title: 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.
authors.affiliation: John B. Pierce Laboratory, Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven 06519.
authors.forename: G W
authors.initials: GW
authors.lastname: Mack
authors.forename: C A
authors.initials: CA
authors.lastname: Weseman
authors.forename: G W
authors.initials: GW
authors.lastname: Langhans
authors.forename: H
authors.initials: H
authors.lastname: Scherzer
authors.forename: C M
authors.initials: CM
authors.lastname: Gillen
authors.forename: E R
authors.initials: ER
authors.lastname: Nadel
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001823:Body Composition; D001834:Body Water / Q000502:physiology; D003681:Dehydration / Q000503:physiopathology*; D004326:Drinking / Q000502:physiology; D005110:Extracellular Space / Q000378:metabolism; D006801:Humans; D007668:Kidney / Q000502:physiology*; D007677:Kidney Function Tests; D008297:Male; D010953:Plasma Volume / Q000502:physiology; D013894:Thirst / Q000502:physiology*; D014870:Water Loss, Insensible / Q000502:physiology; D014882:Water-Electrolyte Balance / Q000502:physiology*