Water is one of the essential nutrients for life, health, and performance, second only to oxygen. 1 You might not even think of oxygen in the context of nutrients, but since you won’t survive many seconds without it, it’s safe to say that it’s a crucial one.
You can go without water for some time, but we’re talking days, not an extended period. If you engage in strenuous exercise, you know when your body gets dehydrated. It doesn’t perform nearly as well when you don’t provide it with enough water.
Water is essential for almost all physiological and biochemical processes. If you go entirely without water, you die within a handful of days or when your body water decrease below 10 % of usual levels.2
The human body is somewhere between a little less than 50% to more than 70% water. The exact amount depends on body composition, sex, and age. Water represents more than 70% of your muscle mass, while your body fat only contains 10% water. That means that trained individuals, on average, are made up of more water since they carry around more muscle and less fat. Females, the elderly, and inactive people usually have more body fat and less muscle than males, young people, and physically active individuals. That means that they also have less body water.
Basically, we’re all walking water balloons.
You lose body water all the time, whether you want to or not.
Every time you let out a breath, you dry up a little. Also, you lose fluids through feces and urine.
The amount of water disappearing through the back door isn’t very, only a couple of deciliters per day unless you have diarrhea. Diarrhea can make you rapidly lose dangerous amounts of water unless you replace it.
How much water you lose through urination mostly depends on how much you drink. If you gulp down enough fluids, you can easily pee a liter of urine per hour. When you exercise or perform some other physically demanding task, the amount of water that reaches your bladder decreases. Less blood goes through your kidneys since your hard-working muscles need it.3 Instead of dehydrating through urination, you lose body water by sweating instead.
Since you lose quite a bit of body water every day, you need to replace it somehow. If you exercise, especially if you do so in a hot environment, you need more water than a sedentary person who spends the day in a cool and air-conditioned room.
Your main sources of water are the beverages you drink and the food you eat.
Plenty of common foods are mostly water. For example, a 100-gram apple gives your more than 80 grams of water. Most fruits are water plus some fiber and sugars wrapped in a peel.
You can count most common beverages towards your daily fluid intake, including coffee and tea. The idea that coffee is a diuretic is pretty prevalent, but no scientific evidence supports this notion.4 Alcohol, on the other hand, is a well-known diuretic, as established many decades ago.5 You shouldn’t count any alcoholic beverages you drink as contributing to your daily fluid intake. The exception would be light beer, which works for rehydration purposes after a workout as well.6
In addition to the water you get from food and drink, your body also creates water on its own. When you oxidize protein, carbohydrates, and fat, you liberate water. That occurs both when you break down the nutrients in the foods you eat and when you use your reserves, for example, your body fat, as fuel. That water is called metabolic water. Humans don’t rely on metabolic water too much, as it only supplies about 10% of our total water needs. However, desert-living animals depend almost exclusively on metabolic water.
Water and Physical Performance
If you exercise regularly, you’re probably quite familiar with dehydration. When you exercise, your body temperature goes up. Your heat production is linked to your exercise intensity. The harder you train, the hotter you get. Athletes need effective cooling to combat the rise in body temperature, and the most effective cooling at your disposal is your sweat.
During a short workout, you don’t get dehydrated enough for any adverse effects. However, in some sports where workouts last more than an hour, sometimes way more, an athlete might lose 5% of their body weight during the training session.7 8 9
That makes fluid intake a much more critical consideration. Both ahead of the training session, as a preventative measure, and during the workout to mitigate the fluid losses and rehydrate. Then, after the workout, drinking enough is crucial to rehydrate. If you don’t hydrate, stay hydrated, and rehydrate, your performance suffers.
Many Different Factors Control Your Sweat Rate
Genetic factors determine how much you sweat to a pretty large extent. Some sweat much more than others during exercise, everything else being equal. You might have glanced enviously at someone looking almost dry and unaffected while you’re dripping with sweat, even though you both are training just as hard in the same temperature.
How much you sweat during exercise depends, to a large extent, on how hot or cold it is. If you exercise in the heat, you’ll be dripping with sweat much sooner than if your training environment is cool or cold. When you’re out running in the summer heat or when squatting in a gym without air conditioning where the walls are heated by the summer sun all day long, it won’t be long before heavy perspiration is a reality.
Your body composition, body size, and clothes all contribute to your rate of water loss through sweating. Let’s take two extremes as an example: heavy American footballers all geared up and cross-country runners. The footballers sweat more than twice as much. In one study, footballers lost 9 liters of water through sweat over a training day, while cross-country runners lost 3.5 liters.10 Both groups trained the same amount and in similar temperatures.
