Age-related dehydration etiology - Reduced fluid intake and increased fluid loss in older adults put them at risk of dehydration. Water deprivation reduces thirst in even healthy older adults. Dehydration is reflected both in low thirst scores during dehydration and in reduced water intake after dehydration.
It has been shown in several studies that older subjects are unable to return to their baseline plasma osmolality or sodium concentration even when water is freely available. Naloxone, an opiate antagonist, modifies fluid intake in young subjects, but not in older subjects, suggesting that the opioid receptor system in older individuals may be deficient, contributing to hypodypsia. Furthermore, presbynephrosis occurs as the elderly age, which contributes to their weakened ability to defend against dehydration as a result of renal concentrating capacity reduction.
Aging seems to cause a multifactorial but specific problem with water homeostasis. Aldosterone and plasma renin levels diminish with age, as does the ability to respond appropriately to sodium deprivation by increasing aldosterone and plasma renin levels. However, plasma antidiuretic hormone (ADH) and arginine vasopressin (AVP) levels are higher in older dehydrated individuals compared to young people, indicating that this volume and tonicity defense mechanism does not diminish as we age.
However, healthy older adults are not different from younger adults when it comes to responding to hypertonicity (induced by intravenous infusion of hypertonic saline). The aging process seems to have more to do with how the body senses volume than how it senses tonicity. The osmolality of plasma and the extracellular fluid volume was altered simultaneously by both dehydration and hypertonic saline infusion until recently, however.
To address this question, a recent study used head-out water immersion, which increases hydrostatic pressure outside the body and drives blood into the thoracic cavity, thus expanding central blood volume by about 700 ml and leading to increased cardiac filling pressure and heart volume without altering plasma osmolality. The increased central volume seems to activate cardiac stretch receptors, attenuating thirst and drinking; this effect is markedly diminished in healthy older subjects, despite a comparable, or greater, increase in central blood volume and atrial natriuretic peptide.
This experiment suggests that a centrally mediated response to volume, but not to osmolality, is reduced with aging. The changes listed above seem to be physiologic since they occur in healthy older subjects in the absence of chronic diseases. However, many degenerative age-related diseases worsen the tendency towards dehydration in older persons.
These include delirium, dementia, diuretic use, swallowing problems, laxative abuse, and problems with hand dexterity or ambulation. In addition, dehydration is a common complication of acute illness in older persons. For example, among Medicare beneficiaries in the USA hospitalized with dehydration, 28% had pneumonia or influenza, 25% had a urinary tract infection and 10% had gastroenteritis.
