This Is What's Really Going On When You're Dehydrated

Illustrated by Mallory Heyer.
By Sally Tamarkin 

We should all be drinking eight eight-ounce glasses of water per day. Unavoidable fact, or thoroughly debunked fiction? It turns out that the old eight-by-eight rule originated from a misreading of a 1945 recommendation from the Food and Nutrition Board — and over the next five decades, just kind of became accepted wisdom.

But, the 64-ounces-in-a-day directive has since been abandoned and replaced with the Board’s 2004 recommendation, which suggests we figure out how much to drink each day by “letting our thirst be [our] guide.” It turns out that between the water we get (from beverages — yup, even caffeinated ones — and water-rich fruits and vegetables) and our bodies’ super-sophisticated mechanism for regulating water balance, maintaining hydration is fairly simple for most healthy adults. Under certain circumstances (like when we get sick, work out extra hard, or exert ourselves in the heat) our bodies have a harder time keeping up with fluid loss, and we can become dehydrated. But, what exactly does that mean?

Our Bodies, Our Cells
The importance of water to our bodies can’t be overstated. After all, it makes up more than two thirds of our body weight and is responsible for a variety of functions, including digestion, blood flow, and body temperature regulation, as well as overall cell health. Fortunately, the fact that we lose between four and nine cups of water per day through breathing, sweating, peeing, and pooping isn’t a problem for most healthy adults because the systems that regulate our hydration are so sensitive. According to CamelBak Hydration Advisor Doug Casa, PhD, evidence shows that the body will compensate for a loss of just 1-2% of the total amount of water in the body by triggering the sensation of thirst — the cue to drink. These cues stay on track and are properly timed because our brains, kidneys, various glands, and hormones work in concert to monitor the amount of water that we’re taking in versus how much we’re losing.

Related: How Much Water Should I Be Drinking?


It all begins with the hypothalamus, the gland responsible for regulating our body temperature and triggering the processes that balance the fluids in our bodies. When the hypothalamus detects too little water in our blood, it signals the release of an anti-diuretic hormone that causes the kidneys to remove less water from the blood. The result? We pee less, and when we do, our urine is more concentrated and darker in color. At this point, the brain also tells us we’re thirsty, and once we sip on some water or consume something hydrating, our water levels return to normal. Similarly, when our body temperature rises either from fever, working out, or being in a warm environment, our bodies try to lower our temperature by sweating: When sweat evaporates from our skin, it takes some heat with it, helping to cool us off.

But, although we lose water when we sweat, pee, and breathe, our bodies are so darn good at triggering thirst and cueing urination that it’s only when we’re losing more water than we can replace — think sweating like crazy, throwing up, having diarrhea, or peeing excessively, or maybe even spending a long night drinking — that we experience dehydration.

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Illustrated by Mallory Heyer.
Hot, Tired, & Thirsty: How Dehydration Feels
When dehydration occurs, we experience a range of symptoms, from dry mouth, headaches, dizziness, and fatigue to lethargy, cool skin, and the inability to pee (because our kidneys have been told not to excrete scarce fluid). Here’s how it works: Water is integral to regulating blood volume, which in turn affects blood pressure and heart rate. When we’re dehydrated, our blood volume drops, and so does our blood pressure, causing our hearts to beat faster. For some, dehydration causes headaches or triggers migraines. Though the link between water balance and headaches is still being researched, one theory is that as our bodies work to maintain fluid levels during dehydration, our blood vessels narrow, reducing the supply of oxygen and blood to the brain, which causes headaches. So, basically, when our cells are deprived of the water they need to function optimally, all systems are especially taxed and must work harder to power us, causing us to feel fatigued or lethargic.

Dehydration affects us cognitively and psychologically, too. Studies have shown that even mild dehydration can cause dips in concentration, memory, and mood. 

Illustrated by Mallory Heyer.
The Takeaway
The amount of water we need varies from person to person and depends on what we’re eating and drinking, the climate, and our activity levels. Healthy adults can pretty much rely on their body’s thirst mechanism to keep them hydrated. If you’re eating fruits and vegetables and drinking fluids, you’re probably getting the water you need.

However, there are circumstances when we need to pay more attention to how hydrated we are: when we’re exercising (hard, in the heat, for long periods of time, or in some combination of these conditions), or when we’re sick with fever or a stomach bug that causes diarrhea or vomiting.

Our bodies are actually so smart about maintaining water balance that when we sweat excessively and thus lose salt and other electrolytes, we’re cued to crave drinks that both quench thirst and contain sodium. During prolonged workouts, Casa recommends going for beverages that have carbohydrates and electrolytes (like sports drinks or coconut water) which will keep you hydrated even when you’re sweating like a champ. Plus, they’ll help take the edge off post-workout fatigue. 

Related: Want To Lose Weight? Drink More Water
 

In helping to replenish valuable electrolytes, sports drinks prevent us from getting overhydrated. When we drink way more water than we need, our kidneys can’t keep up, and we’re unable to urinate enough to get our water levels back in balance. As a result, the sodium in our blood becomes diluted, and water intoxication occurs, causing symptoms like headache, fatigue, nausea, vomiting, confusion, or even seizures. There is no set amount of water that causes intoxication. Instead, Casa says we can avoid overdoing it by drinking “according to the sensation of thirst.” He also recommends getting a handle on how much fluid we’re losing while working out by determining our sweat rate.

To make sure we’re hydrated, our brains, organs, and hormones do the heavy lifting, keeping our fluids balanced without us having to think much about how they do it. But, by knowing our bodies and how we handle exercise, sickness, and climate changes, we can be sure that we’re not doing anything to sabotage the precise and involuntary processes that keep everything flowing and functioning. And, if you’re planning for workouts or trying to get a ballpark estimate of how much water to drink in a day, you can always use a hydration calculator.

Next: 12 Unexpected Reasons to Drink More Water In 2015
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