The concept of hydration is surprisingly contentious. While nobody thinks dehydration is a good thing, there are remarkably different opinions on how much water you need to drink to stay hydrated, whether or not electrolytes are necessary, and even on what exactly dehydration itself is.
The good news is that I haven’t come across any advice that is so bad as to be harmful, but I do think a lot of advice out there misses the difference between simply staying healthy and optimizing performance. If your only concern is to avoid the overt health consequences of dehydration, a lot of what follows in this article is unnecessary. On the other hand, if you’re curious about how better hydration can improve performance, this article’s for you!
First, Debunking Hydration Myths
Myths find a natural breeding ground on the internet; the combination of indelibility and mass access ensure that myths are both easily spread and difficult to erase. Many of these myths about hydration predate the internet, but have tenaciously held on or grown since. Let’s see if we can improve that.
Myth #1: You Need to Drink 8 Cups of Water a Day
This myth dates back to the mid-20th century and is based on… nothing. It was a guideline put forth by the Food and Nutrition Board of the National Research Council who suggested that the average adult needed 2.5 liters of water a day to survive. Importantly, there was no source for this figure, and the board also stated that most people will get this through normal dietary intake including the foods (such as fruits and vegetables) and liquids (such as coffee, tea, milk, etc.) without needing to specifically drink it all in the form of pure water.
It’s difficult to put an exact figure on the amount of water an average person needs, but it’s clear that no one will be harmed by simply drinking according to thirst, and that that may be well fewer than 8 cups. So if you’re thirsty, have some water (or some other water-based liquid); otherwise, don’t fret about reaching some arbitrary intake.
Myth #2: You Can Gauge Hydration Through Urine Color
You’ve probably heard that yellow urine indicates dehydration, but this is misleading at best. It can in certain scenarios be a general guideline but there are also ways to misinterpret or mistake what the color implies. For this reason, I think it’s generally best to avoid using color to judge hydration.
To understand why, we need to understand what urine color is actually measuring, which is the concentration of solutes in the urine solution (the osmolarity). If you increase the amount of solution (water) without increasing the solutes (molecules excreted by the kidneys into the urine, including minerals, amino acids, vitamins, metabolites, etc.), your urine will become lighter. If you decrease the solution without decreasing solutes, it will become darker.
The main thing that determines the amount of water in your urine is how much water you drink, which seems like it would be a good indicator of hydration but glosses over the fact that our body is remarkably adept at maintaining homeostasis and will begin to excrete all excess water once we’re over a certain level—and therein lies the problem. What exactly is the level where the water you drink ceases to expand cell and blood volume and is simply passed into the bladder? It’s difficult to know for sure since it will depend on cell and blood solute levels (more on this later), but we can be certain that it’s well before your urine turns clear.
Urine color as a marker of hydration is further confounded by diet and physiology. For example, the nutrient riboflavin (vitamin B2) causes your urine to turn a bright yellow (a fluorescent yellow to be specific; riboflavin-loaded urine glows under a black light). If you take a vitamin B supplement, chances are your urine will be yellow for upwards of 12 hours after you consume it as your body gets rid of all that excess riboflavin that it didn’t need. Furthermore, some medications alter the concentration of solutes in urine without changing cell or blood solute levels and thus change what “normal” urine looks like. Finally, you can change the amount of water your export to your bladder by changing the level of solute in your blood and cells; since the body wants to maintain the same overall blood and cell osmolarities, you will naturally retain more water and urinate less, making your urine more concentrated without becoming dehydrated.
In short, there’s no hard and fast rule about what color your urine should be, so don’t judge your hydration by its color; instead, focus on volume and comfort. It should never be uncomfortable to pee, and when you do pee it shouldn’t be just a trickle. Basically, you should be drinking enough to give your body the amount of water it needs to safely excrete the stuff it doesn’t need, and as long as you can do that you’re probably okay.
