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Why Can’t We Drink Sea Water?

by Globetrooper Todd | 51 Responses
Why Can't We Drink Sea (salt) Water

Imagine you’re stranded on a desert coastline… Namibia’s, perhaps. The searing heat drains your body of fluids and your mind slows to a snail’s pace. Forget food, forget sunburn, forget your coordinates; your biggest danger here is dehydration. If you don’t find water soon, you’ll perish, simple as that.

As you stand there, you can’t help but look at the trillions of litres of water right in front of you. But it’s salt water, and you know you shouldn’t drink it. You’re not sure why, but deep down you know it’s off limits.

Surely a few drops won’t hurt, maybe just half a cup. You’ll die if you don’t drink something soon, so what’s the worst that can happen?

In these situations, it isn’t enough to just know something’s off limits. When push comes to shove, and when everything’s on the line, you can rationalise your way around anything; even drinking sea water. So the question remains: why can’t we drink sea (salt) water?

Firstly, Why Do We Need Water?

Our bodies are mostly water, between 70 and 80%, depending on whom you talk to. The water in our bodies supports many functions, including waste removal and temperature regulation. Since water is consumed and expelled by these functions, we need to constantly replenish our water reserves to survive.

How Do Our Bodies Regulate Temperature?

In most situations, your body regulates temperature using radiation and convection. In short, this just means your body sends heat to the surface (skin), so cooler air around you can absorb the heat. But this is only effective if the external air is cooler than your body.

Why Can't We Drink Salt Water

Lauren and Adheip riding camels in the Egyptian desert heat

When the outside air becomes hotter than your body, you perspire to regulate temperature. This is not about dampening and cooling the skin; quite the opposite actually. Your body dilates the blood vessels near the skin (sending more hot blood through them) and excretes sweat to effect evaporation. The process of evaporation requires energy, and that energy comes from the heat in your blood and skin. By consuming heat to effect evaporation, your body essentially cools itself, particularly its core (brain and vital organs).

How Do Our Bodies Absorb Water?

You may remember the process of osmosis from science class at school. Osmosis occurs across a semi-permeable barrier (our cell membranes). That barrier allows water molecules to pass in certain directions, depending on the concentration of water solutions on each side (imagine different concentrations of salt water).

Why Can't We Drink Sea (Salt) Water? Isotonic Sate

Our cells aim for this isotonic (equal) state, where the salt concentration is the same on both sides.

In normal circumstances, the saltiness of water in your cells is the same as the saltiness outside your cells. This is called an isotonic state. But when your cells consume water, the solution becomes more concentrated, and the natural process of osmosis allows water from outside your cells to pass into your cells to achieve equilibrium (this is called osmoregulation). That’s how we absorb water as it’s needed; it’s a natural process.

Why Can’t We Drink Too Much Pure Water?

Your cells are isotonic to about 0.9% saline solution. If the salinity of the solution outside the cells decreases, your cells absorb more water to get back to an isotonic state.

Why Can't We Drink Sea Water? Too Much Fresh Water

When we drink too much fresh water, excessive water passes into our cells to try to regulate the concentration (which can become futile). This causes cells to swell and sometimes burst.

When marathon runners and other endurance athletes drink too much fresh water, the solution outside their cells drops rapidly in salinity, so osmosis allows water to pass into the cells as a part of osmoregulation. If they absorb too much water, the cells will swell and burst, which can lead to a quick death. This is why runners drink sports drinks that contain sodium and potassium, to help maintain an isotonic state (and it’s why they’re called isotonic sports drinks).

Why Can’t We Drink Too Much Salt (Sea) Water?

The opposite happens when drinking sea (salt) water. The salinity outside your cells increases rapidly, so osmoregulation effects a movement of water from in your cells, to outside your cells, to achieve an isotonic state. So even though you may be dehydrated, your cells will actually release, rather than absorb, the water around them.

But why would your dehydrated body expel water when death is imminent?  Well, this isn’t a conscious decision by your body. It’s simply molecular physics and osmosis at work. And in osmosis, water passes from low saline concentrations to high saline concentrations, end of story.

Why Can't We Drink Sea (Salt) Water? Too Much Salt

When we drink salt water, water leaves our cells (dehydrating us further) in order to regulate (dilute) the concentration of the salt water we just drank (again, futile).

So, you should absolutely not drink sea or salt water when dehydrated. It’s not just an issue of not absorbing the water, but an issue of osmosis accelerating the release of water and dehydration.

I said in the introduction, “You’ll die if you don’t drink something soon, so what’s the worst that can happen?” Well, what’s worse than dying? Dying much sooner, potentially before help arrives.

Posted in Adventure Travel | September 13th, 2010

51 Responses to Why Can’t We Drink Sea Water?

  1. I always knew Sea (salt) water was bad to drink when dehydrated, but never knew the science behind it… Now it’s definitely off limits (not that I drank it before). :)

    • Hey Norbet, yeah, I figured if I was in that situation and didn’t know exactly why, then I’d probably try it. So it helps to know why I’d be worse off.

