Dehydration causes and symptoms: How to rehydrate the right way
From the desk of Robb Wolf
<p><span style="font-weight: 400">The popular press </span><a href="https://www.ncbi.nlm.nih.gov/books/NBK555956/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">reports</span></a><span style="font-weight: 400"> that 75% of Americans are chronically dehydrated. If we would just drink more water, we’re told, we’d all feel much better.</span></p><p><span style="font-weight: 400">There’s no science behind these claims, but that doesn’t stop millions of people from believing them. If you hear a splashy stat, it’s going to stick in your memory. And most people don’t have time to do their own research.</span></p><p><span style="font-weight: 400">The media counts on that. And their goal is to grab your attention, not educate you. Telling you the truth—that </span><a href="https://www.ncbi.nlm.nih.gov/books/NBK555956/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">dehydration is rare</span></a><span style="font-weight: 400"> among healthy people—wouldn’t scare many people into clicking the headline.</span></p><p><span style="font-weight: 400">But there’s a compelling story at the base of this confusion. To prevent the phantom of dehydration, many folks (especially athletes) </span><i><span style="font-weight: 400">over</span></i><span style="font-weight: 400">-hydrate. And this leads to a far more dangerous problem.</span></p><p><span style="font-weight: 400">The problem is low serum sodium, or hyponatremia, and most people haven’t heard of it. Exercise-associated hyponatremia </span><a href="https://www.mdalert.com/article/brief-review-of-the-literature-on-hyponatremia-death-and-injury-in-endurance-athletes" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">affects</span></a><span style="font-weight: 400"> about 15% of elite endurance athletes, causing symptoms that range from headache to muscle cramps to brain damage.</span></p><p><span style="font-weight: 400">Since these symptoms mimic the symptoms of dehydration, it’s hard to tell them apart. And the consequence of this confusion is a story worth sharing.</span></p><p><span style="font-weight: 400">Today I’ll give you the full scoop on dehydration: the symptoms, the causes, and how to rehydrate properly to prevent exercise-associated hyponatremia. I think you’ll learn something useful.</span></p><h2><strong>Dehydration 101</strong></h2><p><span style="font-weight: 400">Dehydration means net water loss from the body. If you’re losing more water than you’re taking in, you’re </span><a href="https://www.ncbi.nlm.nih.gov/books/NBK555956/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">considered dehydrated</span></a><span style="font-weight: 400">.</span></p><p><span style="font-weight: 400">Dehydration is often conflated with hypohydration, the low body water state that results from net water loss. Most people, when they talk about dehydration, are talking about hypohydration. Because of this, I’ll be using the terms interchangeably.</span></p><p><span style="font-weight: 400">There are three main types of dehydration:</span></p><ol><li style="font-weight: 400"><b>Isotonic dehydration:</b><span style="font-weight: 400"> When sodium and water loss occur roughly equally. This is the most common form of dehydration—think sweat.</span></li><li style="font-weight: 400"><b>Hypertonic dehydration:</b><span style="font-weight: 400"> When water losses exceed sodium losses. (Respiratory water loss, for example).</span></li><li style="font-weight: 400"><b>Hypotonic dehydration:</b><span style="font-weight: 400"> When sodium loss exceeds water loss, mostly due to diuretics or regimens (like the keto diet or fasting) with a diuretic effect.</span></li></ol><p><span style="font-weight: 400">Contrary to popular belief, healthy people are rarely dehydrated. Older people are slightly more likely (20-30%) to suffer low body water due to problems regulating thirst, diuretic drugs, and mobility issues—But widespread dehydration isn’t really a thing.</span></p><p><span style="font-weight: 400">This wisdom hasn’t yet penetrated our medical system. Let me share a quick story to illustrate.</span></p><p><span style="font-weight: 400">A few years ago I met with the dietetics staff of a medium-sized hospital. Super well-intentioned people, but too focused on chronic dehydration. They thought that drinking more water was the crucial intervention for their obese and diabetic patients. When I asked them how that approach was going in terms of patient outcomes, things got a little… tense.