Myth #3: Caffeine Is Dehydrating
I did write about this in my Caffeine Supplement Guide, but it bears mentioning here as well: caffeine-containing drinks are not dehydrating. Caffeine is at best an extremely minor diuretic, but for most people it will have virtually no effect, especially in the context of exercise. The reason we tend to urinate more after drinking coffee or tea is that we’re consuming liquids and liquids in general cause us to pee.
Myth #4: You Can’t Overhydrate
I don’t know if this is fair to call a myth, but I’m putting it in here anyway. Technically, you can’t really “overhydrate” if we define hydration as maintaining an adequate blood and cell volume and osmolarity—but this would involve being smart about your hydration choices. What you can do is “over-water” yourself wherein you drink so much water that you drive your blood sodium concentration to a dangerous low and induce hyponatremia. This is what most people call “overhydration”, but as mentioned, I don’t really think it’s fair because it’s not really a state of hydration so much as dyshydration, and can be avoided by either A) drinking less water or B) drinking salted water.
What Is Hydration (and Dehydration)?
With the myths about hydration out of the way, let’s discuss what hydration really is: blood and cell water volume. On the surface, this is a very simple concept; when you dehydrate food, you take the water out of the cells until it’s drier and tougher. What makes it complicated is understanding how hydration works on a small scale where you’re not trying to mummify things, but rather are trying to increase water content to either a “normal” level or a supernormal level.
In this way, hydration becomes more than just water—it’s the balance of water to the concentration of solutes in the places it matters (i.e., blood and cells). There are numerous solutes in our blood and cells, but the two most important ones are sodium (for blood) and potassium (for cells); both are electrolytes. If you drink a liter of water but your blood sodium remains the same, you will not become more hydrated, you will simply filter the excess water out into the bladder because your body wants the osmolarity of your blood to remain in its happy place. The same is true for your cells and their osmolarity. Therefore, increasing blood or cell volume (hydration) can only be achieved in one way: by increasing levels of both the solution (water) and the solute (sodium or potassium).
On the other hand, dehydration can be achieved in two ways: by losing water or by losing electrolytes. Loss of either alone will trigger a subsequent loss of the other, and in athletes, both methods happen simultaneously in most cases.
First, we lose a lot of water through our sweat. All of that water originates from our blood, and as the water leaves the blood our total blood volume shrinks, making it harder to transport things effectively. In average conditions, the average athlete loses around half a liter of water an hour from sweating.
Second, we lose sodium through our sweat. Since our body exports heat through sweat (making water loss through sweat a necessity), and since our body prioritizes blood osmolarity, sodium loss is inevitable—it has to go somewhere since it can’t stay in the blood! Different people excrete different amounts of sodium in their sweat, but on average most people lose 50–60 millimoles of sodium per liter of sweat (roughly 1,100–1,400 milligrams).
Thus, dehydration in athletes is really a two-fold problem which requires a two-fold solution. If you only replace the water you lose, you will only transiently increase blood volume before the excess is lost to the bladder; if you only replaced the sodium, you wouldn’t improve blood volume and the excess sodium would be lost through sweat or urine. Only by replacing both can dehydration be effectively combated.
Of course, our body isn’t so poorly designed that once we start down the cycle of dehydration we can only break it by finding a source of sodium—that would have been challenging before the advent of readily available sodium. Instead, we do store a limited amount of sodium on the surfaces of the bones ready to bolster our dropping levels. And in most scenarios, this store is perfectly adequate. Where we run into problems is when an athlete is not yet heat-acclimated, when an athlete’s diet is too low in sodium to replenish those stores, and when we discuss whether these stores bring blood volume up to an optimal level performance-wise or just a healthy level.
This is where electrolyte-replacement comes in. By replacing the lost water and sodium, we’re able to keep blood volumes not only at a healthy level, but possibly at a slightly higher-than-normal level —much like keeping blood glucose slightly higher in order to boost performance. We also prevent diet-related hyponatremia, so we can be less concerned with getting enough sodium through our diet (though given the preponderance of the mineral in our diet today, this is really only of concern for those who completely eschew processed foods).
