Question of the Week

Would a sea water enema help hydration?

Sun, 5th Dec 2010

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Burgess John asked:

Dr Chris,


I hope the subject might get your attention but I guess as a Doc....! a matter of survival and in dire need of fluids without potable drinking water would an anal infusion of sea water work?† If it would how much fits down there, a glass, a litre?


I know of an occurrence where brackish water was administered that way to avoid the stomach upset and consequent problems drinking it would have resulted in.


Great show, Dave's kitchen science gets my vote.





John Burgess


We put this to Miles Parks, Gastroenterologist, and Ari Ercole, Intensivist, both from Addenbrookes Hospital...

Miles -   Using the rectum as a means of administering fluid replacement in dehydrated individuals is an interesting idea.  The colon or large bowel functions primarily to absorb fluid, normally doing so of course as material enters to the cecum from the small intestine, the material which has not been digested, fibrous products and so on, together with a substantial amount of fluid and solute enters into the colon, and the fluid is then sucked out as the material goes around the 4 feet or so of the colon to form up the stool.  And so, you can see the colon is well designed for absorbing liquid but it really needs to do so in the context also of Some clyster (enema) syringestransport of solute, thatís to say of sodium and chloride ions, and so on.  Itís the absorption of these, of the solute itself which then creates the osmotic gradient which sucks fluid across the lining of the bowel, and into the bloodstream.  So, bearing these things in mind, I do think that water enemas on their own or water on its own is perhaps unlikely to be absorbed in a clinically significant quantities, and of course, itís just likely to come out of the rectum again whereas, I think administering saline or something of that type would potentially lead to quite a significant absorption of fluid.

Diana -   Water on its own probably wouldnít do very much, but a saline solution could give one a better chance of a drink.  But what if you're stuck for saline drips and all you've got is a much saltier seawater?

Ari -   Unfortunately, giving yourself a seawater enema for hydration if you've had the misfortune to be stranded at sea is a complete thermodynamic nonstarter.  The problem is that the salt in seawater is much more concentrated than the concentration of all the various solutes found in body tissues.  Since, to a first approximation at least, the gut can be thought of as a sort of semipermeable membrane, this will lead to water molecules tending to move from the body through the gut wall, and into the seawater to reduce the concentration difference.  This process where water moves along its own concentration gradient across a semipermeable membrane is called osmosis, and itís very important in biology.  In this case it will actually result in you becoming increasingly dehydrated.  The situation is reversed with freshwater which would be successfully absorbed.  Having said that, neither procedure would be very safe especially if the water was dirty.  So, giving yourself a seawater enema if you're trapped at sea is likely to make you lose water.  Of course, giving yourself a seawater enema if you're trapped at a cocktail party is likely to make you lose friends as well.


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When I saw the title of this question, I was sooooo tempted to write 'shove it up your a**e and find out', but I shall refrain from such an undignified and ungracious way to treat a guest. Don_1, Tue, 29th Sep 2009

Do I understand your question correctly? You are asking if, Knowing that you can't drink sea water, but being in danger of dehydrating, could you survive by shoving it up your rectum?

I don't know the answer, but I suspect the water would have to pass through your digestive tract to do any good, which you can't do with sea water.

I think the real trick would be to force yourself not to pee. Sucking on a pebble or something to make you salivate would help. I believe that after a while, the body would start recycling its fluids. Of course, this is only good for a certain length of time; eventually, you would need to be rescued. rhade, Tue, 29th Sep 2009

It seems that some people may have done the experiment in a way.

" "There's no way you'd get me waterski jumping," he says. "I tried water-skiing once on holiday and it was like having a salt water enema. I didn't want to let go, so I was just dragged along."

The colon does remove a lot of water from the material passing through it so it might be able to get effectively "fresh" water from seawater.
On the other hand if it didn't work I suspect it would dehydrate you rather quickly.
I doubt the research has been done.
Bored chemist, Tue, 29th Sep 2009

My tortoises can 'drink' (perhaps not the best term to use here) via their cloaca (vent). Whether they could filter out salt in the process, I really don't know.

I suspect that the body would reabsorb water from the bowel if it were necessary, and that such reabsorbed water would be 'filtered' to remove the undesirable elements, so maybe that might include salt. But to what degree this filtration would be...... well, erm...... nope, I'm stumped. Don_1, Wed, 30th Sep 2009

This forum's great; you guys ask the strangest questions. peter125, Thu, 18th Mar 2010

I've heard that if you are unfortunate enough to drop from a great height into water, it's a good idea to try to go "feet first" and it is also imporant to clench your butt as tight as possible on entry. Geezer, Thu, 18th Mar 2010

* thrown out of Starbucks head-first ?  RD, Thu, 18th Mar 2010

I believe that much of the problem with drinking salty sea-water is that people don't do it until they're already dehydrated, by which time the body is too dehydrated to deal with the salts. LeeE, Fri, 19th Mar 2010

There is a TV show on the Discovery TV Channel were a former British Special Forces member (or whatever they are called) named Bear Gryll is the host. It is named "Man vs. Wild"

He did a survival program on surviving at sea. This was one of the techniques that he used to survive. The technique was actually demonstrated. The water was actually fetid water at the bottom of his raft - have no idea what the salt content was. JimBob, Fri, 19th Mar 2010

There's no way I'm clicking on either of those links.

"fetid water at the bottom of his raft"

No sh*t!