    • i dont get it

      • With a name like that I’m not surprised

  2. Someone just sent me a tip to try a salt water cleanse, master cleanse style, but only if you’re going for the cleaning out your colon effect!

    Nice write-up & graphics!

    • Hey Bessie, thanks for dropping by and many thanks for the kind words. I didn’t see the salt water master cleanse on your list of 29 things to do by 29. Just make sure it doesn’t coincide with extreme dehydration in the desert. May run into a few problems :)

  3. Nice article you’ve got there
    I’ve always known that eating anything salty is extremely bad when you’re thirsty, but never knew the actual reason till I read that article !

  4. Really interesting article. Earlier this year I spent three months at sea and had to use a desalinator to convert salt water in to drinking water. There were a couple of days when we didn’t have enough power to run it though and after reading your article very pleased I resisted the urge to drink straight from the sea…

    • Thanks for dropping by Luke; I just checked out your site… what an adventure that was?! If you ever consider running your own expeditions with clients, let us know because we’re slowly moving into that area. We’d love Globetrooper to become a hub for finding participants for unique adventures.

      Ripley Davenport listed his Gobi 2011 expedition on Globetrooper and I believe he got a handful of clients as a result (including Lauren and me).

      • Hi Todd – the Gobi ’11 expedition sounds incredible. I was really interested in getting involved but think I contacted Ripley too late about it.

        I will definitely be using Globetrooper for any future expeditions.

  5. Todd.
    Thanks for the details in the article. As an engineer, I research things by nature. And now as a runner, I’ve been looking into this more. I remember the issue of not drinking sea water, and that it will dehydrate you faster. My question is, “At what concentration does the drink go from good-for-you to bad-for-you?” I just finished training for and running a marathon which featured Gatorade Pro; which almost doubles the amount of sodium (200mg/8 oz). I remembered the issue from my biology class and your article, and was wondering, where is the Gatorade Pro in relation to sea water? Could I be drawing water out of my body by because of the salinity of the drink?
    Thanks,
    Pete.

    • Hey Pete, The sodium concentration of human blood is 0.9%. So Gatorade Pro concentration seems quite a bit under that, say 200mg / 224g. Sea water is over 3% concentration. Interesting though, because some of those drinks taste really salty. The last time I drank Gatorade on a competition run it made me feel sick and I think I lost about 5 minutes. Just this week we had an electrolyte solution from the pharmacy that didn’t contain sugar and it was much better in terms palatability. Don’t get me wrong, I like Gatorade in normal circumstances, but in competition the taste is too sickly sweet for me. I much prefer water. The run I’m taking about is only 14km and I’m drinking less than 1 litre, so probably no risk of brain swelling or whatever happens to marathon runners after too much water. I wonder why they don’t make Gatorade at 0.9% salinity then? Probably trying to avoid what you mentioned because sodium is consumed through other sources.

  6. Very interesting! I’m a stewardess and a part of our training course and every day exams is survival in a desert and sea, too. We were always told NEVER drink sea water but never knew all the details why. Now I know, thanks :)

    • hehe, glad I could help. I was always told the same, but figured I couldn’t live in the dark anymore and had to find out why :)

  7. Great information here! I’m like everyone else. Always knew we weren’t supposed to do it, but never knew why. I also had no idea it would SPEED UP the dehydration process, just always thought it wouldn’t help it. Great post!

    • Thank Abbey, great to hear from you. Blog is looking awesome too.

  8. Great informative article and site. ties in nicely with sports drinks which I was originally looking for .

    Thanks.

  9. Bio-teacher here. i was looking for info specific to the effects of salinity (as in sea water) on the ‘sodium pump’ in the loop of Henle`in the kidney tubules, how it affects the hormone Aldosterone -doesn’t this increase salt uptake by the blood, which by osmosis, increases water re-absorption… from the CELLS I suppose, and the collecting ducts would still have too much salt so would NOT let water be re-absorbed by the kidney medulla or by the blood… so urine would still be produced. Another question: How does the chlorine in tap water affect the sodium pump in the Loop of Henle`? -and how does this affect osmoregulation?

  10. Felicia again, -seeing your pic of the camels; Did you know that camels have much longer Loops of Henle` in their kidney nephrons than other mammals like us? -An adaptation to survival in the desert; they re-absorb more water in the kidneys and produce less urine which is highly concentrated as a result. they also store fat, not water, in their humps -fat metabolism produces water…

  11. In case you don’t want to die in that situation http://www.survival-homestead.com/distilling-water-from-the-sun.html

  12. Good article, except for this sentence:

    “But when your cells consume water, the solution becomes more concentrated, and the natural process of osmosis allows water from outside your cells to pass into your cells to achieve equilibrium (this is called osmoregulation).”

    I’m not sure what ‘the solution’ is referring to (inside or outside the cell). I suspect the problem is with your choice of the words ‘consume water’, which perhaps should be ‘expel water’, or ‘are in need of water’.

  13. I don’t believe it. Listen – sea water is water and salt. When I am dehidrayted I don’t have much water left in my body. So when I drink sea water, I console SOME water. Therefore I will have more water in my body than before. I will no longer be dehydrated.

