This comprehensive water balance study is the first to document water input and output in younger and older men and women. The results indicate that chronologic age does not influence the ability of a healthy person to consume sufficient fluid to achieve water balance and to maintain indexes of hydration within clinical normalcy.
The observation that age did not influence total water intake is consistent with some previous research. De Castro et al (17
), using 7-d food diary data from 262 healthy men and women grouped into 20–34 y, 35–49 y, 50–64 y, and 65–80 y age ranges, reported that total water intake, regardless of source, was not different among the 4 age groups. They also reported that age did not influence water intake from “drinks,” defined as an item ingested in liquid form and not normally considered a food (ie, items such as soups and instant breakfast beverages were excluded). In contrast, Raman et al (18
), using 2
H-labeled water turnover data from 458 men and women grouped into 40–49 y, 50–59 y, 60–69 y, and 70–79 y age ranges, reported that preformed water intake, defined as water consumed orally from beverages and food, was lower in the 70–79-y-old men than in the 40–49-y-old men and lower in the 70–79-y-old women than in the 40–59-y-old women. The dietary total water intakes of ≈3300 mL/d in older women and 3700 mL/d in older men in the current study are somewhat higher than those Raman et al (18
) reported for the 70–79-y-old women (2330 mL/d) and men (2750 mL/d) and are 135–160% of recommended intakes (13
). The strict dietary control of the present study provides an opportunity to document that age did not influence ad libitum water consumption and that it accounted for ≈44–50% of dietary total water intake among the 4 groups when water intake from food and beverage sources was controlled. The ≈1720, 1830, 1350, and 1640 mL/d ad libitum water intake by the YM, OM, YW, and OW, respectively, represents 91%, 96%, 71%, and 86% of the recommendation to consume 8 glasses (8 oz or 237 mL each) of water each day (≈1900 mL/d) (32
Improved education of older adults about guidelines for water intake is required. For example, the modified food guide pyramid for adults aged ≥70 y (32
) depicts 8 glasses of water as the foundation of the pyramid. However, Russell et al (32
) indicated in the description of the pyramid that older adults are encouraged to consume ≥8 servings of noncaffeine, nonalcohol “fluids” per day, but not specifically water. Nutrition education directed toward older adults should continue to emphasize the importance of adequate water intake, but it should more clearly state that this need may be met by consuming a variety of fluids, not exclusively 8 glasses of water. Indeed, little scientific support is found for the recommendation to consume 8 (8-oz or 237 mL) glasses of water (33
). The overconsumption of water may cause water intoxication and nonfatal hyponatremia, especially in older adults as a result of an age-associated decline in the kidneys to dilute urine (2
The current study and previous research by de Castro et al (17
) and Raman et al (18
) used healthy subjects who were not purposefully exposed to conditions known to alter fluid balance, such as water deprivation, overhydration, altered metabolic state (eg, induced hypertonicity), altered environmental conditions (eg, high temperature or altitude), and prolonged exercise (5
). Under stressed conditions, thirst responses are controlled by homeostatic mechanisms (36
). Older adults exhibit a different thirst response and reduced fluid intake (36
), which are associated with an age-related shift in the set point that controls body fluid volume and composition (37
). The ability of the older subjects in the current and past research studies (17
) to maintain water intakes comparable to those of groups of younger subjects suggests that water consumption was not controlled by homeostatic mechanisms (17
). That is, the experimental conditions were such that the main determiners of water intake were unregulated factors, such as the amount and timing of food intake, food and beverage preferences, food and beverage availability, and the consequences of food and beverage intake (eg, urine production). De Castro et al (17
) emphasized that the apparent lack of homeostatic control of fluid intake under unstressed conditions does not imply that homeostatic mechanisms are not important in older adults, but only that they are not likely called on when water intake exceeds requirement.