Where Ideas of Hydration Clash
So far, nothing I’ve written is contentious. Everyone—or at least everyone at least attempting to base their decisions in science—agrees that hydration is a mingling of water and sodium. Where ideas about hydration begin to get challenged is when and what you should drink. For example, some people argue that thirst is a good indicator even for athletes and that you should just drink when you’re thirsty. In a similar vein, some argue that there’s no need to replace sodium because our body will keep sodium levels at adequate levels naturally.
Is there merit to either of these ideas? Yes. Thirst is a perfectly adequate way to maintain hydration, and assuming you’re not suddenly jumping out and climbing on a hot day unacclimated or drinking liters of water an hour, you probably won’t develop low blood sodium (hyponatremia). What these hypotheses fail to address, in my opinion, are the performance benefits of not only maintaining blood volume, but expanding it. If you drink according to thirst and don’t salt your drinks, you’ll never be as hydrated as you could be—you’ll always be only as hydrated as your body accepts as “normal”, which is not always performance-optimal.
An easy comparison here is to the role of dietary glucose in sports drinks. Our body has adequate stores of glucose in the form of muscle glycogen to allow us to exercise for a few hours at normal intensities without dipping into hypoglycemia. If you’re going out for an hour jog, or hitting the gym to casually climb routes for a couple hours, you probably won’t benefit much from external glucose. On the other hand, if you’re exercising intensely for a long period of time, that external glucose adds an extra energy source and increases your total energy pool, allowing you to exercise harder, longer, and better. There’s no question that adding glucose (and fructose) to sports drinks improves performance, and we can easily see that athletes who have higher blood glucose levels that are only attainable through in-the- or just-before-the-moment dietary carbohydrate consumption also burn more calories—meaning they do more work.
If you drink only according to thirst and don’t add salt to your water, it’s akin to relying solely on muscle glycogen to power your exercise—you’ll be fine health-wise, but your performance will be slightly limited depending on the situation. But if you drink salted water, you’ll expand your blood volume as much as possible and increase your blood’s ability to transport nutrients, oxygen, etc.
What Does the Science Say?
Given the contentious climate surrounding hydration, it shouldn’t be surprising that there’s no singular consensus pointing either way. For every study you find that suggests exercise is impaired by dehydration or “drinking to thirst” only, you can find others that show performance to be unaffected or even increased during periods of dehydration or when athletes drink only according to thirst. This shouldn’t be taken as an indication that there isn’t a real answer, but rather that we have to tease out what’s really happening.
To begin with, there are many factors that affect an individual’s ability to stay hydrated. Externally we have the weather, type of exercise, and intensity; hotter and more humid weather both cause greater rates of sweating, as does the overall intensity of the exercise and length of exercise. We can control these factors to a certain extent, but we can’t extrapolate much from a “cool weather” experiment to a “hot weather” experiment—we need to perform both.
Internally, different individuals have different levels of heat acclimation, sweat rates, and sodium concentrations in their sweat. For example, one study found that amongst 29 men, sweat rate varied from a low of only 1/10th a liter per hour to a high of one liter per hour, and that sweat sodium concentration varied from less than 20 millimoles per liter to 110-120 millimoles per liter. While most people fell between these two extremes, it’s important to understand that there exists both people who sweat very little and excrete little sodium and also people who release buckets of hyper-salty sweat.
Then, there’s the challenge of instructing someone to “drink to thirst” because there is no universally understood feeling of “thirst”—e.g., different people understand “thirst” to feel differently. Some people get uncomfortable the moment they get a tickle in the throat while others only accept thirst to mean a parched mouth. Thirst is also influenced by the drinks available; we get “thirsty” for drinks that contain chemicals which make us happy such as glucose, ethanol, and caffeine more than we get thirsty for regular water, and if you provide two groups with either a sweetened sports drink (that they enjoy) or water and instruct them to “drink to thirst” the group with the sweetened beverage will consume more. Ostensibly, both groups would have the same hydration needs, but other chemical triggers influence our desire to consume fluids beyond the ones involved in thirst regulation.
