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a Geriatric Research Education and Clinical Center (GRECC), St. Louis VAMC, and Division of Geriatric Medicine, Saint Louis University, Missouri
THERE is now good evidence that older persons fail to have an appropriate thirst response to dehydration (1)(2). Dehydration represents a common problem in older persons in hospitals (3) and in nursing homes (4)(5)(6). This is often associated with the onset of febrile illnesses or diarrhea. Although dehydration can develop with extraordinary rapidity in nursing home residents, it is considered a critical indicator for poor care in nursing homes. Dehydration is associated with a variety of complications including death, delirium, renal failure, orthostatic hypotension, pressure ulcers, falls, reduced salivary flow and periodontal sepsis, constipation, urge incontinence, and hyperosmolar diabetic coma (7)(8)(9)(10). These concerns were incorporated in the Modified Food Guide Pyramid for people over 70 years of age with the suggestion that daily fluid or water intake should be at least 8 glasses per day (11).
Surprisingly there is little data concerning the normal fluid intake of community-dwelling older adults and whether this is associated with a higher prevalence of dehydration. In this issue of the Journal, Lindeman and colleagues (12) report that they could find no evidence of dehydration in community-dwelling elders ingesting 6 glasses of fluid per day. In a study of older inner-city African Americans, the prevalence of an elevated blood urea nitrogen/creatinine level (
18) was 10% (13). Elevated levels of this measure of dehydration were associated with functional and cognitive decline and an increase in falls. DeCastro (14) could find no differences in overall fluid intakes of subjects ranging from 20 to 80 years of age. Older persons ingested more coffee and less soda and alcohol than younger persons. Younger persons tended to increase their fluid ingestion later in the day while older persons obtained most of their fluid by coingestion of solids at mealtimes. Overall these studies suggest that the majority of healthy older persons ingest sufficient fluids to maintain an adequate hydration status.
If this is the case, do older persons have any risks associated with ingesting excessive fluids? The major risk appears to be the propensity many older persons have towards developing hyponatremia. Miller and colleagues (15) found that nearly half of residents in nursing homes developed hyponatremia during the year of their study. The majority of these residents had a forme fruste of the syndrome of inappropriate antidiuretic hormone (SIADH) which was triggered by increased fluid administration. Others have reported similar findings in nursing homes (16)(17), and 11.6% of an ambulatory geriatric clinic population were found to be hyponatremic (18). Whether these mild degrees of hyponatremia have an impact on the older person by producing fatigue or impaired function or cognition remains to be determined.
Nocturia and associated nocturnal incontinence and sleep disturbance is a major problem for many older persons. The nocturia is due to an impairment of the circadian rhythm of arginine vasopressin with aging (19). Unnecessary daytime fluid intake will lead to an increase in nocturia, aggravating the associated problems. Obviously, older persons with heart failure often need to limit fluid intake to allow maintenance of appropriate cardiovascular function.
Older persons have complex physiological changes. The regulation of fluid balance in older persons appears to be similar to many other situations where the system remains in homeostasis until it is stressed. Thus older subjects appear to have impaired thirst responses to dehydration, heat acclimation, and possibly hypernatremia (1)(20)(21). In addition, they fail to reduce their fluid intake to head out water immersion (22). These changes appear to be due, in part, to a failure of the mu opioid fluid drive in the hypothalamus (23). These changes in thirst are balanced by an increase in atrionaturetic factor and blunting of the response of the kidneys to arginine vasopressin as well as an altered ability to concentrate urine and a decline in glomerular filtration rate in many, but not all, older persons (24).
Obviously, stressors play a key role in the pathogenesis of dehydration. Recognition of and an appropriate increase in fluid intake in response to these stressors is a key to prevention of dehydration. Some of these stressors are less obvious than others. One of these is the economy class syndrome, that is, thromboembolic events occurring during or immediately following long-distance flights. In these cases dehydration and drinking alcohol during the flight (which inhibits arginine vasopressin release) are common precipitating features (25).
It is important for modern geriatric medicine to try to be evidence based whenever possible. This is particularly important when we make health recommendations to the healthy elderly population. Based on the available evidence, it would seem that healthy elderly people should receive a recommendation to drink 6, not 8, glasses of fluid daily and to increase their fluid intake by 2 glasses under stressful conditions such as febrile illnesses, heavy exertion, excessively hot weather, and for our frequent flyer seniors, before and during long-distance air flights.
It may be similarly necessary to rethink the recommendations of the Modified Food Guide Pyramid (11) for older persons to increase the amount of energy-rich food intake. This recommendation is based on the evidence that there is a physiological anorexia of aging and the poor outcomes of older persons suffering from protein energy undernutrition (26). However, protein energy undernutrition is a problem of the minority of healthy elderly people and is often associated with an underlying stressor such as depression, medical illness, or social problems (27). On the other hand, Roberts (28) showed that older persons have dysregulation of caloric intake and if they over-ingest calories they are more likely than younger persons to develop obesity. Thus, recommending a higher caloric intake to healthy older persons may lead to an increase in the fat frail (sarcopenic obesity) (29).
Unfortunately, as the discussion in this editorial shows, the appropriate care of older persons requires the wisdom of Solomon. Rarely is it a case of one size fits all. The gerontological health care provider needs to focus continuously on the individual's needs rather than providing group recommendations. Those developing health recommendations for older persons in general need to evaluate the evidence carefully and recognize the heterogeneity of the population we are all fortunate enough to work among.
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  J. E. Morley and J. H. Flaherty Editorial It's Never Too Late: Health Promotion and Illness Prevention in Older Persons J. Gerontol. A Biol. Sci. Med. Sci., June 1, 2002; 57(6): M338 - 342. [Full Text]
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D. R. Thomas, W. Ashmen, J. E. Morley, and W. J. Evans Nutritional Management in Long-Term Care: Development of a Clinical Guideline J. Gerontol. A Biol. Sci. Med. Sci., December 1, 2000; 55(12): 725M - 734. [Abstract] [Full Text]
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