Is dehydration raising your blood pressure? Here’s the science

From the desk of
Robb Wolf
ScienceIs dehydration raising your blood pressure? Here’s the science

The more we learn about the human body, the more links we discover between hydration and heart health. Today you’ll learn about one of those links: the link between dehydration and blood pressure.

When people are dehydrated, they’re low on body water. Low body water means low blood volume, which generally leads to low blood pressure. I’ll elaborate later, but that’s the Twitter-friendly version of why low blood pressure is a common dehydration symptom.

But can dehydration cause high blood pressure? The story isn’t entirely clear, but the correlational data is there. That’s right. Through different mechanisms, dehydration may cause both low AND high blood pressure.

It’s not intuitive, but reality isn’t calibrated to align with our intuitions. If something seems intuitive, we should pause and revisit the basic assumptions upholding that intuition. Despite the vigorous head bobbing that follows any mention of Occam’s razor, the simplest explanation isn’t always the correct one.

But don’t worry, I won’t overcomplicate this topic. I’ll simplify the nuance as best I can, and occasionally add a dash of humor to help keep you with me.

Stick around for 5 minutes to learn about dehydration and low blood pressure, dehydration and high blood pressure, other causes of high blood pressure, and how to stay properly hydrated. First things first: let’s cover some basics.

What Is Dehydration?

Dehydration is a state of net water loss from the body. This water loss, in turn, results in a condition of low body water called hypohydration. (Note: dehydration and hypohydration are technically distinct, but most people use them interchangeably.)

The symptoms of dehydration include thirst, dark urine, low urine volume, dry skin, fatigue, muscle cramps, dizziness, nausea, constipation, headache, and irritability. Some of these symptoms are linked to low blood pressure, while some reflect other fluid balance disruptions.

Anything that causes water losses can cause dehydration, including:

  • Excessive sweating from exercise, hot climates, etc.
  • Respiratory water losses from COPD or asthma
  • The use of diuretics, laxatives, and other medications
  • Water losses from kidney disease, hyperglycemia, or other medical conditions
  • GI water losses from diarrhea or vomiting

Inadequate fluid intake can also cause dehydration, but healthy adults rarely make this mistake. Not drinking enough water is primarily a problem for older folks with mobility issues and unreliable thirst mechanisms. If it’s hard to tell you’re thirsty and even harder to get to your water, you’re simply less inclined to drink.

This is a crucial point. Most people drink enough water, and exceeding our natural water needs can be dangerous. I’ll return to this topic later.

It’s a cliche to say that we’re mostly made of water, but it’s true. (We’re about 60% water weight.) All that H2O resides inside cells, outside cells, and in-between cells as interstitial fluid.

Today, our focus will be on the extracellular fluid—the water that adds volume (in plasma) to your blood. And blood volume influences your blood pressure.

Blood Pressure 101

First, a definition: Blood pressure (bp) refers to the pressure of your blood against the walls of your blood vessels. Pretty straightforward, right?

Many factors influence your blood pressure: the pumping of your heart, your electrolyte status, your nervous system health, various hormones, and (as we discussed) your blood volume. Lording over this system is an ancient brain structure called the hypothalamus.

We measure blood pressure with two numbers:

  1. Systolic blood pressure: your bp as your heart beats (top number)
  2. Diastolic blood pressure: your bp between heartbeats (bottom number)

Combined—as in, 110/70—these two figures represent your blood pressure status. Here’s how the American Heart Association classifies different blood pressures:

  • Normal blood pressure: systolic under 120 AND diastolic under 80
  • Elevated blood pressure: systolic 120-129 AND diastolic under 80
  • High blood pressure: systolic 130 or greater OR diastolic 80 or greater

What about low blood pressure? There’s no consensus, but it generally refers to pressures below 90/60.

Dehydration and Low Blood Pressure

One sign of moderate-to-severe dehydration is low blood pressure. When blood volume drops due to water losses, blood pressure drops along with it.

It’s basic physics. A lower volume of a substance (in this case, blood) will exert less pressure on the vessel (blood vessels) that contains that substance. Think of blowing up a balloon. The less air you blow, the lower the air pressure inside the balloon. Less volume, less pressure.

Having low blood pressure is generally undesirable. Why? Because blood shuttles oxygen and other nutrients to your tissues, and maintaining normal blood pressure helps push those nutrients to the right places.