The findings that dietary protein intakes that spanned the range of adequacy did not influence water input, output, and balance corroborate those of Luft et al (40
). They reported that water intake (food, beverage, and ad libitum consumption) and urinary excretion were not different when 8 young men consumed either 80 or 180 g protein/d during 7-d controlled feeding periods. De Castro et al (17
) observed that, for 20–80-y-old men and women, protein intake positively correlated with total fluid intake. However, this apparent relation did not remain when other dietary factors (ie, carbohydrate, fat, and sodium) were considered with multivariate analyses, and the researchers concluded that the primary determinant of fluid ingestion was the amount of solid ingested, not a specific macronutrient. The current findings that urine specific gravity and plasma osmolality were not influenced by protein intake or subject age, coupled with the lack of change in ad libitum water consumption, support the conclusion that healthy older adults with clinically normal kidney function are able to successfully respond to changes in urea production. These conclusions should not be generalized to high-protein diets (40
) or protein deficiency (41
) without further research.
The apparent positive net water balance () was expected, because water output by sweat was not accounted for in the water balance equation. The accurate assessment of 24-h sweat loss is notoriously difficult to quantify and will vary considerably from person to person. The greater net water balance in men than in women may be attributed to greater sweat losses in men during resting and exercise states (42
). The questionnaire-based assessment of physical activity suggested that the 4 groups performed comparable amounts of work, activities, and exercise. This finding would support the conclusion that the differential net water balance between men and women was not due to differences in habitual activities. This conclusion should be drawn cautiously, because the questionnaire was designed to assess patterns of physical activities during the previous month (28
) and not on the specific days of water balance assessment. In addition, this questionnaire was designed for use in older adults and is better suited to evaluating the energy expenditure of physical activity for groups of subjects than for individual subjects (43
). The stability of body weight during the water balance periods suggests that the positive water balance was not due to body water retention and that these subjects were able to tightly regulate their body water.
The difference in water balance between the men and the women is not likely due to the difference in sodium intake. Luft et al (40
) reported that water intake and urinary water excretion in young men were not influenced when sodium intake was set at 10, 200, or 400 mEq/d, and de Castro et al (17
) showed that sodium intake did not relate to fluid intake among 262 men and women aged 20–80 y.
The observation that urinary water output was not different in the younger and older men and women is generally consistent with the observation of Raman et al (18
), who reported no age-related difference in urinary excretion among women aged 40–69 y, but they reported ≈20% greater urine excretion in 60–69-y-old men than in 40–59-y-old men. In the current study, urinary water excretion accounted for ≈60, 63, 64, and 68% of total water output in the YM, OM, YW, and OW, respectively. These values are comparable to the 66% value reported by Raman et al (18
) and higher than the 50% value previously assumed (15
). Stool water excretion accounted for ≈4% of total water output among the 4 groups in the present study. The measured stool moisture content was 79%, which is higher than the 72% assumed by Raman et al (18
The assumed constant hydration of FFM is among the most widely known and applied body-composition constants (44
). The biological importance of an age-related change in FFM hydration, as observed in this study, is not well established, but it may relate to age-associated shifts in body composition (including osteoporosis and sarcopenia). The biological importance of the FFM hydration is also applicable to growth, sex, body size, and acute or chronic catabolic illness (44
). As reviewed by Wang et al (44
), controversy exists as to whether FFM hydration is influenced by age in the adult human. The current observation that FFM hydration (TBW:FFM; ) was higher in the older men and women than in the younger men and women supports previous findings in men (46
) and women (47
). In contrast, some studies showed no age-related change in FFM hydration (8
). Wang et al (44
) commented that these contradictory findings may relate to differences in health status, levels of physical activity, body mass, and other differential population characteristics. The distribution of body fluid within the intracellular and extracellular compartments is important physiologically but was not assessed in this study.
In summary, the results of this comprehensive water balance study in younger and older men and women suggest that chronologic age does not compromise the ability of an apparently healthy person to consume sufficient water to maintain hydration status. These data provide the first documentation that ad libitum water intake is comparable among younger and older adults who are adapted to controlled diets that provide protein intakes ranging from 63% to 125% of the recommended dietary allowance. These findings are limited to healthy people who are not acutely exposed to stressors known to compromise hydration status. Further research into the effect of age on water balance with the direct measurement of water input and output in older adults under uncontrolled dietary conditions is needed.