Body fluid balance in dehydrated healthy older men: thirst and renal osmoregulation

Abstract

We examined osmotic control of thirst and free water clearance in healthy older (65+, n = 10) and younger (Y, n = 6) subjects during a 3-h rehydration period after an approximately 2.4% decrease in body weight. Plasma volume (PV), plasma osmolality (Posm), renal function, and thirst were measured before and after dehydration and during rehydration. In 65+, baseline PV was lower (43.1 +/- 1.6 vs. 48.1 +/- 2.5 ml/kg), Posm was higher (287 +/- 1 vs. 281 +/- 2 mosmol/kgH2O), and perceived thirst was lower than in Y. During dehydration, the osmotic threshold for increased thirst was shifted to a higher Posm in 65+. Total fluid intake was greater in Y than in 65+ (16.6 +/- 4.1 vs. 8.9 +/- 2.0 ml/kg); however, the relation between thirst and the rate of fluid intake was identical. Thus the blunted rehydration in 65+ is related to a lower overall sensation of thirst. The stimulus-response characteristics of osmotic control of free water clearance was similar in 65+ and Y; however, 65+ operated around a higher Posm and on a less-steep portion of the stimulus-response curve. These data support the hypothesis that the hyperosmotic hypovolemic state of healthy older individuals is not a result of a simple water deficit but represents a shift in the operating point for control of body fluid volume and composition.

Effect of aging on regional cerebral blood flow responses associated with osmotic thirst and its satiation by water drinking: A PET study

“…In young people, the volume of water imbibed within a short time after a period of dehydration is usually sufficient to replenish body f luids (9). Older people tend to drink less water when dehydrated than their younger counterparts.…”

“…Irrespective of the factors giving rise to thirst, older people tend to drink less than their younger counterparts when dehydrated (8,9). There has been considerable speculation about the effect of aging on the neural processes involved in the genesis and satiation of thirst.…”

Water-loss dehydration and aging

“…This reduced ability to retain fluid by concentrating urine may mean that urinary markers of dehydration are less useful in older people, and dehydration risk can be exacerbated by the use of The thirst response is less effective with ageing as older people have to experience greater serum osmolality before thirst is stimulated, although they respond to thirst in the same way as younger people (Davies et al, 1995;Mack et. al., 1994).…”

The prevalence and risk factors for age-related macular degeneration in rural–urban India, Sankara Nethralaya Rural–Urban Age-related Macular degeneration study, Report No. 1

“…Even healthy older adults feel less thirst in response to water deprivation; this is evidenced both in a lower self-reported thirst score during dehydration and reduced intake of water after the dehydration period (78). In several studies, older subjects were not able to return to baseline plasma osmolality and sodium concentration despite free availability of water (78,79). Fluid intake in young subjects, but not in older subjects, is modified by injection of naloxone, an opiate antagonist, suggesting that the opioid receptor system may be deficient in older adults and may contribute to the hypodypsia seen in older persons (80).…”

“…Fluid intake in young subjects, but not in older subjects, is modified by injection of naloxone, an opiate antagonist, suggesting that the opioid receptor system may be deficient in older adults and may contribute to the hypodypsia seen in older persons (80). In addition, with ageing there is a reduction in renal concentrating capacity in response to dehydration known as presbynephrosis, which contributes to the blunted ability of the elderly to defend against dehydration (79).…”