments.
Many swear by the advice that for proper body
hydration, 64 oz of water should be consumed each day. Other scientists and
researchers disagree with that long held belief, recommending that one
should only consume water “when thirsty.”
Why should we be concerned? For one, water shortages
may be the next great crisis faced by a planet with limited natural
resources and exploding population growth. In March 2003, the United
Nations issued a report stating that more than 2.7 billion people will face
severe water shortages by the year 2025 if the world continues consuming
water at the same rate. Wars have erupted over water rights; famine and
mass starvation have resulted from climate changes that have turned gardens
into deserts. Soon we will all be concerned about how much water we
really need.
There is no question that water is vital to the body’s
overall health. We use water for transporting nutrients and
wastes, lubrication, temperature regulation, and tissue structure
maintenance. In addition, plentiful fluid consumption may be
protective against diverse medical conditions, including kidney
stones, constipation, colorectal cancer, premalignant adenomatous
polyps, and bladder cancer. Water deprivation results in
life-threatening dehydration within a few days. Loss of body
water exceeding five percent of body weight leads to decreased
endurance, culminating in heat exhaustion. Older vs. younger individuals
have been shown to have a higher risk of developing dehydration
than younger adults, which may be attributed to decreased total
body water (TBW) with age, impaired renal fluid conservation, and
physiological hypodipsia or insensible thirst.
Despite the physiological importance of water to life,
little is known about water intake and excretion patterns in free-living
individuals, because fluid intake, particularly from noncaloric,
nonalcoholic, and noncaffeinated beverages is poorly documented. The
1977-1978 National Food Consumption Survey is one of the few sources of
information on water intake, but the data are limited by unaccounted for
water found in foods and the use of a single 24-hour dietary recall.
Moreover, nonquantitative intake from water fountains and the likelihood
that many people consume fluids with little thought leads to underreporting.
One method that does not depend on self-reported intake
is the use of hydrogen-labeled water turnover, a method used by comparative
animal physiologists for decades to objectively measure water turnover in
wild animals. The procedure begins with a bolus administration of
isotopically labeled water, such as nonradioactive 2H oxide.
Within two to three hours, this tracer equilibrates with body water and
provides a measure of the volume of the TBW pool. The labeled water is then
excreted from the body through all routes of water loss and is diluted by
unlabeled water through all routes of input. The time course of labeled
water dilution provides a measure of water turnover (input and output) per
unit of time.
A New Study
A new study combined data from two studies in healthy,
free-living American adults across a broad age range to which 2H-labeled
water was administered to measure total energy expenditure (TEE) using the
doubly labeled water (DLW) technique. In one of these studies, two 24-hour
urine collections were made from many of these same participants using p-aminobenzoic
acid (PABA) to confirm completeness.
The data obtained are among the first objective
assessment of water turnover in American adults and provide documentation of
both the average and range of water input and urine production.
This study, “Water Turnover in 458 American Adults
40-79 Years of Age,” is a collaborative effort by
Aarthi Raman and Dale A. Schoeller from
the Department of Nutritional Sciences, University of
Wisconsin-Madison, Madison, WI; from divisions of the National Institutes of
Health in Bethesda, MD, are Amy F.
Subar and Richard P. Troiano (Division of Cancer Control and
Population Sciences, Applied Research Program);
Arthur Schatzkin, (Division
of Cancer Epidemiology and Genetics, Nutritional Epidemiology Branch,
National Cancer Institute);
Tamara Harris and Douglas Bauer (National Institute on
Aging); and James E. Everhart (National
Institute of Diabetes and Digestive and Kidney Diseases). Additional
participants include Shiela A.
Bingham, at the MRC-Dunn Human Nutrition Unit, Cambridge,
United Kingdom; Anne B. Newman, at
the Division of Geriatric Medicine, University of
Pittsburgh, Pittsburgh, PA; and
Frances A. Tylavsky from the Department of Preventive
Medicine, University of Tennessee, Memphis, TN. Their findings appear in the
February 2004 issue of the American Journal of Physiology—Renal
Physiology.
Methodology
Despite recent interest in water intake, few data are
available on water metabolism in adults. To determine the average
and range of usual water intake, urine output, and total body
water, the researchers administered 2H oxide to 458
noninstitutionalized 40- to 79-yr-old adults living in temperate
climates. Urine was collected in a subset of individuals (n =
280) to measure 24-hour urine production using p-aminobenzoic
acid to ensure complete collection. Preformed water intake was
calculated from isotopic turnover and corrected for metabolic water and
insensible water absorption from humidity.
Results
Preformed water intake, which is water from beverages
and food moisture, averaged 3.0 l (liters)/day in men (range: 1.4-7.7 l/day)
and 2.5 l/day in women (range: 1.2-4.6 l/day). Preformed water intake was
lower in the 70-79 age group (2.8 l/day) than in 40- to 49-yr-old men, and
was lower in older age group (2.3 l/day) than in 40- to 49- and 50- to
59-year-old women. Urine production averaged 2.2 l/day in men (range:
0.6-4.9 l/day) and 2.2 l/day in women (0.9-6.0 l/day). Other results
indicated no age-related differences in women, but men 60-69 years old had
significantly higher urine output than 40- to 49- and 50- to 59-year-old
men. Additionally, only the 70- to 79-year-old age group included
sufficient blacks for a racial analysis. Blacks in this age group showed
significantly lower preformed water intake than did whites. Whites had
significantly higher water turnover rates than blacks as well.
Conclusions
These results
demonstrate that water turnover is highly variable among individuals
and that little of the variance is explained by anthropometric
parameters. A key aim of this research was to test
whether the elderly had low intakes of water that might predispose them to
chronic dehydration. The results found that, on average, the oldest group of
individuals had a preformed water intake that was 98 percent of the younger
group of individuals when expressed per kilocalorie of energy expended.
There was no evidence of dehydration in the 70- to 79-year-old group,
despite the majority of the individuals having intakes less than the
commonly used suggestion of eight 8-oz glasses of water each day.
Furthermore, recommendations to increase fluid intake to eight 8-oz glasses
of water in the elderly may not be prudent because the elderly have an
elevated risk of overhydration due to the weakened physiological movement of
water through the system. Instead, the researchers suggest that fluid
intake for the elderly be increased during periods of acute thermal stress.
-end-
Source: February 2004 edition of the American Journal of
Physiology—Renal Physiology.
The American Physiological
Society (APS) was founded in 1887 to foster basic and applied science, much
of it relating to human health. The Bethesda, MD-based Society has more than
10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals
every year.
***
Editor’s Note: A copy of the research article is available in pdf
format to the press.
Members of the press are invited to obtain a pdf copy of the study and
to interview members of the research team. To do so, please contact Donna
Krupa at 703.527.7357 (direct dial), 703.967.2751 (cell) or
djkrupa1@aol.com.
