Just how much water do we really need? The answer may depend on our age

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.