Causes of dehydration

Causes of Dehydration

What Causes Dehydration? 

Are you wondering what causes dehydration? Dehydration occurs when we lose more fluid from our bodies than we take in. Every day we each lose an average of 2.5 liters of fluids from our bodies through sweating, going to the toilet, and even breathing [1]. Therefore, in order to stay hydrated, you need to consume at least 2.5 liters of fluids each day to replace what you’ve lost. This volume increases with exercise and illness, as extra exertion causes you to lose more fluids than normal. If you don’t replace the lost fluids quickly enough, you’ll begin to feel dehydrated.



Sweating and Dehydration 

sweating is a cause of dehydration

Of the 2.5 liters you lose on average per day, about 500 milliliters of water is lost through your skin [1]. This may not seem like a lot, especially because most of it is unnoticeably lost during normal activities. You can sweat just by sitting in a warm room, as your body automatically works to cool itself down. The warmer you are, through exercise or environment, the more you need to cool down, so the more you sweat.

You can estimate how much water you’ve lost during exercise by weighing yourself before and after exertion. One litre of water weighs 1 kilogram, or about 2.2 pounds. For every pound (about 500 grams) lost during exercise, you lose about 500 milliliters, or 16-20 fluid ounces of water [2]. But, if you don’t want to do math after exercise (like most of us!),  you can ensure you’re getting the proper amount of fluid by drinking a full water bottle (like this one!) during your recovery.



Vomiting and Dehydration 

When you vomit, your body expels the content of your stomach before you’ve had a chance to absorb any of the water in it. Not only does vomiting cause a huge loss of water all at once, but it also makes it harder to get water into your body to replace the lost fluids, as nausea and vomiting can make it difficult to drink and keep things down. This makes it hard to to get fluids into your system, leading to dehydration.



Diarrhea and Dehydration

diarrhea cause of dehydration

While food is traveling through your gut, your body removes as much water as possible from it. If you get diarrhea (because of illness, or something you’ve eaten), food passes through your gut too quickly, and your body doesn’t get the opportunity to take enough water out of it, so the water is lost from your body. As diarrhea can come on quickly, and last days, weeks, or even months in cases where there is an underlying problem, it can lead to serious water loss, and significant dehydration [3].



Stomach Flu and Dehydration 

stomach flu dehydration

Contagious stomach flu (also known as gastroenteritis) is when a bacteria or virus gets into your gut, and causes it to become inflamed and irritable. You’ll feel this inflammation as nausea, vomiting, or diarrhea. Stomach flu is therefore a major cause of dehydration. Not only do you lose fluids through diarrhea and vomiting, but the nausea can make it difficult to drink and keep fluids down. This combination of symptoms is a sure way to cause dehydration [4].



Proper Hydration is Key

Once you’ve become dehydrated, through exercise, vomiting, or diarrhea, it’s important to replace the lost fluids quickly. To do this, you need to consume fluids of the right volume and with the right balance of salts, to match the salt balance in your body. In vomit, diarrhea, and sweat, you don’t just lose water, but important salty-nutrients too. Proper hydration needs proper fluids. Drinking water is great, but not always sufficient. Be sure to also consume drinks with good balance of electrolytes (like in Hydrant!), and avoid caffeine and alcohol. 

Writer: Ailsa McKinlay
Editor: Elizabeth Trelstad,


[1] “Regulation of Water Balance”, section 7.3 from the book An Introduction to Nutrition (v. 1.0). This textbook extract provides a great summary of water intake and loses from the body.
[2] WebMD. Water Tips for Efficient Exercise. WebMD provides concise information about health care: it offers simple pieces of advice informed by scientists. This article discusses why water is crucial during exercise. 
[3] Diarrhoea. Written by doctors, this website provides a clear and comprehensive guide to diarrhea. 
[4] WebMD. Gastroenteritis. WebMD provides helpful, concise information about the stomach flu.

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