(er, well that might not be quite right either.) Geezer, Fri, 19th Mar 2010 RD, Fri, 19th Mar 2010

Come on Geezer - Live LARGE! JimBob, Fri, 19th Mar 2010

How does this answer the salt water question? JimBob, Fri, 19th Mar 2010

How does this answer the salt water question?

I took "fresh water" to mean freshwater ...
RD, Fri, 19th Mar 2010

Interesting thought.
I'll let you try the experiment.

As far as drinking water...
If your kidneys can concentrate the urine more than the salt water intake, then it is hydrating.  If the kidneys can't concentrate the urine more than the salt water, it would likely be dehydrating (which I assume is the case for seawater).

Essentially your body produces carbon dioxide + water out of hydrocarbons.  But, the human body isn't very efficient and looses a lot of water through respiration and urine.  Some desert animals are supposed to be able to survive on essentially no water intake.

Anyway, back to the original question.
I think if you exposed intestinal cells to seawater, you would create an osmotic gradient from the intestinal cells into the intestine.  And, there would be a net outward flow of water. CliffordK, Mon, 29th Nov 2010

I agree with CliffordK. I think, if the experiment were done with seawater, the high salt concentration would result in an osmotic diarrhoea owing to the net movement of water OUT of the body and into the intestinal lumen to dilute the hypertonic solution.

Some laxatives actually work via precisely this mechanism; they are usually water-binding macromolecules - such as complex carbohydrates - that are too large to be absorbed and are not degraded by digestive processes. In the large bowel they hang onto water, increasing the hydration fraction and thus the bulk and softness of the contents, making the faeces easier to pass.

So salt enemas would actually be a squit-ticket to die for - quite literally... chris, Mon, 29th Nov 2010

I think the idea is to use urine granpa, Tue, 30th Nov 2010

But, in dehydration states, urine's always going to be hypertonic (saltier) than body fluids, thus accelerating the dehydration...

Chris chris, Tue, 30th Nov 2010

Your kidneys will really slow down their output in a dehydration state.  What you do have will be very concentrated (although perhaps less than seawater).

Only drink the urine if you know one of your shipmates is a diabetic in a hyperglycemic crisis CliffordK, Thu, 2nd Dec 2010

Or better still, diabetes insipidus... chris, Fri, 3rd Dec 2010

so if you are stuck at sea the better thing to do would be to try to catch a seagull or fish and drink the blood? JnA, Sat, 4th Dec 2010

The best thing to do is to try to find a way to capture, recover, or distill fresh water including capturing the morning dew if there is some.

Actually...  I would really consider getting some kind of a portable desalinator put in my liferaft  :)  But, I guess one may not always have control of the situation leading to being stranded.

I would be a bit worried about the "disposables"   What if one needs more water.  If they only do a quart or two, it might be best just to stock the liferaft with a few gallons of clean water instead.

I was thinking a bit about the new sailboats with bilge tanks.  There certainly would be a reason to fill the bilge tanks with Fresh Water rather than Salt Water, especially if it could be compartmentalized and consumed one tank at a time.

Here is a "solar still" on Wikipedia.

The idea looks pretty simple, and should be able to be adapted to a boat assuming one had some kind of pastic or tarp available.  The main difference would be to either distill bilge water, or distilling it from moistened clothing.

And, yes, FOOD is also good.  But, I think I'd consider a bird, fish, or a piece of seaweed as just food, and not trying to suck their blood.  Heck, if I thought I could eat bird feathers, I'd probably chew on them too.

Keep in mind that saltwater fish may have saltier blood than humans. CliffordK, Sat, 4th Dec 2010

I thought I'd look up some of the numbers...  it is a little hard to find everything, but, looking at Sodium and Chloride ions.  Note, the urine also is the primary balance for Nitrogen (Urea), and other water soluble toxins.  But regulation of Sodium (salt) is important when considering it with respect to sea water.

Sodium in Blood 136Ė145 mEq/L
Hypernatremia is supposed to have a 70% mortality rate with levels greater than 160 mEq/l with the highest levels in literature being 180 or 190 mEq/l
Sodium in Urine max of about 270Ė290 mEq/l
Chloride (similar to Sodium) in Sweat < 40 mEq/L  (while you think it is salty, that is only because of evaporation, it is actually hypotonic.

Sodium in Sea Water is 468 mEq/l
Sodium in Great Salt Lake or Dead sea would be about 10x that much.

Drinking sea water will replace fluid in your body...  but it is at an extreme cost as your body won't be able to recover its salt balance.
The sea water enema will likely cause some salt to be absorbed, and create an osmotic gradient likely causing a tendency to loose water.  However, it seemed like a good idea, somewhat like the very poor man's peritoneal dialysis.
Your bladder doesn't concentrate fluids, so injecting the salt water up your willie wouldn't help either.  And the best way to get fluids to the kidneys is by ingesting them.
Your own urine may have a lower concentration of sodium than the sea water...  But it will have a higher nitrogen concentration, and it is still at the point where your kidneys can't make it any more concentrated.

As mentioned, there are diseases that affect the urine.
The Classic uncontrolled Type 1 Diabetic will have urine resembling Gatorade including sugars, and because of an osmotic gradient, less salt.
Diabetes insipidus also causes unconcentrated urine.

The problem is that the individuals with (primary or secondary) urinary disorders causing dilute urine would be in very very very bad shape.
CliffordK, Sun, 5th Dec 2010

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