Finally, there’s a whole psychological overlay to the problem: indoor trials such as cycling ergometer time trials tend to demonstrate a greater benefit to hydration than outdoor, “real world” trials. While a number of factors have been suggested (such as improved cooling when riding a real bike vs. a stationary cycle), a major factor is likely to be motivational—simply put, strong athletes who are competing are more likely to “play through the pain” in suboptimal conditions when they can snatch a victory than when they’re outside a competitive setting such as in a lab. This isn’t even considering that “real world” settings are uncontrolled, and often involve a wide range of athletic ability and a wide range of interventional strategies; the winning cyclists who finished dehydrated were not necessarily aided by their dehydration (as is sometimes “witnessed” in real world trials) so much as they had other advantages that overcame the disadvantage of being suboptimally hydrated.
When you put all of these problems together along with the not-particularly solid criteria for euhydration (normal state of hydration) vs. dehydration, you can see why we might get so many conflicting reports. So what to do?
Measure Your Hydration!
Even if we cannot measure hydration through urine color or thirst sensation reliably, we can attempt to maintain a “normal” level of hydration via monitoring our weight. In this scenario, we assume that before we begin exercise we are adequately hydrated and that the only weight we will lose during exercise is the weight associated with calories (i.e., glycogen, fat, and protein) and with water. All things considered, glycogen loss will have the largest caloric impact (because every gram of glycogen is matched with a few grams of water), and given the average size of our glycogen stores we shouldn’t expect more than a 1-2% loss of body weight over the course of an exercise session (depending on length and intensity). That’s one to two pounds (0.5–1.0 kilograms) for most people, which we’ll take as our baseline “acceptable” amount of weight to lose before dipping into loss of hydration.
Now comes the measuring. Before a normal indoor climbing session, void your bladder, weigh yourself, and write down the number—I guess you could do this outdoors, too, but it would involve dragging a scale up to the crag with you. Climb or train as normal, drinking your usual amount of water (preferably slightly salted to ensure adequate retention), and then after your session void your bladder again and weigh yourself again. Compare your after weight to your before.
If you weigh the same or only about a pound or two less (0.5–1.0 kilograms) (for longer sessions), then the amount of liquid you’re consuming is likely adequate to maintain hydration. If you dropped more than this, then you likely finished your workout in a state of lesser hydration than when you began and should drink some more water next time—specifically, for every pound lost drink about fifteen ounces more water,* or for every kilogram lost drink one liter extra. This will be your new standard for “average” conditions.
EXAMPLE: You weigh 75 kilograms before climbing, then climb for two hours and drink one liter of water. After climbing, you weigh 73 kilograms. Assuming a normal loss of 1 kilogram from glycogen and glycogen-associated water (which incidentally aids in hydration), you lost about a liter of water, and should bump your normal hourly intake from a 1/2 liter to a full liter.
*It might seem strange that a pound of water—16 dry ounces—does not weigh 16 ounces wet. This is due to a fascinatingly infuriating aspect of the US measurement system wherein a fluid ounce measures volume whereas a “normal” ounce measures weight. They are not the same, and while they are similar, the difference is enough to be annoying. Another victory for the metric system, where a kilogram of water is also a liter of water—very easy to translate!
Other Variables to Consider
Measuring is good, but it only tells us about our hydration needs in average conditions. When conditions vary, we need to consider what other variables will affect our needs.
Variable #1: Time
Are you going to the gym on your lunch break to get a quick thirty minutes in? Are you waking up before dawn to hike two hours to a crag for a sunrise to sunset outing? Your hydration needs are going to differ.
The shorter the session, the less important it’s going to be to adequately replace your fluids. You can get by without water for a half hour (or even an hour) and not lose performance. You can drink suboptimal amounts of water even for longer sessions of a couple hours without noticing. As time drags on, however, the impact of suboptimal hydration compounds—e.g., if you lose an average of 500 milliliters of water per hour, you’ll have lost 4 liters of water by the end of the day (often 5% of your body weight or more). This will measurably impact your performance.
Suffice to say, the longer you exercise, the more aware of hydration you should be.
