Water serves a vital role in the everyday operation of cells within the body. The maintenance
of water balance is essential for health and is determined by a sophisticated scheme that
balances water input (drinking) and water output (perspiration, urine) from the body. Specific
mechanisms interact to control thirst, drinking behavior, and the output water from the
kidney when the body is faced with a water deficit. However, restoring body water balance
following dehydration ultimately depends on mechanisms that regulate fluid intake or
drinking. Despite these sophisticated defense mechanisms, the restoration of water balance
following dehydration is usually slow and incomplete; a problem termed “involuntary
dehydration.” Thus, even healthy young adults should be advised of the benefits of complete
water replacement following dehydration.
More importantly, it has become increasingly clear that the ability to regulate fluid balance in
response to fluid deprivation or dehydration is compromised in older individuals. Decreased
ability to regulate water balance can adversely affect the aging population, leading to
increased risk of dysfunction, morbidity, or mortality. In addition, these problems in body
fluid regulation are often exacerbated by the presence of other chronic diseases associated
with aging, such as hypertension or cerebrovascular disease. As such, the aging population
is considered at greater risk for developing dehydration and any associated complication.
Dehydration refers to the process of reducing body water either through illness, physical
exertion, thermal stress, or water deprivation. Generally, during dehydration, body water is
conserved by defense mechanisms, which act to reduce water output by the kidney. This
process is mediated by the release of an antidiuretic hormone and its action on the
functional part of the kidney (the nephron) to conserve water. Antidiuretic hormone is
released during dehydration by signals from blood that reflect the tonicity (salt
concentration) and volume of blood held within the blood vessels. The secretion of
antidiuretic hormone during dehydration is not decreased with aging, yet the ability of the
kidney to conserve water is reduced. There are several possible explanations for this
problem. First, the number of nephrons per kidney begins to decrease by about 10% per
decade after the age of 40. Second, nephrons from older kidneys appear less responsive to
a given level of antidiuretic hormone than younger nephrons. Thus, both a reduction in
number of nephrons and their sensitivity to antidiuretic hormone limit the ability of older
individuals to conserve water and predisposes the older individual to dehydration.
The ability to maintain water balance is highly dependent on thirst, a sensation thought to
provide the drive for fluid ingestion. Clearly, it is only through the ingestion of fluid that a
water deficit can be replaced. Fluid ingestion is characterized by two types of drinking
behaviors – primary drinking, which is driven by deficits in body water and acts to restore
those deficits, and secondary drinking, where drinking occurs when no apparent need is
present (thirst associated with excessive talking). Primary thirst is regulated by plasma
tonicity and the volume of blood within the blood vessels (similar to antidiuretic secretion).
Unlike antidiuretic hormone secretion, however, the increase in thirst following dehydration
decreases with aging. During dehydration older individuals report less thirst despite having
similar increases in plasma tonicity as younger individuals. This lower perceived level of
thirst is associated with reduced level of fluid ingestion. As mentioned earlier, most people
will not drink enough water to replace their original fluid deficit, and this problem is
exacerbated with age. Thus, thirst cannot be used as a reliable indicator of the fluid
requirements of older individuals. This is probably an expected conclusion since a variety of
sensory functions (e.g. hearing, vision, smell and touch) are also generally reduced with
aging.