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Your body is about 70% water by weight. If you weigh 150 lbs, that means 105 lbs – or about 12.5 gallons – of your weight is attributable to water. Although that may sound like you have a lot of water in reserve, a 2-3% loss can compromise your performance. Water loss of 5-10% can be dangerous or even life-threatening. Water serves many functions in the body, including your ability to perform well during training and on race day.

When we talk about ‘hydration’ we are referring to maintaining adequate levels of fluid in the body so that biological processes can be occur efficiently. Water (and proper hydration) plays two major roles during exercise. The first is maintaining the volume of blood in the body, and the second is regulating body temperature. We’ll talk a bit about both.

Blood Volume

Although it may look like it’s just a red liquid when you cut your finger or scrape your knee, blood is fairly complex. It contains various substances that support processes ranging from fueling metabolism to fighting infections. From a broad perspective, it serves as the delivery service for things that need to be moved around inside our bodies. The two main components of blood (although there are many other molecules) are red blood cells and plasma. Red blood cells help carry oxygen throughout our body. This oxygen is used by our organs to produce the energy they need to function. Red blood cells function in our bodies for a few months before being replaced, so their numbers aren’t directly impacted by hydration. Plasma is like the water in a river. It helps carry things throughout the body including red blood cells, immune cells, glucose, hormones, and a number of other molecules. The level of plasma is impacted by hydration status. As you start to become dehydrated, water will be pulled from plasma to try and maintain essential biological functions. This reduction in blood volume is referred to as hypovolemia (aka “low volume”). The problem with reduced blood volume is that it will make your heart work harder to try and maintain the delivery of things like oxygen to your organs. If blood volume is reduced, it will impair your performance. If blood volume is reduced too much it can result in cardiac distress. So where does that water from your plasma go you might ask? If you’re exercising, particularly in a hot environment, it will be used to produce sweat.

Body Temperature

Normal body temperature ranges from 97.5°F to 98.9°F. If the temperature is above or below this range impairments in performance, damage, or even death can occur. The most obvious factor that can impact body temperature is how hot or cold our surroundings are. Beyond environmental temperature, the heat we produce by contracting our muscles also impacts our body temperature. Producing heat is very useful in cold environments (think shivering), but heat production needs to be managed in warm and hot environments.

Exercise involves a lot of muscular contraction. Since a byproduct of muscle contraction is heat, exercise results in the production of excess heat. The heat produced during exercise must be removed from the body to maintain internal body temperature within the tolerable range. Our body can utilize water from our plasma to produce sweat. The sweat is excreted from sweat glands on our skin and evaporates. The process of sweat evaporation cools our skin and blood passing by the skin which in turn cools our body.

Sweating is the main way our body removes heat during exercise. Several factors play into how efficiently sweating removes heat including: a) hydration status, b) environmental temperature, and c) environmental humidity.

Hydration Status

If you begin exercise in a dehydrated state, then you won’t be able to effectively cool yourself by sweating. This relates back to the discussion above about blood volume since there will be less water available to contribute to sweating.

Environmental Temperature

Environmental temperature is simply how hot or cold it is outside. The warmer it is in the environment, the more sweating will be required to effectively cool the body down.

Environmental Humidity

Environmental humidity is how much moisture is in the air. We are probably all familiar with those sticky summer days and how they can feel hotter than the temperature reported on our weather app. One of the reasons for that sticky-hot feeling is that water in the air makes it more difficult for the sweat on our skin to evaporate. This reduces the cooling ability of sweating and should be taken into consideration when exercising in hot humid conditions.

Additionally, the type of clothing being worn can impact the ability for sweat to evaporate and cool our bodies down. Thick or heavy clothing that is useful in the cold will obviously retain moisture and make it more difficult for sweat to evaporate.

Hydration Strategies

Now that we’ve discussed some of the factors that impact hydration, let’s look at how we can work to optimize your hydration during training and for race day! The approach you take will depend on the duration of the event, as well as the environmental conditions. Tuning in your hydration strategy should not be left until race day. Figuring out your optimal approach to hydration is a critical part of your race preparation so please don’t take it for granted.

Before Exercise

Consume about 0.5L (16oz) two hours before exercise. This will allow time for any extra fluid to be processed by the body and excreted as urine. Needing to make a pit stop at mile 2 won’t help your race time, and it isn’t uncommon for there to be a line at the Porta Potty station. If needed, take small sips of water leading up to the race.