Variable #2: Weather
We sweat primarily to cool off. If you don’t need to cool off, you sweat less. Obviously, temperature is a major factor here—hot days increase core temperatures during exercise much more rapidly than cool days—but humidity is a big factor, too.
When the air is dry, it can easily wick the sweat from your skin, cooling you down; when the air is already saturated with moisture, it cannot wick moisture from the skin nearly as easily and your body sweats more to compensate. This means climbers in tropical Thailand are likely to need more water on average than climbers here in arid Colorado.
Conversely, on cool days you may need less water—but don’t underestimate the amount you sweat! Winter athletes are notorious for dehydration partially because they don’t perceive themselves to sweat as much (and partially because it’s a pain in the ass to go to the bathroom in a snowsuit), and would do well to drink more.
Variable #3: Intensity
This goes hand in hand with the last factor: like the weather, exercise intensity plays a large role in core temperature increases. The harder you’re climbing, the more waste heat your body produces and needs to export through the sweat. Therefore, when you’re training or climbing hard you should drink extra fluids.
Salted vs. Unsalted Water
No article about hydration would be complete without considering one final topic: electrolyte replacement. As I mentioned briefly above in the section defining hydration, the only electrolyte most people need to worry about is sodium; we lose a much smaller amount of potassium than sodium because it’s sequestered in the cells, and calcium and magnesium are lost in only minor amounts. Technically we also lose a significant amount of chloride, but since table salt—the easiest way to add sodium to your drink—contains chloride already in the exact ratio we need, it’s a bit redundant to talk about.
Everyone has sodium stores on their bones which are adequate for replacing small amounts of sodium lost during exercise, or large amounts on occasion. The problem is that a “normal” intake of sodium (according to health guidelines) is actually extremely low for an athlete and will eventually lead to depleted sodium stores and low blood sodium levels. Consider that a typical athlete will lose an average of 500 milligrams of sodium per hour during exercise in normal conditions. If you only exercise an hour a day, this is easy to get and replace through diet. If you exercise for 8 hours, though, you’re now looking at 4,000 milligrams of sodium loss—approximately double the recommended sodium intake. If you’re exercising in hot weather, it could be even more, and if you happen to sweat more than normal or have saltier-than-normal sweat, it could be even higher.
For healthy athletes, any excess sodium consumed through salted water is unproblematic—it’ll just be excreted through the urine or sweat. On the other hand, the benefit of drinking salted water is blood volume maintenance and the complete avoidance of hyponatremia. To me, it seems worthwhile to salt your water, even if only slightly (e.g., 1.25 grams of salt per liter or about 1/4 teaspoon, which will provide around 500 milligrams of sodium). At this level it’ll be mostly untasteable and only really replace the sodium you’re losing anyway.
There’s one more reason to use salted water: when you salt your water, you’ll keep your thirst sensation sensitive and will be more likely to drink to thirst at an adequate level. This means you’re more likely to stay hydrated even when conditions are different than your measured norm. And, you’ll pee less frequently to boot.
Four thousand words later, here we are at the end of the article; you wouldn’t think a topic as deceptively simple as water would take so long to discuss! I think it’s perhaps because it is so deceptive that it takes so much, but that’s just my opinion.
If there’s one thing I want you to take away from this article, it’s that I think you should salt your water when you’re exercising—especially if you’re going to be exercising for long periods of time in conditions where you’ll be sweating a lot. Most of the other controversies surrounding hydration are solved (or dodged) just by using salted water. A quarter teaspoon of salt per liter of drinking water—that’s about 1.25 grams of salt, giving 500 milligrams of sodium—is adequate, though upwards of a half teaspoon is plausible as well for those who know they sweat a lot (and don’t mind slightly salty tasting water).
The other takeaway is this: hydration is synonymous with blood volume. You can feel okay thirst-wise but have reduced blood volume simply because you’re maintaining blood osmolarity, and you better believe that will affect your performance over the course of a day.
That’s all! Stay cool out there, and stay hydrated.