[PHNUTR-L] Just how much water do we really need? answer may depend on age

[Kathrynne Holden, MS, RD]

Just how much water do we really need? The answer may depend on our age
http://www.eurekalert.org/pub_releases/2004-02/aps-jhm022404.php

(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.

###

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.
-- 
Kathrynne Holden, MS, RD
"Ask the Parkinson Dietitian"  http://www.parkinson.org/
"Eat well, stay well with Parkinson's disease"
"Parkinson's disease: Guidelines for Medical Nutrition Therapy"
http://www.nutritionucanlivewith.com/