The symptoms of low blood pressure are dizziness, fatigue, lightheadedness, nausea, and blurry vision. In severe cases, low blood pressure can lead to irregular heartbeat and shock.

On a practical note, hydrating with water PLUS electrolytes is crucial for maintaining a normal blood volume. If you drink too much plain water, you may cause an electrolyte imbalance and exacerbate blood pressure issues. More on that soon.

Can Dehydration Cause High Blood Pressure?

The science is thin, but chronic dehydration is linked to high blood pressure. For instance, observational data suggest that folks with diabetes and hypertension (high bp) also tend to have low urine output, a sign of dehydration.

One possible mechanism involves a hormone called vasopressin. (Vasopressin is also called antidiuretic hormone because everything in physiology must have at least two names to maximize human bewilderment.) Allow me to explain the physiology.

Vasopressin is a fluid-retention hormone. Secreted by the hypothalamus (the brain region that’s always monitoring hydration status), vasopressin acts on the kidneys to suppress diuresis. More vasopressin, less pee.

But vasopressin has other effects, like making your blood vessels constrict. Imagine a balloon shrinking, but without releasing any air. The air pressure increases. Similarly, dehydration-induced vasopressin release may spike blood pressure.

Still, dehydration is one of the last causes I’d blame for high blood pressure. Oftentimes there are more likely culprits at play.

More Likely Causes of High Blood Pressure

High blood pressure continues to bewilder the medical community. About one-third of Americans have this heart disease risk factor, but most cases are labeled “essential hypertension” because we can’t identify a specific cause.

But we do know which factors increase hypertension risk: smoking cigarettes, alcoholism, a sedentary lifestyle, sleep deprivation, genetic factors, a high-sugar diet, obesity, and electrolyte imbalances. Let’s double-click on the last two.

When people are obese, they’re also more likely to suffer from chronic inflammation. Unfortunately, this immune-system confusion can dysregulate hormones that govern blood pressure.

Electrolyte status (especially for sodium and potassium) is also crucial for healthy blood pressure. Consider the following:

  • Excess sodium consumption can elevate blood pressure, but so can sodium deficiency. That’s because sodium-retention, blood-pressure-raising hormones like renin and aldosterone kick in on low sodium diets.
  • Low-sodium diets in line with government recommendations are correlated with higher blood pressures in people with otherwise normal blood pressure.
  • Higher potassium intakes are uncontroversially linked to lower blood pressures. Unfortunately, only 20–40% of Americans achieve the National Academy of Medicine (NAM) adequate intake of 3.4 g/day in men and 2.6 g/day in women. Adequacy is great and all, but it appears that the range of 3.5–5 g/day could produce even more benefits with little to no downside if you have healthy kidneys.

So if you want to improve your blood pressure, you’ll want to get your electrolytes dialed in. Let’s review a practical hydration strategy now.

How To Stay Hydrated

The goal of your hydration strategy should be two-pronged:

  1. Get enough fluids
  2. Get enough electrolytes, especially sodium

Most people focus on the first prong only. Following homespun wisdoms like the 8×8 rule (8 glasses of 8 ounces of water per day), they drink beyond thirst in a misguided effort to prevent dehydration.

Unfortunately, this can disrupt the second prong. Why? Because drinking plain water beyond thirst dilutes blood sodium levels, leading to an electrolyte imbalance called hyponatremia.

As you’ll recall, sodium disturbances drive blood pressure disturbances. Healthy hydration isn’t just a matter of drinking adequate fluids per thirst, but also consuming enough sodium to meet baseline needs (about 5 grams per day) and to replace sweat losses (up to double or triple the baseline).

Drinking electrolyte water is a snappy solution to this problem. When you add sodium to your fluids and drink to thirst, you simultaneously prevent dehydration and electrolyte issues. Simple as that.

Hey, that’s why we created LMNT. It’s a convenient electrolyte drink mix with enough sodium to move the needle for your hydration needs. LMNT tastes so good you won’t believe it’s sugar-free. My whole family (including my daughters) is addicted to the stuff.

The bottom line? When you get your hydration handled, many problems (including a minority of blood pressure problems) may handle themselves. I’ll drink a tall glass of electrolyte water to that.

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