How to know you're dehydrated (Signs of early, moderate, and severe dehydration)

If you follow my work, you know I don’t spend much time worrying about dehydration. I worry more about dangerously low sodium levels that result from over-hydration.

Unfortunately, low serum sodium (hyponatremia) is often misdiagnosed as dehydration. The symptoms are similar. But when you treat hyponatremia as dehydration—when you drink water well beyond thirst—you only make it worse.

But even if dehydration is less dangerous than hyponatremia, it doesn’t mean we shouldn’t address it. If you continually lose more water than you take in, it will lead to a suboptimal state. We call that state dehydration.

I wrote this article to be a nuanced guide to determining your hydration status. I’ll show you the signs of dehydration, but you have to take them with a pinch of salt since the symptoms can mimic the symptoms of low sodium.

I mean that literally. Take your water with a pinch of salt. Or a stick of LMNT. That’s how you prevent both low water and low sodium. Now let’s talk about the low water part.

What Is Dehydration?

Dehydration is defined as losing more fluids than you’re taking in. It’s net water loss from the body.

You lose water through the lungs, skin, kidneys (urine), and GI tract. If these losses exceed the quantity of fluids you’re consuming, you’re said to be dehydrated.

The result of dehydration is a low-water state called hypohydration. Hypohydration is considered “severe” when total water losses exceed 5% of body weight, though mild hypohydration can be a 1% water loss or less. Hypohydration and dehydration are often used interchangeably, so I’ll be following that convention here.

Healthy adults rarely become dehydrated. When someone has access to water, they drink adequate fluids. Often, they drink too much. More on that later.

There is no established test for dehydration. Clinicians often measure serum osmolality (the concentration of electrolytes in the blood), but this isn’t practical for most people. Another potential sign of dehydration is weight loss exceeding 3% over seven days, but weight loss alone doesn’t indicate hydration problems.

Older adults are 20-30% more likely than the general population to develop dehydration. Why? Probably because older adults are more likely to suffer from an impaired thirst mechanism, diabetes, or renal disease. They also may have mobility issues that make water access a challenge.

Other potential causes of dehydration include:

  • Heavy sweating due to heat, exercise, or skin disease.
  • Excess water loss via urine due to kidney disease, diuretic drugs, Addison’s disease, high blood sugar (hyperglycemia), and several other conditions.
  • Losing water through the GI tract: diarrhea, vomiting, laxatives.
  • Excess respiratory losses due to asthma or COPD.

This is just a partial list. The human body is about 60% water, and this water can escape through every orifice. Normally, however, our fluid balancing system works impressively well.

Fluid Balance 101

Thirst is a simple urge that results from a complex system. All the time, specialized molecules called osmoreceptors are measuring the hydration status of your blood. If your blood volume drops too low—a condition called hypovolemia—these receptors trigger thirst in a brain region called the hypothalamus. Then you drink something and the balance is restored.

But thirst is just one way the brain regulates fluid balance. It also controls fluids by controlling your kidneys.

When the hypothalamus senses blood volume dipping, it pings the pituitary, a pea-sized gland at the base of your brain. The pituitary is told, in no uncertain terms, to release antidiuretic hormone (ADH) into circulation.

ADH is the opposite of a diuretic. It tells your kidneys to retain more fluids. ADH is why you pee less when you’re dehydrated.

Beyond thirst and ADH, there are other fluid balancing forces at play. For instance, drops in blood pressure (which may accompany dehydration) activate the renin-angiotensin-aldosterone system—a trio of hormones that, put simply, increase water and sodium absorption through the kidneys.

So yes, your body is adept at preventing dehydration. But it’s wise to know the signs and symptoms so you can see it coming and take corrective action if needed.

Signs of Mild, Moderate, or Severe Dehydration

Dehydration is typically diagnosed through signs and symptoms. Many of these symptoms mimic the symptoms of other conditions (like hyponatremia), so it’s important to be cautious with aggressive fluid replacement.

Thirst

Thirst or dry mouth is an early sign of mild or moderate dehydration. It’s your brain telling you the volume of fluids in your blood has dipped too low.

