How Dehydration Affects Your Body and When to Get Help

Emily Boynton Fact Checked
Woman drinking water
© Kike Arnaiz / Stocksy United

You’ve heard the stat: on average, humans are made up of about 55-60% water.  

Water helps with your digestion, regulates your body temperature, and aids joint movement and organ function. Despite its crucial role, many people don’t drink enough water and end up dehydrated — especially in the summer months.  

“Dehydration is when you are having more fluid output than input,” says Dr. Marie Vrablik, an emergency medicine physician at Harborview Medical Center. “When you have a negative balance of fluid, your body has to shift priorities and decide which functions it’s going to forgo.” 

Put simply, dehydration occurs when you don’t have enough water for your body to function properly. 

Mild dehydration: feeling thirsty

Some causes of dehydration include not drinking enough water and losing water quickly due to sweating, vomiting or diarrhea.

Mild dehydration sets in when you’ve lost 1-2% of your body’s water. Symptoms include dry mouth and feeling thirsty and tired, Vrablik says. Some people may also get a headache or feel dizzy. 

“Your body is telling you to drink something,” she says. “It’s a basic drive. We can go weeks or days without food, but we can’t go long without water, especially in the heat.” 

You can treat mild dehydration on your own by drinking water or a sports drink with electrolytes. Just be sure to avoid alcohol and caffeinated drinks like tea, soda and coffee as these can make dehydration worse.  

Moderate dehydration: problems with kidney function 

If you continue to lose (and not regain) water, you enter moderate dehydration. This can set in after a couple of hours, especially if you are losing fluid due to sweating, Vrablik says. 

“Once you progress from mild to moderate, that’s when you start to see effects to your kidney function,” she notes. 

Your kidneys help filter metabolic waste from your urine and blood. When you’re dehydrated, your kidneys try to hold on to as much water as possible. As a result, less water is used to flush out waste, making your urine more concentrated (which is why your pee is darker when you’re dehydrated). 

Symptoms of moderate dehydration include decreased urination and darker color urine. You may have increased body temperature and a faster heartbeat. And, while it seems paradoxical to expel liquid, the buildup of metabolic byproducts can also make you feel nauseated and can lead to vomiting, Vrablik says.  

Similar to treating mild dehydration, you treat moderate dehydration by getting fluids back into your body. Take a break from what you’re doing to let yourself rest and cool down. Vrablik also cautions to drink slowly, as drinking too much too fast can shock your system and cause you to vomit.

Severe dehydration: risk of kidney failure 

The more dehydrated you are, the more your body begins to shut down. It takes a period of more than 24 hours to enter severe dehydration, though Vrablik notes it takes less time if you are losing fluid because you are sweating. 

“As your kidneys shut down the rest of your body starts to suffer: your brain, liver and, eventually, your heart,” Vrablik says.  

She explains that in severe dehydration, your kidneys don't have enough water to filter your blood adequately, which can lead to kidney insufficiency or kidney failure. In turn, you may become uremic, aka have toxins build up in your blood. In extreme cases, this can lead to your heart stopping (cardiac arrest). 

Symptoms of severe dehydration include dark urination or not urinating, not sweating, light headedness and fainting, as well as experiencing confusion, irritability or anxiety, and becoming unresponsive.  

“If you’re able to drink fluids and have access to fluids, then hydrate. If you’re dehydrated and you’re nauseated or vomiting, or you’re not able to drink, that’s when you need medical care,” Vrablik says.  

Severe dehydration is a medical emergency. If you are experiencing signs of severe dehydration, call 911 or go to the ER to get IV (intravenous) fluids.  

How much water do you need to stay hydrated? 

When it comes to dehydration, prevention is key. And this means drinking adequate amounts of water. 

“There’s an old adage of eight cups a day, but it’s not true. That’s too much for some and too little for others. The right amount depends on your activity level, body size, and whether you’re pregnant or nursing,” Vrablik says. 

Instead, aim to drink water throughout your day, and listen to your body: If you’re thirsty, you need water.  

It also helps to plan for exercise or extended time in the heat by drinking water before your activity starts and bringing plenty with you as well. If you’re caring for children or older adults, you want to be mindful of their water intake as well.  

“Kids will have a hard time expressing their thirst, and some older adults’ sense become blunted, so they may not realize they’re thirsty,” Vrablik says.  

In short? You can prevent dehydration by drinking water when you’re thirsty, bringing fluids with you to activities and planning for those who may need some extra support.