</span></p><p><span style="font-weight: 400">I’m not saying that dehydration is never a problem, but I am saying it’s a smaller problem than is commonly believed. The logical next question is: how do you know you’re dehydrated?</span></p><h2><strong>Dehydration Symptoms</strong></h2><p><span style="font-weight: 400">One way to check for hypohydration is to draw blood and measure its osmolality. If the concentration of electrolytes is high, you might be dehydrated. But not only is this test imperfect; it’s also highly impractical.</span></p><p><a href="https://www.ncbi.nlm.nih.gov/books/NBK555956/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">Clinicians</span></a><span style="font-weight: 400"> also look at weight loss. Losing 3% of your body weight over 7 days might indicate dehydration, but it could also indicate water loss from glycogen depletion (if transitioning to keto or fasting) or fat loss. Not a great test either.</span></p><p><span style="font-weight: 400">That leaves you with symptoms. Individually, each symptom of dehydration (like headache) has dozens of potential causes. But if enough symptoms come together, it strengthens the case that you’re dehydrated.</span></p><p><b>Dehydration Symptoms</b></p><ul><li style="font-weight: 400"><span style="font-weight: 400">Thirst</span></li><li style="font-weight: 400"><span style="font-weight: 400">Dry skin and lips</span></li><li style="font-weight: 400"><span style="font-weight: 400">Dark urine</span></li><li style="font-weight: 400"><span style="font-weight: 400">Decreased urinary volume</span></li><li style="font-weight: 400"><span style="font-weight: 400">Headache</span></li><li style="font-weight: 400"><span style="font-weight: 400">Mood and memory disruptions</span></li><li style="font-weight: 400"><span style="font-weight: 400">Muscle cramps</span></li><li style="font-weight: 400"><span style="font-weight: 400">Fatigue</span></li><li style="font-weight: 400"><span style="font-weight: 400">Constipation</span></li><li style="font-weight: 400"><span style="font-weight: 400">Lower blood pressure</span></li><li style="font-weight: 400"><span style="font-weight: 400">Nausea</span></li><li style="font-weight: 400"><span style="font-weight: 400">Dizziness or fainting</span></li><li style="font-weight: 400"><span style="font-weight: 400">Rapid heartbeat (tachycardia)</span></li></ul><p><span style="font-weight: 400">The last few symptoms—low blood pressure, dizziness, fainting, tachycardia—are associated with severe hypohydration, sometimes </span><a href="https://pubmed.ncbi.nlm.nih.gov/22150427/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">defined</span></a><span style="font-weight: 400"> as over 5% body water loss.</span></p><p><span style="font-weight: 400">As I mentioned earlier, some dehydration symptoms—headache, fatigue, and muscle cramps—mimic the symptoms of hyponatremia. Mixing the two up, however, can be hazardous and even </span><a href="https://pubmed.ncbi.nlm.nih.gov/10091501/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">fatal</span></a><span style="font-weight: 400">. If you want to exacerbate hyponatremia, just give the patient sodium-free water.</span></p><p><span style="font-weight: 400">Anyone can mix up these symptoms though. So at the end of the article, I’ll provide a simple hydration strategy to address both dehydration and hyponatremia. No osmolality measurements needed.</span></p><h2><strong>What Causes Dehydration?</strong></h2><p><span style="font-weight: 400">There are four main causes of dehydration:</span></p><ol><li style="font-weight: 400"><span style="font-weight: 400">Losing water through skin</span></li><li style="font-weight: 400"><span style="font-weight: 400">Losing water through the GI tract</span></li><li style="font-weight: 400"><span style="font-weight: 400">Losing water through urine</span></li><li style="font-weight: 400"><span style="font-weight: 400">Inadequate water intake</span></li></ol><p><span style="font-weight: 400">Let’s briefly review these.</span></p><h3><strong>#1: Water loss through skin</strong></h3><p><span style="font-weight: 400">When you sweat, you lose both water and sodium. This leads to isotonic dehydration.</span></p><p><span style="font-weight: 400">Both warm and cold climates increase sweat loss, and therefore increase dehydration risk. It’s obvious why you sweat more in the heat. Sweat is how your body keeps from overheating. Without out, humans wouldn’t have survived long as a species.