The world is aging rapidly and more so in the developed countries where between 13% and 20% of the population is 65 years or older.1 Of this older population, the 85-year-or-older age group is growing the fastest. Although age alone is not an independent risk factor for dehydration, converging age-related factors including blunted thirst, loss of lean muscle mass, increasing inability to respond efficiently to physiological stressful events where dehydration results, and renal changes including a reduced capacity to handle water and sodium efficiently may contribute to increased risk for dehydration in older adults.2–4 If chronic medical conditions and the medications that are used to manage these conditions are factored into the equation, older adults are at even higher risk for hydration difficulties, specifically dehydration.
To highlight these points, Stookey and colleagues5,6 completed a secondary analyses of hydration markers in 2 large population-based surveys, NHANES III (National Health and Nutrition Examination Survey) and the EPESE (Established Populations for Epidemiological Studies of the Elderly). In both analyses, they found nearly identical results where 40% of community-dwelling older adults were underhydrated by plasma tonicity (295–300 mOsm/L), and 20% were dehydrated (>300). Also, those with elevated tonicity were more likely to be older and of African or Hispanic descent and have comorbid conditions of diabetes, hypertension, obesity, and functional impairment.
This article frames the problem of dehydration in older adults by exploring several key issues contributing to complexities of hydration issues in older adults: blunted thirst response, decreasing water intake with age, difficulties with assessment of hydration status, and hydration habits that may contribute to risk for dehydration.
THE ROLE OF THIRST
Most work on thirst including older persons has been done in laboratory settings where progressive dehydration has been induced and has included only men.4 There is sparse information on thirst in older women. A synthesis of research conducted by Kenney and Chiu,4 examining thirst across the life span, suggests that older men have a normal response to osmotic thirst but have a blunted thirst response to hypovolemic stimuli. When required to subjectively report their thirst, they rate themselves as either “thirsty” or “less thirsty” than younger men, and they do not rehydrate themselves adequately. Age-related changes in receptivity to osmotic and hypovolemic stimulation and hormonal changes in sensitivity to antidiuretic hormone and increased secretion of atrial natriuretic peptide appear to offer some explanation of thirst alterations in men 65 years or older.
Another hypothesis suggests that aging is associated with changes in satiation that hinders adequate rehydration in response to hyperosmolarity. In a study that examined cerebral blood flow in response to a dehydration/rehydration protocol, positron emission tomography scanning of older and younger adults suggested that the anterior midcingulate cortex, which responds to drinking behavior, responded differently in older men.7 There was a greater reduction in blood flow to this area in older men despite an inadequate amount of ingested water to mitigate fluid losses. The authors conclude that this response may be due to sensitized responses of the anterior midcingulate cortex to afferent inflow from sensory receptors in the mouth, throat, and gastric regions. Regardless of the mechanism, it is clear that thirst is not a reliable indicator of hydration status in older adults.
DECREASED INTAKE IN THE OLDEST OLD
In addition to changes in thirst, older adults consume less water from food and fluids relative to younger adults.8 Zizza and colleagues’9 study of American adults found that relative to the 65- to 74-year age group, the middle-old group of 75 to 84 years and the group older than 85 years had significantly lower intakes, and 81% of the group 85 years or older did not meet an adequate intake. Similar results were found in German older adults; however, the authors note that the intakes on average were within the recommendations set by the German referent value. However, in the oldest group, 51% did not meet this value (1990 mL).10
Although there are standard daily values for fluid intake in adults, the requirement varies across countries and applies to older adults 70 years or older. This is noteworthy because one can understand that the requirements for a 70-year-old could differ drastically from an adult who is 85 years or older. The Institute of Medicine recommends a total daily fluid intake from foods and fluids of 3.7 and 2.7 L for elderly men and women, respectively.11 The European Food Safety Authority recommends a little less at 2.5 L for older men and 2 L for older women.12 Both standards do not factor the activity level or health status of the older adult into their recommendations.
To further complicate these age-related factors, assessment of hydration status in older adults is also difficult. Although urine and serum measures of hydration status can be used with accuracy in younger persons, age-related changes in kidney function make these measures less reliable in older adults. Similarly, clinical indicators such as changes in vital signs, skin turgor, and the oral cavity can be confounded by age-related changes, disease states, and medications. This lack of a criterion standard for dehydration in older adults has spurred researchers to develop risk models using composite measures, such as clinical signs/symptoms or medical history for detecting dehydration or risk for dehydration. Several such attempts to quantify risk for dehydration in older adults are (1) questionnaires, such as the Dehydration Risk Appraisal Checklist,13 which is a 17-item checklist including medical conditions, medications, and clinical characteristics that have been correlated with dehydration; and (2) statistical risk models using clinical signs indicative of dehydration,14 such as those described by Vivanti and colleagues,14 where tongue dryness was associated with poor hydration status in older adults admitted to a rehabilitation unit. However, these efforts are imprecise because they are based on the assumption that dehydration risk can be quantified based on specified characteristics, such as diseases, medications, or clinical signs in older adults. Given the heterogeneity of health status in the older adult population, it is likely that these measures give us a less-than-adequate assessment of any individual older adult’s hydration status. An additional underlying assumption is whether the risk for dehydration can even be assessed, given the unknown consequences of chronic suboptimal hydration and whether this state poses a risk for dehydration. Perhaps a more accurate way to understand dehydration risk in older adults requires that we know their hydration habits coupled with an objective biochemical profile. Therefore, this article concludes by looking at an oral hydration typology that classifies older adults into 4 groups based on hydration habits: “can drink,” “can’t drink,” “won’t drink,” and “end of life,” to better understand dehydration risk.
A typology of hydration habits was developed using qualitative methods in a study that examined the hydration habits of 35 residents of nursing homes over a 3-month period.15 The typology included 4 groups including those who “can drink,” those who “can’t drink,” those who “won’t drink,” and those who are at the end of life. The “can drink” group was divided into 2 subgroups: those who were generally younger and functionally independent, but who did not know how much they should drink per day. The second subgroup included those who were functionally independent, but were forgetful and so, although able to drink independently, required cuing to do so.
The second category were those who “can’t drink” either because of an increased choking risk (dysphagic subgroup) or because they were either physically incapable of getting fluids independently or unable to manipulate drinking utensils because of physical limitations (physically dependent subgroup). These subgroups were both highly functionally dependent in activities of daily living.
The third category included persons who “won’t drink” secondary to an often lifelong habit of never drinking very much—“sippers” and those persons who fear being incontinent of urine. This category had more individuals who were either hospitalized for dehydration, received intravenous fluids, or who had a blood urea nitrogen–creatinine ratio of greater than 25. The incontinent group was also the most cognitively intact, had the highest average urine specific gravity, and drank the fewest fluids at mealtime.
The last category included persons at the end of life. At end of life, decisions about stopping eating and drinking or decreased intake become a personal and family matter, where dehydration may be the preferred state.
The typology offers support for hydration habits as risk factors for dehydration in frail older adults, but needs to be prospectively tested in more functionally able, community-dwelling adults and incorporate serial biochemical measures of hydration. Work on dehydration risk needs to consider both person-centered risk factors, such as discussed in the hydration typology, and factoring in biochemical indicators of hydration status. Given the fluctuating nature of the biochemical indicators in response to changing hydration status and the unknown effect of chronic suboptimal hydration, future directions for research should focus on the temporal sequence of underhydration to adverse health events in order to better determine optimal requirements for water in older adults.
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