I was flipping through an old textbook on nutrition in the elderly and saw this:

Impaired Thirst

A physiologic change that significantly influences fluid homeostasis in older people is altered thirst. Medical textbooks over the years have pointed out that older patients do not ask for fluids. Research studies with healthy older men (ages 67 to 75) and healthy young controls (ages 20 to 31) have confirmed a blunting of the thirst mechanism. After 24 hours of water deprivation, resulting in about 2% loss in body weight in both groups, the older subjects still experienced little thirst, whereas the younger subjects were extremely thirsty.

A major concern was the response of the older men when unlimited amounts of water were made available to them. The older individuals consumed only 3.4 ml per kg of body weight, whereas the younger subjects consumed 8.5 ml per kg of body weight. In other words, the younger men consumed the amount of fluid required to raise their plasma volume to predeprivation levels. The older subjects did not replace the water they had lost in spite of a significant increase in plasma sodium concentration, which normally stimulated thirst. Older people are less able to measure serum osmolality and appropriate fluid replacement levels, leading to chronic dehydration in even healthy elderly.

This loss of thirst creeps up on you. It’s not that you have adequate thirst when you’re 66 but suddenly lose it when you’re 67. Even healthy people need to pay attention to fluid intake as they age.

And it’s not just lack of thirst that contributes to dehydration, but also “the kidneys’ decreased ability to conserve water.” Less is going in; more is going out. I keep forgetting that “more is going out” part.

They note these consequences of low body water:

• Body water is a diluent for medications, and a dehydrated individual is more vulnerable to drug toxicity.
• Temperature regulation is less efficient with a decreased level of body water to act as a thermal buffer.
• Physical signs of dehydration include dry tongue, flushing, reduced turgor of the skin, and, as the condition increases in severity, confusion or delirium.
• Serum sodium and blood urea nitrogen levels usually are elevated.
• Reduced fluid intake exacerbates a constipation problem and puts a greater burden on the kidneys, particularly among those with high intakes of protein and electrolytes (e.g. sodium).

This text, and various other sources, say we should be drinking about 2000 ml daily. That’s 2 liters or 8 and a half cups of non-alcoholic fluid a day.

Related:
Don’t Depend On Your Thirst Sensation To Guide Your Fluid Intake, Especially If You Are Older