The most significant loss of fluids through sweating documented in science in an athlete was during the Olympic games of 1984. The marathon runner Alberto Salazar sweated 3.7 liters per hour during preparations for the race and during the race itself.11 It was sweltering, and he was an extreme case who sweated a lot when he exercised in the heat.
Dehydration Makes Your Performance Decline
If you don’t replace the water you lose during prolonged exercise, or if you initiate your training session more or less dehydrated, your performance suffers. A minor to moderate dehydration, no more than a couple percent of your body weight, is enough for noticeable adverse effects.
Dehydration has pronounced adverse effects on your cardiovascular system, mainly because your plasma volume decreases in response to heat and a reduction in body water. Plasma volume is what would remain in your blood if you removed the blood cells. When you get dehydrated, your plasma volume decreases.12 How much and how fast depends on the situation. In extreme cases, it happens very quickly. For example, firefighters participating in drills in extreme heat lose 15% of their plasma volume in less than 20 minutes.13 Of course, your workouts won’t have such a dramatic effect, even if you exercise in the heat, but your plasma volume can still decrease enough for a noticeable performance hit.
Each percent of your body weight that you sweat off during exercise increases your heart rate by about seven beats per minute. At the same time, your stroke volume, meaning the amount of blood your heart pumps out with every beat, decreases.14 15
Your more rapid heartbeat compensates for your lower stroke volume, but only up to a point, not indefinitely. When you perform an intense training session in the heat, your muscles don’t get all the blood they need. That makes you feel fatigued, leading to a decline in performance. If you continue exercising, losing more and more water through sweating, you eventually overheat and become forced to stop.16
Also, you can’t perform your best if you’re already partially dehydrated when you begin training. If you start your workout with a body mass 1.5 to 2% lower than usual because you haven’t been drinking enough, you fatigue quickly. It’ll take you much longer to cover the same distance. One study on competitive runners found that even a small to moderate dehydration caused 1,500-meter runners to finish 10 seconds slower than a hydrated state. During a 10,000-meter race, dehydration made the runners more than 90 seconds slower.17
During exercise in the heat, you notice the adverse effects of dehydration even more quickly. When you exercise in a cooler environment, 20 degrees Celsius (68 degrees Fahrenheit) or below, moderate dehydration of up to 2% of your body mass doesn’t affect your performance very much, as long as your workout lasts 90 minutes or less. If you work out longer than that, you can expect a performance loss from moderate dehydration, even in cool environments.18 During exercise in the cold, mild dehydration has little to no effect on performance.19 Your body doesn’t have to cool down through mechanisms of its own since the air temperature is low enough for optimal cooling.
The combination of exercise in the heat and dehydration can have dramatic detrimental effects. Your performance can drop by 60% depending on how dehydrated you are and your tolerance to physical activity in the heat.20 Your motivation to exercise also helps or hinders your performance, whether you want it or not. That motivation is linked to your heat tolerance. If your heat tolerance is low, your brain tells your body to slow down, making you unable to perform your best.
Strength-Training and Other Forms of High-Intensity Exercise
Strength training and other types of high-intensity exercise, like running sprints, is called anaerobic exercise. Anaerobic means “without oxygen,” and when you perform an anaerobic exercise, your cardiovascular system can’t supply your muscles with enough oxygen to keep up with their demands.
Your anaerobic performance is also dependent on hydration. One review concluded that you lose 2–3% of your strength and up to 10% of your muscle high-intensity endurance if you lose 3–4% of your body weight by sweating while exercising.21
Research also shows that exercise-induced dehydration makes you lose some of your motor skills.22 23 That happens somewhere around a loss of 2 % of your body weight, and the effects get more noticeable the more dehydrated you get.
Also, the loss of strength you experience while dehydrated might increase your risk of injury. Your joint stability suffers, meaning dehydration during a squat workout could lead to a greater risk of injury if your knees lose stability.24
Mental Effects of Dehydration
Your muscles aren’t the only things negatively affected by dehydration. Your brain also functions less efficiently. Suppose you exercise in the heat and lose around 2% of your body mass through sweating, your short-term memory declines. In that case, you can’t pay proper attention to what happens around you, and your ability to handle mathematical challenges deteriorates.25 Both males and females experience low moods and find it harder to concentrate in a dehydrated state.26 27
Can You Rely on Your Thirst to Tell You When to Drink?