Drinking to thirst is an effective way to prevent dehydration. Drinking beyond thirst, however, can lead to electrolyte imbalances.

Urine color and output

The shade of your urine is a useful indicator of hydration status. The darker your urine, the more dehydrated you probably are. Conversely, clear urine may mean you’re over-hydrated.

Also, urinary volume decreases when you’re dehydrated. You can thank ADH for helping you retain these fluids.

Dry skin and lips

Water keeps skin moist, so dry skin or cracked lips can indicate mild, moderate, or severe dehydration. But since dry skin has dozens of potential causes—zinc deficiency, vitamin A deficiency, a cold or dry climate, or reactions to chemicals like chlorine—don’t assess hydration status by skin moisture alone.

Fatigue

Some research suggests dehydration causes exercise-related and everyday fatigue. But the evidence goes both ways. For instance, a 2019 systematic review found athletes drinking to thirst lost significant body water but performed comparably to athletes replacing fluids more aggressively.

Headache

Even mild dehydration can cause headaches. Headache is also a hallmark of hyponatremia, so it’s crucial to replace sodium along with fluids. Otherwise, sodium-free water will make the headache worse and may lead to further complications.

Muscle cramps

Both dehydration and hyponatremia can cause muscle cramps. Many athletes confuse one for the other, and then wonder why they keep getting charley horses. Too much water!

Constipation

Constipation is clinically defined as having two or fewer bowel movements per week. It’s associated with hardened feces and decreased quality of life.

Being dehydrated means less water is available to loosen stools. This shouldn’t be an issue if you’re drinking to thirst, but in certain cases—high altitude, diuretic drugs, etc—you may need to replace fluids and electrolytes more proactively.

Lower blood pressure

Water comprises most of your blood volume. When you lose too much water, your blood volume decreases, which in turn decreases blood pressure. Mild dehydration probably won’t cause a drop in blood pressure, but severe dehydration might.

Nausea, dizziness, and fainting

Closely linked to blood pressure drops are nausea, dizziness, and fainting. In a severely dehydrated state, less blood reaches the brain. This often presents as lightheadedness when transitioning from sitting to standing.

Rapid heartbeat

To compensate for lower blood volume, the heart may begin beating rapidly. It must work harder than usual to get blood to the tissues. Rapid heartbeat (tachycardia) is a sign of severe dehydration.

Mood and cognition

In a study from the British Journal of Nutrition, dehydrated men were more tense, anxious, and tired during physical and cognitive tests. They also showed mild memory impairments during the cognitive tests.

Another study found that mild hypohydration (about 1% net water loss) resulted in more errors during a monotonous driving simulation. The magnitude of these errors was reportedly similar to a mildly intoxicated driver with a 0.08% BAC.

Why Drinking Too Much Water Is Dangerous

For healthy people, preventing dehydration is simple. Just drink to thirst.

But that’s not the message we’re raised with. We’re told to drink 8 glasses of water per day, thirst be damned.

There was never any science behind this canard, but it stuck anyway. As a result, people believe that drinking more water is always a healthy choice.

But drinking too much water can lead to a dangerous electrolyte disturbance called hyponatremia. Many athletes have perished from hyponatremia. How many sports-related dehydration deaths have been reported? None.

It’s tricky, because the symptoms of hyponatremia mimic the symptoms of dehydration. Because of this, you see a lot of tragic misdiagnoses.

What To Do If You Think You’re Dehydrated

If you treat dehydration by swilling sodium-free water, you’re asking for trouble. You’re asking for hyponatremia. That’s why I recommend a strategy that addresses both dehydration and hyponatremia concerns.

The strategy is simple: Drink electrolyte water to thirst.

Certain scenarios—excessive sweating, cold climates, or elevation—may require more diligent fluid replacement, but as a rule, drinking electrolyte water to thirst will solve 95% of hydration problems.

If you think you’re dehydrated, put electrolytes in your water and sip the mixture until thirst subsides. And if you want to sip beyond thirst, that’s okay too, since you’re including sodium.

Follow this basic guideline and you generally won’t have to worry about dehydration. It’s nice to have one less thing to worry about, isn’t it?