How does aging increase risk of dehydration?

Aging weakens the regulatory systems for maintaining water balance, necessitating more conscious oversight of fluid intake to prevent dehydration.

By Peter Attia on

Many of us are all too familiar with the changes that come with aging – sagging skin, loss of muscle mass and bone density, more frequent memory lapses, the list goes on. But of all these changes, perhaps one of the least familiar nevertheless has an enormous impact on healthspan and longevity: the decline in our ability to stay sufficiently hydrated.

The importance of hydration

In AMA #33, I did a deep dive on hydration in which I touched on some of the consequences of dehydration and volume depletion. Acutely, these include dizziness, fatigue, confusion, and a drop in blood pressure, while more sustained or severe dehydration can cause kidney damage, exacerbate hypertension, and increase the likelihood of deep vein thrombosis.

A little over a year ago, I became rather painfully aware of the importance of hydration when extreme volume depletion and consequent low blood pressure caused me to temporarily lose consciousness and fall into a table. Fortunately for me, my long-term consequences were limited to a couple of minor scars, but for many – and the elderly in particular – similar episodes can lead to debilitating injuries or death.

But the elderly aren’t just at greater risk of serious consequences when they happen to get dehydrated – they are also more likely to get dehydrated in the first place.

Hydration is a balance of inputs and outputs

The body’s fluid balance is regulated by two broad categories of processes: those that control water coming in, and those that control water going out. On the intake side, we have thirst. As we become more dehydrated or volume-depleted, we experience a conscious, increasing drive to consume more fluids, and by satisfying that drive, we counteract the fluid loss.

On the output side, we have a number of pathways, including perspiration, respiration, water loss in stool, and of course, urination. Of these, urination is subject to particularly strict regulation for maintaining fluid balance, such that we can remove water when we have excess or retain water when our body stores are depleted: when we become dehydrated, the kidneys can reabsorb some of this water to limit further loss. This process of concentrating urine is the reason why urine becomes darker in color when we drink too little water.

The balance of intake and output is a fairly dependable system in most day-to-day circumstances for most adults. Thirst and urine concentration generally do a decent job of keeping us sufficiently hydrated to avoid negative effects, as long as fluids are readily available and we don’t ignore thirst when it arises. 

However, there are a few important circumstances when regulatory systems tend to break down on either the intake or output side. When we lose large quantities of fluid very quickly – for instance, in hot, dry environments, during prolonged exertion, or in my case, by donating blood – perceived thirst and voluntary drinking frequently fail to compensate adequately for the deficits. Consumption of alcohol, on the other hand, impairs our ability to concentrate urine, resulting in dehydration through excessive water excretion. But another risk factor – one we can’t avoid so easily – catches us coming and going, compromising both intake and output regulation: advanced age.

Aging impairs fluid intake and output regulation

Elderly adults have long been known to have blunted thirst responses to dehydration. A classic 1984 study compared perceived thirst in response to water deprivation in healthy men aged 67-75 and aged 20-31 and showed that the older group reported less thirst and consumed less water following the deprivation period, despite exhibiting greater objective measures of dehydration. Similar results have since been observed in response to other thirst-inducing stimuli, such as hypertonic saline infusion or heat stress, but the precise mechanisms underlying this age-associated defect in thirst responses have yet to be fully elucidated.

Further elevation of dehydration risk arises from an age-associated decline in urine concentrating ability. While young adults decrease urine output in response to water deprivation, previous work has demonstrated that this reduction in excretion is somewhat diminished in individuals aged 40-59 and almost completely absent in those aged 60-79. One reason for this is a decrease in the kidneys’ ability to respond to antidiuretic hormone (also known as vasopressin) – a hormone responsible for stimulating water reabsorption from urine, among other roles in maintaining fluid balance. (Vasopressin is also inhibited by alcohol, which is why alcohol consumption paradoxically produces dehydration.)

Low muscle mass exacerbates the problem

In the past, I’ve frequently commented on a more familiar change associated with aging: a shift in body composition. Lean mass typically decreases while fat mass increases, and because muscle cells contain far more water than fat cells, this transition results in a substantial reduction of total body water. With less total body water, the effects of any deviation from optimal fluid balance are magnified, such that even mild water deprivation can cause clinically significant dehydration.

In addition to this direct impact of lean mass loss on fluid balance, low muscle mass can also have an indirect effect by impairing one’s ability to seek water in response to thirst. Frailty compromises the mobility required for everyday tasks of life, including walking to the kitchen to get a glass of water. When quenching thirst requires great physical difficulty or assistance from others, we may be less motivated or able to respond to thirst when it arises, as has been suggested by studies among adults in assisted living or residential care.

Preventing dehydration – and its consequences – as we age

When it comes to fluid regulation, it’s clear that aging constitutes an assault on multiple fronts. Fortunately, the first line of defense is simple: consciously drink more fluids. When we’re young, hydration doesn’t require much thought; our bodies tell us to drink water when we’re dehydrated and are effective at preventing excess water loss. But as we reach middle age and beyond, running on autopilot likely isn’t enough. While there’s little we can do about losing the ability to concentrate urine, we can be more deliberate in our fluid intake as a means of maintaining adequate hydration. (For my recommendations on the best ways to hydrate, check out AMA #33.) 

We can also reduce the impact of dehydration on healthspan and longevity by building and maintaining muscle mass. Increasing lean mass increases total body water, thus limiting the likelihood that mild water loss might cause dizziness or other negative effects, and if lightheadedness does arise, additional strength may make the difference between a catastrophic fall and a minor lapse in balance.

A hard-learned lesson

Aging is accompanied by countless physiological changes and longevity hazards, many of which have long captured my attention. To recognize the importance of hydration among that multitude, I needed it to hit me in the face – literally. With any luck, this newsletter might spare others from learning the hard way a lesson which I won’t soon forget – and one that will only grow in significance as the years pass and hydration requires increasing diligence.

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