Short workouts lasting no more than an hour in a relatively cool environment do not require any special preparations or any planned fluid intake during the activity itself. You can let your thirst guide you, both before and during training.28 That’s enough for a healthy fluid balance. If you work out first thing in the morning, though, having a drink of water beforehand is a good idea, whether you feel thirsty or not. After a night in bed without any fluid intake, you’ll be partially dehydrated.
If you engage in endurance exercise and sweat a lot during training, you benefit from planning your fluid intake instead of relying solely on thirst for several reasons.
Thirst is a delayed reaction to dehydration. You don’t feel thirsty before you have already lost 1–2% of your body mass through sweating.29 In other words, delaying fluid intake until you’re already negatively affected by dehydration is probably not a great idea.
Also, your thirst goes away before you have restored your fluid balance by drinking.30 You can’t always rely on thirst alone for peak performance. If your workout lasts a long time and you sweat a lot, thirst is not a reliable indicator of an optimal fluid balance.
An easy way to find out how much water you lose in the form of sweat while exercising is to weigh yourself before and after training. Do so naked, since workout clothes soaked in sweat weigh more than dry ones. If you’re wearing heavy, wet clothing when weighing yourself post-workout, the result is inaccurate. Also, you need to abstain from drinking during this particular test workout. Your pre-exercise body weight minus your post-exercise body weight shows you how much water you’ve lost in the form of sweat.
How Much Should You Drnk Before Working Out?
Drinking 500 mL of water a couple of hours before working out is a good idea. If you’re already hydrated, it won’t do any harm, and if you’re not, it’s the difference between a good and a bad workout. This particular fluid intake is extra important if you exercise in the heat and sweat profusely.31
If you plan to exercise in the heat for more than an hour, you can also drink a couple of dL more before initiating the workout, maybe 15 minutes before. Like we said before, your kidneys partially shut down urine production during exercise, so this fluid intake won’t fill up your bladder while you’re working out.
Large amounts of fluids during the hours before a workout gives you a little better fluid balance and makes you more heat resistant. At the same time, that water might run through you and end up in the bladder, which can more or less ruin your workout or competition. If you’re in a gym with access to a bathroom, it doesn’t matter at all. It might be annoying if you’re out on a training run, but not much more than that, as long as you can do your business behind a bush or something. However, if you’re in the middle of a competition, a full bladder can mean the difference between the gold medal and finishing dead last.
Fluid Intake During Exercise
Your main goal of drinking during exercise is to prevent a loss of body water exceeding 2% of your body mass and maintain your electrolyte concentrations.32
There is no one-size-fits-all water intake during a training session. How large you are, the type of exercise you’re engaging in, and how much your sweat all affect your fluid requirements. As we described earlier, the best way to make sure is to weigh yourself before and after training.
Even though that’s not hard or time-consuming, most people won’t do it anyway. If you don’t want to go through that hassle, you can follow the standard recommendation of drinking 1.5 mL of water per 1 kcal of energy you expend.33
In other words, if you burn 500 kcals during your workout, 750 mL of water would be a good drinking target. That should be enough regardless of the type of exercise or how much you sweat, maybe except for really intense training in extreme heat.
Drinking 250 mL every 20 minutes works fine.34That practice keeps you hydrated even when the sweat is pouring.
The above is something to keep in mind when exercising in the heat and you sweat profusely. If you’re pumping biceps in an air-conditioned gym, following the same recommendations won’t do any harm, but neither do you get any benefits by drinking that much. Most likely, the only effect you’ll get is having to go to the bathroom more often.
Plain water is good enough in most instances, but athletes engaging in extreme training benefit from adding sodium and carbohydrates to their intra-workout water.
Sodium is an “osmotically active” substance, helping your body retain fluid and rehydrate more efficiently after losing substantial amounts of body water. Along with electrolytes like potassium and calcium, sodium regulates your body’s fluid balance, both at rest and during and after an intense and sweaty training session.
The average person, or indeed the average athlete, doesn’t have to ingest sodium during training. A balanced mixed diet gives you enough, often more than enough. However, if your workouts last four hours or longer, consider adding 0.3 to 0.7 grams of sodium to your beverage of choice.35 That’s how much sodium you get, from 0.75 to 2 grams of regular salt. By doing so, you prevent excessive losses of salts from sweating and maintain your electrolyte balance. You reduce the risk of muscle cramps and hyponatremia, low levels of sodium in the blood.