</span></p><p><span style="font-weight: 400">Cold climate sweating is less intuitive, but think about it: once you start moving, you get hot and sweaty under those layers. Not only that, our thirst mechanism isn’t as accurate when we’re cold. Read my blog on </span><a href="https://drinklmnt.com/blogs/health/why-winter-hydration-is-important" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">winter hydration</span></a><span style="font-weight: 400"> to go deep on this topic.</span></p><p><span style="font-weight: 400">Other factors affecting sweat loss include:</span></p><ul><li style="font-weight: 400"><span style="font-weight: 400">The intensity of exercise</span></li><li style="font-weight: 400"><span style="font-weight: 400">If there’s a breeze</span></li><li style="font-weight: 400"><span style="font-weight: 400">Humidity</span></li><li style="font-weight: 400"><span style="font-weight: 400">Various medical conditions—including hypoglycemia (low blood sugar), hyperthyroidism (overactive thyroid), and acromegaly (excess growth hormone)</span></li></ul><p><span style="font-weight: 400">The more you sweat, the more likely you are to become dehydrated.</span></p><h3><strong>#2: Water loss through the gut</strong></h3><p><span style="font-weight: 400">You lose water out both ends of the GI tract. If you have an infection that causes vomiting or diarrhea, it’s crucial to continually replace both fluids and electrolytes. Also, certain drugs—along with excess magnesium—can have a laxative effect. These fluids will need to be replaced too.</span></p><h3><strong>#3: Water loss through urine</strong></h3><p><span style="font-weight: 400">Your kidneys are the key organs for </span><a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5760509/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">regulating bodily fluids</span></a><span style="font-weight: 400">. If body water gets too low, your kidneys absorb more water through specialized cells called nephrons. If body water gets too high, the kidneys increase urinary volume.</span></p><p><span style="font-weight: 400">In healthy people, this system works fantastically. (Not so much in those with kidney disease). And it’s not just the kidneys, but the brain, a bunch of hormones, and the thirst mechanism that keeps things running smoothly.</span></p><p><span style="font-weight: 400">Anything with a diuretic effect, however, may throw this system out of balance. This includes not only diuretic drugs, but diuretic eating patterns like the keto diet and intermittent fasting.</span></p><h3><strong>#4: Inadequate water intake</strong></h3><p><span style="font-weight: 400">Thirst is an excellent guide to hydration needs. Below the conscious threshold, osmoreceptors in your brain are always monitoring your body water. When water gets too low, the osmoreceptors ping your hypothalamus to activate thirst. Now you’re consciously thirsty.</span></p><p><span style="font-weight: 400">Drinking to thirst is usually sufficient to prevent dehydration, but here are a few situations where it might not be:</span></p><ul><li style="font-weight: 400"><a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4212586/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">Activity at altitude</span></a></li><li style="font-weight: 400"><span style="font-weight: 400">Exercise that causes excessive sweating</span></li><li style="font-weight: 400"><span style="font-weight: 400">Impaired thirst mechanism due to age-related disease</span></li></ul><p><span style="font-weight: 400">Older folks, </span><a href="https://www.ncbi.nlm.nih.gov/books/NBK555956/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">in fact</span></a><span style="font-weight: 400">, run a higher risk of dehydration than everyone else. This is partly due to less effective thirst mechanisms, and partly due to immobility. If you can’t get up for a drink of water, you’re more likely to become dehydrated.</span></p><h2><strong>How Not To Rehydrate</strong></h2><p><span style="font-weight: 400">Dogma holds that you should drink eight glasses of water per day to prevent dehydration. It’s the poster you see hanging up on the middle school guidance counselor’s wall.</span></p><p><span style="font-weight: 400">But you won’t find any scientific references on that poster. There simply is no evidence supporting 8-glasses-per-day.