    • It is very true that you would die. You can try it if you want!! Too much salt will cause you to die, the end.

      • When a cell takes in seawater, it also takes in salt. Salt will dehydrate the cell so if the cells dehydrate, you dehydrate. Get it??

  14. How do fish do it? I guess their cells could have a higher natural salinity, but it doesn’t taste that way.

  15. I used to accidentally drink sea water all the time when I was a kid (when I was swimming).

  16. Drinking very limited amounts of seawater, in desperate situations, can keep you alive for a little longer. Your body can process a bit of salt water, but certainly not enough to quench your thirst.

    So if you’re on a raft in the middle of the ocean, and you have no other options (rain, condensation, fish, etc…), and you have the discipline to keep your seawater intake very low, it might extend your life by a day or two, which could be enough to be rescued.

    You’d have to be pretty desperate, though. This stuff is definitely not good for you, and it will make you crazy if you drink more than small amounts.

  17. I was always fascinated by the Kangaroos and Wallabies that could survive on seawater, like the Macropus eugenii. Seems like an adaptation we should have taken on a long time ago.

  18. what kind of ‘pathway’ informs the brain that water is missing ? a salinity threshold triggers some hormones ? or a relative increase of another molecule ? .. just curious

  19. Water is considered highly saline if it contains anywhere from 10,000-35,000 ppm of dissolved salts. Water is considered fresh if it contains less than 1,000 ppm of salt. In some regions of the United States, slightly saline water is used for tasks like crop irrigation. In regions where an abundant supply of clean, fresh water is difficult to come by, desalinization technologies convert seawater to drinkable fresh water. This is still a very expensive process, although costs are starting to drop, and methods, such as reverse osmosis, are improving. Today, the towns of Avalon and Santa Barbara in California, and Tampa Bay in Florida, are working on desalinization projects.

    But that still doesn’t answer your question. So, we typed “why can’t humans drink salt water” directly into the search box. The U.S. government came through again with an answer from the Department of Energy’s Ask a Scientist web site. Prof Bill’s response is brief and to the point:

    Humans can’t drink salt water because the kidneys can only make urine that is less salty than salt water. Therefore, to get rid of all the excess salt taken in by drinking salt water, you have to urinate more water than you drank, so you die of dehydration.

    As is so often the case, this answer raises some new questions. How much salt is too much? Salt, like water, is a key ingredient of life on earth. The right amount of sodium chloride (common table salt) is essential for human health. In fact, our blood is 0.9% salt and our body weight is about 1/400 salt. Living cells depend on sodium chloride to maintain the chemical balances required for complex processes and reactions that take place at the microscopic level.

    Insufficient salt intake can lead to fatigue, illness, and death, although it is more common nowadays to hear about health problems associated with too much salt. According to the Salt Institute’s encyclopedic site about salt, the National Academy of Sciences’ recommended daily dose is 500 mg/day — though most Americans consume closer to 3,500 mg/day. (A teaspoon of salt equals about 2,000mg.)

    Optimal salt intake varies — it depends on a person’s genetic makeup, where they live, how active they are, and other lifestyle factors. However, nobody, except for a saltwater fish, is designed to drink saltwater.

  20. Do fish drink?

    • No, not like we drink. By they absorb water through osmosis. They also have very different physiology, so not really comparable to humans when it comes to salinity.

  21. Does this means none of the salty drinks get absorbed in our body..??

    • Hi piya, No, it just means that consuming very salty foods/drinks dehydrate us because they cause water to be expelled from our cells. Hope that helps.

  22. Great read.
    It is a quasi oxymoron to “die of thirst at the sea”.

  23. So for survival purposes. If I am stranded at sea and I have a gallon of water. Would it make sense to double my water supply by mixing 50/50 seawater/pure water? Reducing the salinity to 1.5%? IF it’s too much, would it make sense to have a higher ratio of pure water first so as to slowly increase the salinity inside your cells?

    • That’s a good question. I think it would be case-by-case and depend on how dehydrated you already were, which would be difficult to determine.

  24. Thanks for the detailed explanation Todd!

  25. thanks for the article.
    very interesting and familiar to me;
    did a lot of research myself and travelled 3 years on camelback through deserts.
    but someone tried and succeeded? (drinking salt water floating on the ocean)
    http://en.wikipedia.org/wiki/Alain_Bombard

    • Hi Peter,

      H didn’t just drink sea water. He drank rain water and ate fish, which is a good source of fresh water.

      While it’s certainly possible to consume some salt water, and millions of people drink salty Gatorade daily, many more people die from trying to hydrate with salt water when in difficult situations.

      I guess the point is that drinking sea water to hydrate is a bad idea. I think even Alain would agree.

  26. Helpful info. Fortunate me I found your site by accident, and I’m shocked why this twist of fate did not came about earlier! I bookmarked it.

  27. thank u so much im not that big im just 11 yeats old but i lernd alot for my final test

  28. Sea water can be used to cool your body, in the same way as sweat does. By emersion or splashing your body in sea water the cooling effect will help to reduce sweating, and thus help to reserve your water. May not be enough to keep you alive for long though.

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