If you’ve been fasting for an extended time before exercising, you also benefit from adding a little salt to your water. If not, the salt and electrolytes you get from your regular diet are perfectly adequate.
Do you find it hard to drink enough during workouts? Try adding the amounts of salt mentioned above to the water you drink, even if your workouts don’t last for hours. It won’t hurt but drives thirst and helps you drink more.
During long and intense training sessions, ingesting carbohydrates offers several benefits. Not only do you get an energy boost, but you also improve your fluid balance. You absorb more water from your small intestine with the addition of carbs.36 Also, you will probably find it easier to drink large amounts with the help of a bit of sweetness compared to plain water.
Combining the two simple sugars, glucose and fructose, increase the amount of fluid you absorb compared to relying only on glucose.37 However, large amounts of fructose easily upsets the stomach, so limit fructose to no more than 20–30 grams per 1,000 mL of water.38 39 Between 30 and 80 total grams of carbohydrate optimizes fluid absorption. Larger amounts might have the opposite effect.40
Don’t Forget to Rehydrate Post-Workout
You should replace the water you lost through sweating with 1.5 times as much during the 4 hours following the workout.41 That amount includes both beverages you drink and the water you get from any food you eat during that time. You need to drink more than you lost for a couple of reasons. First, you continue to sweat after you stop exercising. Maybe not enough for you to notice sweat running down your body, but you keep losing body water for a time after training. Second, your urine production takes a while to start up again. But when it does, you won’t get to keep all the fluids you drink in your body.
Plain water is perfectly adequate for most athletes who don’t sweat profusely during training and don’t exercise for more than 60–90 minutes. However, if your training sessions last longer than that or sweat a lot, you can add 0,3–0,7 grams of sodium per 1,000 mL of water you drink to replace the salt you’ve lost.42
Carbohydrates and protein are helpful post-exercise because they replenish your spent muscle glycogen and kickstart your muscle protein synthesis, i.e., your muscle-building processes. You don’t have to drink your carbs and protein, though. A regular meal of solid foods is just as good. However, you might find your appetite depressed after working out intensely in the heat. If that’s the case, a protein shake or a dedicated recovery beverage is both convenient and gives your body what it needs without having to force down a lot of food.
Milk might be the perfect recovery beverage. Milk restores your fluid balance after a workout just as well or even better than commercial sports beverages. In addition, you get both carbohydrates and muscle-building protein simultaneously, along with sufficient amounts of sodium to replace what you’ve just lost.43 44 45
What Should You Drink to Rehydrate?
It doesn’t matter too much what you drink, as long as you get the same amount of water.46 Everything from plain old water to fruit juice to sports beverages to tea and cola is pretty much equally effective. Coffee and beer are a little less so, but not by as much as most people probably think. Milk tops the lift of post-exercise beverages and rehydrates you more efficiently than water.
Over the day as a whole, moderate amounts of caffeine up to 5 mg per kilogram of body weight won’t dehydrate you.47 Replacing the regular beer with light beer lets you enjoy a post-workout brewski and still rehydrate.48
- Water is essential for both your health and your performance.
- If you don’t replace the water you lose through sweating, your performance drops.
- Losing more than 2% of your body mass is detrimental to performance. Further dehydration exacerbates the adverse effects.
- Dehydration during a training session overloads your ability to regulate your body temperature and your cardiovascular system. Your perceived effort increases, you get less motivated to continue, and it doesn’t take long until your performance takes a hit.
- Try to drink enough during your workouts to keep your body mass loss below 2 % of your body weight.
- Individual water requirements determine how much you need to drink. If you want a concrete number, weigh yourself before and after a workout to determine how much water you’ve lost. If you often train in various temperatures, you need to repeat the procedure for each condition.
- After working out, rehydrate by ingesting 1.5 times as much water as you lost the following hours through drinking and eating regular food.
- Plain water is enough for workouts lasting an hour or less.
- Longer workouts, especially in the heat, benefit from adding salt and carbs to the beverage you drink. Up to 80 grams of carbs and 0.3 to 0.7 grams of sodium per 1,000 mL of water give you a good balance between hydration and performance while keeping the risk of gastrointestinal issues to a minimum.
Water is essential, and drinking enough of it benefits your training.
If you train long and hard, especially in the heat, take the time to plan your fluid intake and maybe even enhance it with a few select nutrients.
However, plain water is all you need if you don’t work out for hours or train in an air-conditioned gym. You can let your thirst tell you when and how much to drink, and you get a hydrated body and one less thing to worry about at the same time.
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