Can you drink too much water? How to prevent overhydration

Many sources advise drinking beyond thirst, but that advice can do more harm than good. Drinking too much plain water can dilute blood sodium levels, which can cause headaches, fatigue, confusion, and worse. This is common in endurance athletes, but anyone can over-hydrate if they’re ignoring their body’s signals and drinking more than they need. It’s especially something to be aware of in the hot summer months or similar extreme conditions.

I have an aversion to alarmist content, but I think it’s important to talk about this stuff. The truth is, overhydration has generally proven to be more dangerous than dehydration. And it can be hard to discern between the two because the symptoms can be quite similar, which makes it extra important to hydrate smartly.

If you take nothing else away from this article — or simply have someplace else to be — the main takeaway to hydrate safely is to drink electrolyte water to thirst. This will simultaneously prevent overhydration and dehydration.

If you’re sticking around for a while, we’re going to debunk bad hydration advice, share the symptoms of overhydration to look out for, and then explore how to prevent and treat it. Ready to get to it? Let’s dive in.

Bad Hydration Advice

The problem of overhydration is rooted in decades-old health recommendations. In the 1940s, the US National Academy of Sciences started telling people to drink 84 ounces of water per day. About twenty years later, a physician named Irwin Stillman recommended a weight loss diet that included eight glasses of water daily.

Neither recommendation had any science behind it, but the “more water is healthier” meme stuck like a bad political slogan. Consequently, many people believe healthy hydration is synonymous with preventing dehydration at all costs.

Sites and resources touting the inaccurate hydration advice are still rampant today. In a 2016 paper, researchers used common Google searches — “hydration guidelines,” “drinking fluids,” etc. — to uncover 141 hydration-related websites. Over one-quarter of the sites were related to scientific organizations, peer-reviewed literature, or medical professionals, but nonetheless they dished out impractical advice. Here are the two most egregious bits.

Bad hydration advice #1: Drink beyond thirst to stay hydrated

What the science actually says: Drink to thirst to stay hydrated. Our thirst mechanism has been finely tuned over millions of years. When we need water, we get thirsty. When we’re hydrated, we’re not thirsty. For most folks, it really is as simple as that.

The science: Thirst is controlled by a region of the brain called the hypothalamus. This brain region is sensitive to receptors that are perpetually monitoring your blood volume, triggering thirst if blood volume gets too low from, say, heavy sweating.

The problem with drinking plain water beyond thirst is that excess plain water dilutes blood sodium levels. As sodium levels fall, confusion, brain fog, weakness, and other neurological symptoms can occur.

Exercising is one of the most common scenarios where folks might overhydrate. It can be a tricky situation because exercise increases levels of antidiuretic hormone, making it harder to excrete excess fluids through urine while being active. Too much water coming in and not enough going out — that’s a recipe for overhydration.

The standard advice when it comes to hydrating during exercise is to drink water to replace weight loss (only 7.3% of relevant websites advised drinking to thirst to hydrate for exercise). Weight loss is supposed to be an accurate proxy for water loss (dehydration), but unfortunately, it’s not. One study found that runners lost 5.5 pounds during a 56 km race, but total body water only dropped about 3 pounds. This suggests your body loses weight from non-water sources (like body fat) during exercise. It also suggests that drinking to replace weight loss replaces too much water.

In a perfect world, you’d replace sweat losses 1:1 with a mixture of water and salt, the main constituents of sweat. That way, sodium levels would stay stable during fluid replacement. You can calculate your fluid and sodium losses through sweat if you want to be precise (read our guides to sweat rate and sweat sodium concentration), but listening to the body and drinking salty fluids to thirst works just as well for many in our community. Adding about 1 gram of sodium per liter (32 ounces) of water is a reasonable estimate to replace the sodium lost through sweat.

To recap, whether you’re exercising or just looking to hydrate smartly: Listen to your body. Drinking to thirst (and including electrolytes in your water) prevents the dangers of overhydration and dehydration simultaneously.

Bad hydration advice #2: Drink extra water during exercise to prevent heat illness

What the science actually says: Dehydration can contribute to heat illness, but drinking excess water won’t prevent it. Heat illness during exercise can occur with or without dehydration.