During Exercise

Individuals typically do not consume enough water and begin drinking too late during an event. When you feel thirsty, you are likely ‘behind’ in maintaining hydration. Consuming 0.2-0.4 L (7-14 oz) every 20 minutes, even before you feel thirsty, will help you maintain your hydration during a race or training session. The frequency or volume may need to be increased in a hot environment, and the optimal balance of volume/frequency will depend on the individual. You should attempt to keep a steady intake of water without consuming so much that you feel uncomfortable (avoid that sloshing feeling). This is also a good time to determine if you are able to run with a water bottle, or if you will depend on fluid stations on the race course.

After Exercise

Following training or a race, it’s important to rehydrate. Despite your best efforts, it is likely you will have lost water during exercise. The most important factor in rehydration is the volume of fluid consumed. The more you drink, the better you will rehydrate. You should attempt to drink more fluid than you lost. Additionally, a modest amount of sodium will enhance your ability to rehydrate following an event. Sodium is an element we need for a variety of biological processes, including hydration. For the vast majority of people, this can be provided through normal dietary patterns.

How do you know if you’ve drank enough to effectively rehydrate? Great question. There is a pretty simple way that anyone can track your hydration status. Just weigh yourself before and after exercise. The amount of water you’ve lost in liters (L) by sweating is effectively the difference in your body weight measured in kilograms (Kg).

Sweat Loss (L) = Pre-exercise BW (Kg) – Post-exercise BW (Kg)

Let’s say our 68Kg (150lbs) person in the example at the start of this blog weighed 66Kg (145.5lbs) after a run. If we plug that into the formula above we get:

68Kg – 66Kg = 2Kg

That means they lost 2Kg (4.5lbs) from sweating during their run. To replace that water loss, they should drink at least 2L (68oz) of water. But preferably more. This doesn’t have to be done immediately, but should be part of the exercise recovery process.

If you’d like to switch between pounds (lbs) and kilograms (Kg) divide weight in lbs/2.2. You can also enter “150 pounds to Kg” in Google and it will do the conversion for you.

Warning signs for heat distress and heat stroke

Being mildly dehydrated (2-3% weight loss from sweating) may impact performance but isn’t inherently dangerous. As you become more dehydrated (5% or greater weight loss from sweating), the risk of compromising your ability to cool off by sweating increases (not to mention making your heart work harder). Extreme dehydration, especially in hot temperatures, can be dangerous and even life-threatening. With this in mind, it’s important to be aware of some signs that allow you to monitor hydration in the moment. Although it would be nice, albeit impractical, to have a bathroom scale every 3 miles on your training or race course to calculate sweat loss, your body will give specific warning signs if you are becoming too dehydrated and in danger of overheating.


As stated earlier, sweating is a healthy way of maintaining normal body temperature, however, if you have flushed skin and are excessively sweating more than you normally do during exercise, you may be overheating. This is your body’s attempt to cool itself down by ramping up sweating. If you are experiencing these symptoms, stop exercising. Find shade and a cooler place, and consume water.

Heat Stroke

Heat stroke is imminent when the skin is hot and dry. At this point your body stops sweating in an attempt to conserve water for essential life-support functions. This is very dangerous. People will become disoriented and can lose consciousness. If you experience hot dry skin while exercising, stop immediately. Cool off and seek medical attention.

Take away tips

· Consume about 0.5L (16oz) of water 2 hours before race. If needed, small sips up until the start of the race. This will allow time for any “extra” water to be removed by urination.

· During exercise, try to consume 0.2-0.4 L (7-14 oz) every 20 minutes. Experiment with this during your training to figure out the best frequency and volume that works for you.

· Rehydrate after exercise. Your goal should be to consume more water than you lost. You can monitor this by weighing yourself before and after exercise.

Bottling it up

If you’ve made it all the way to the end of this post, you may be asking yourself; “Self, why didn’t he talk about sports drinks like Gatorade or Powerade?” Well friends, that is the topic of an upcoming post. We’ll discuss if they are needed, and some strategies to make sure you get the most benefit from consuming a sports drink if you choose to consume them, so stay tuned!

Author, Paul Burghardt, PhD

Contributor, Rachel Golaszewski RD, PhD Candidate

Contributor, Maria Dinh, RD, PhD Student

Wayne State University

Department of Nutrition & Food Science