Water need depends on age

Fluid intake for the elderly should be increased during periods of acute thermal stress, say experts.

health and fitness Updated: Feb 25, 2004 18:30 IST

Just how much water do each of us really need? Not to swim in, or diet with. Not to respond to marketing claims or to counter salty foods or to cope with dry environments, but to drink.

There is no question that water is vital to the body's overall health. A plentiful consumption of fluid may be protective against diverse medical conditions, including kidney stones, constipation, colorectal cancer, premalignant adenomatous polyps and bladder cancer.

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 non-caloric, non-alcoholic and non-caffeinated beverages is poorly documented.

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 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.

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 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.

Preformed water intake, which is water from beverages and food moisture, averaged 3.0 l (liters)/day in men and 2.5 l/day in women.

Preformed water intake was lower in the 70-79 age group averaged 2.8 l/day than in 40 to 49-yr-old men, and was lower in older age group averaged 2.3 l/day than in 40-49 and 50 to 59-year-old women.

Urine production averaged 2.2 l/day in men ranged 0.6-4.9 l/day and 2.2 l/day in women.

Other results indicated no age-related differences in women, but men 60-69 years old had significantly higher urine output than 40-49 and 50 to 59-year-old men. Additionally, only the 70 to 79-year-old age group included sufficient blacks for a racial analysis.

These results demonstrate that water turnover is highly variable among individuals and that little of the variance is explained by anthropometric parameters.

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.

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.

First Published: Feb 25, 2004 18:30 IST