</span></p><p><span style="font-weight: 400">In fact, the </span><a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4212586/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">evidence</span></a><span style="font-weight: 400"> goes the other way. The evidence says that drinking too much water is the real problem for folks, especially </span><a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5334560/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">elite endurance athletes</span></a><span style="font-weight: 400">. Why? Because drinking too much water dilutes blood sodium levels. It causes hyponatremia.</span></p><p><span style="font-weight: 400">I talked about this earlier. Exercise-associated hyponatremia is serious business. Many athletes have died from low sodium levels, but none—</span><a href="https://www.mdalert.com/article/brief-review-of-the-literature-on-hyponatremia-death-and-injury-in-endurance-athletes" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">according to MDAlert.com</span></a><span style="font-weight: 400">—have died from dehydration.</span></p><p><span style="font-weight: 400">Even so, organizations like the American College of Sports Medicine (ACSM) continue to </span><a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4212586/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">recommend</span></a><span style="font-weight: 400"> aggressive fluid replacement tactics. They’re a big reason why you see water stations every couple miles of a marathon course. And they’re a big reason why so many distance runners end the race in a lethargic, discombobulated state. (Cognitive symptoms are a hallmark of hyponatremia).</span></p><p><span style="font-weight: 400">The average person won’t develop severe hyponatremia, but I see plenty of subclinical low sodium that results from:</span></p><ol><li style="font-weight: 400"><span style="font-weight: 400">Drinking too much water</span></li><li style="font-weight: 400"><span style="font-weight: 400">Not consuming enough sodium</span></li></ol><p><span style="font-weight: 400">We can fix both those problems with a little knowledge and preparation.</span></p><h2><strong>Hydrating With Electrolytes</strong></h2><p><span style="font-weight: 400">If you care about your health, you need to hydrate the right way. That means including electrolytes in your hydration strategy.</span></p><p><span style="font-weight: 400">Sodium (from salt) is the crucial electrolyte to manage. I just talked about what happens when sodium levels drop too low, but f</span><span style="font-weight: 400">olks also tend to be deficient in potassium and magnesium. Both these minerals have dozens of crucial functions, so I recommend optimizing those as well.</span></p><p><span style="font-weight: 400">Here are some science-backed daily electrolyte intakes (diet and supplements combined):</span></p><ul><li style="font-weight: 400"><span style="font-weight: 400">4–6 grams sodium</span></li><li style="font-weight: 400"><span style="font-weight: 400">3.5–5 grams potassium</span></li><li style="font-weight: 400"><span style="font-weight: 400">400–600 mg magnesium</span></li></ul><p><span style="font-weight: 400">Active, sweaty, and low-carb folks should probably bump up the sodium by a gram or two. A sweaty athlete can lose up to </span><a href="https://pubmed.ncbi.nlm.nih.gov/22150427/" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">7 grams of sodium</span></a><span style="font-weight: 400"> while hustling in the heat.</span></p><p><span style="font-weight: 400">To meet your electrolyte needs, consume </span><a href="https://science.drinklmnt.com/electrolytes/electrolyte-rich-foods/" target="_blank" rel="noopener" data-wplink-edit="true"><span style="font-weight: 400">electrolyte-rich foods</span></a><span style="font-weight: 400">, be generous with the salt shaker, and put electrolytes in your water.</span></p><p><span style="font-weight: 400">Put electrolytes in your water? Yes. Putting electrolytes in your water—and drinking that electrolyte water to thirst—simultaneously prevents dehydration and hyponatremia. </span><span style="font-weight: 400">We recommend mixing your own </span><a href="https://science.drinklmnt.com/electrolytes/best-homemade-electrolyte-drink-for-dehydration/" target="_blank" rel="noopener"><span style="font-weight: 400">electrolyte homebrews</span></a><span style="font-weight: 400"> or using </span><a href="https://drinklmnt.com/products/lmnt-recharge-electrolyte-drink" target="_blank" rel="noopener noreferrer"><span style="font-weight: 400">LMNT</span></a><span style="font-weight: 400">.</span></p>