The science: Only 3.4% of hydration websites correctly identified that dehydration is “not generally a cause of heat illness during exercise.” Although dehydration does reduce sweat rate and therefore your ability to cool yourself, non-dehydrated athletes often develop heat illness nonetheless. Proper hydration doesn’t protect you from heat illness if it’s hot enough and the exercise is intense enough.

For example, one study found that dehydrated athletes stopped exercising at lower core body temperatures than hydrated controls during hot, intense exercise sessions. In other words, dehydrated athletes stayed further away from heat exhaustion and heatstroke, which are defined by elevated body temperature.

This finding doesn’t mean you should seek out dehydration during physical activity. Again, dehydration can impair your ability to stay cool. But it does suggest dehydration isn’t the primary cause of heat illness during exercise. More likely causes include the environment (temperature, humidity, breeze), exercise duration, and exercise intensity.

Beyond exercise, most cases of heat illness occur in babies or seniors. Why? Because these populations have less functional sweat mechanisms, regardless of hydration status.

A final point? The focus on avoiding dehydration to prevent heat illness often leads to overhydration, which can mimic dehydration symptoms. This confusion occasionally has tragic consequences.

Again, the key here is to drink to thirst, and adding in electrolytes can help replace sodium sweat losses plus prevent overhydration. And the most effective heat illness preventative is staying out of the heat.

Now that we’ve debunked some common hydration myths let’s cover the symptoms of overhydration so you know what to watch for.

Overhydration Symptoms

When I talk about the symptoms of overhydration, I’m talking about the symptoms of low blood sodium levels, medically known as hyponatremia. These symptoms fall into two buckets:

1. Mild to moderate hyponatremia symptoms
2. Severe hyponatremia symptoms

Mild cases may resolve without medical treatment, but many cases require the help of health professionals. Seek medical advice if you’re not sure.

#1: Mild to moderate overhydration symptoms

The early symptoms of hyponatremia include dizziness, fatigue, muscle cramps, mild nausea, and bloating. As the condition worsens, the hyponatremic person may also experience:

• Headache
• Confusion
• Irritability
• Light sensitivity
• Vomiting
• Drowsiness
• Difficulty breathing
• Altered cognition

These symptoms are common in distance athletes who over-hydrate with plain water, but various medical conditions (like heart or kidney issues), illnesses that cause vomiting or diarrhea, and drugs (like diuretics or NSAIDs) are also risk factors.

#2: Severe overhydration symptoms

Severe overhydration symptoms include seizures, coma, abnormal posturing (a sign of brain damage), and dilated pupils.

In these cases, seek immediate medical attention. Urgent treatment with IV hypertonic saline fluids (saltwater) can help prevent brain damage or death.

Preventing & Treating Overhydration

So there you have it: More water is NOT always better. To recap, here are the practical takeaways for preventing overhydration:

• Drink to thirst. Drinking to thirst prevents both overhydration and dehydration. Your body knows when to stop consuming fluids. Tune in to it.
• Don’t drink to replace weight loss. Not all weight lost during exercise is water weight. You’ll likely over-hydrate if you replace every pound you lose with plain water.
• Don’t expect water to keep you cool. Drinking extra water doesn’t prevent heat illness. Staying out of the heat does.
• Put salt in your water. Sip salty fluids to thirst throughout the day to prevent dehydration and low sodium. Athletes lose about 1 gram of sodium per hour of sweaty activities. That’s one of the reasons there’s 1 gram of sodium in each stick pack of LMNT Drink Mix and can of LMNT Sparkling.

In terms of treatment, it’s always best to consult a medical professional. There are three main treatments:

1. Restricting sodium-free fluids. This “stops the bleeding,” metaphorically speaking. When you cut off plain fluids, your body can excrete excess water to normalize sodium levels.
2. Administering intravenous (IV) saline. This is the default medical intervention. Medical professionals typically administer an IV solution containing 3% NaCl (salt), or about 1.2 grams of sodium per 100 mL of fluid.
3. Drinking salty water. There’s some controversy over oral saline (saline = salt water) vs. IV saline for treating hyponatremia, but research suggests it’s a viable alternative in some instances. In one trial, oral saline was as effective as IV saline for reversing hyponatremia in runners after a 161-km race. It’s best to let a medical professional dictate hyponatremia treatment, though.

All right then, we’ve just replaced inaccurate hydration advice with science-backed knowledge. If you forget some of the hairy details, that’s okay. Just remember the fundamental rule of hydration: drink salty fluids to thirst.