# could you really filter saltwater via your rectum? **URL:** \ **Category:** Factual Questions\ **Created:** [January 22, 2011, 7:54am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547 "2011-01-22T07:54:22Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![MisterBik](https://avatars.discourse-cdn.com/v4/letter/m/8e8cbc/32.png) [@MisterBik](https://boards.straightdope.com/u/MisterBik)\ **Post date:** [January 22, 2011, 7:54am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/1 "2011-01-22T07:54:22Z")
I recently read an TSD article entitled “Is strength of will in fighting illness a factor in whether you live or die?”. At the end of the article Cecil writes > [@](#): > > Finally, let’s not overlook the story of Dougal and Lynn Robertson. While stranded at sea for 38 days with only small amounts of rainwater to drink, they gave brackish-water enemas to themselves and others on their life raft on the theory that their rectums would filter out the salt and absorb enough water to keep them alive. The book I found this story in (Surviving the Extremes: A Doctor’s Journey to the Limits of Human Endurance by Kenneth Kamler, 2004) doesn’t make clear how important rectal desalination was to the Robertsons’ survival. As a ‘survival’ fan, I was curious about this, and want to know if this is something that could be used if stranded at sea / on an island, with no freshwater. Could someone survive by way of seawater enemas? ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [January 22, 2011, 10:27am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/2 "2011-01-22T10:27:51Z")
I don’t see how. The colon’s natural function (one of them) is to absorb water _and salts_ from pre-fecal material. But, pretending for a moment that the colon is permeable only to water… Water purification by reverse osmosis, which appears to be the mechanism here, requires physical pressure to counteract the osmotic pressure, which for seawater is roughly 26 atmospheres. I don’t think that kind of pressure can be generated in the rectum, much less the colon. ---
**Author:** ![cochrane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cochrane/32/10441_2.png) [@cochrane](https://boards.straightdope.com/u/cochrane)\ **Post date:** [January 22, 2011, 11:15am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/3 "2011-01-22T11:15:59Z")
Whether or not this is actually effective, Bear Grylls did this in one of his Man vs Wild episodes in a raft in the middle of the ocean. ---
**Author:** ![6ImpossibleThingsB4Breakfast](https://avatars.discourse-cdn.com/v4/letter/6/e19adc/32.png) [@6ImpossibleThingsB4Breakfast](https://boards.straightdope.com/u/6ImpossibleThingsB4Breakfast)\ **Post date:** [January 22, 2011, 11:50am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/4 "2011-01-22T11:50:14Z")
I would have thought that water moves from the most diluted to the least diluted, when separated by a membrane. So a saltwater enema would actually dehydrate you. Other than Bear Grylls being a total tosser, who carries an enema in their survival kit? ---
**Author:** ![Simple\_Mind](https://avatars.discourse-cdn.com/v4/letter/s/7cd45c/32.png) [@Simple\_Mind](https://boards.straightdope.com/u/Simple_Mind)\ **Post date:** [January 22, 2011, 12:00pm UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/5 "2011-01-22T12:00:06Z")
I read the Book where Dougal and Lynn Robertson, tell the story of their ordeal after being attacked on their Boat by Killer Whales off the Galapagos Islands, there was also a Student on Board that did not belong to the Family that refused to have the enemas, at the time of the rescue that did not seem to have made a difference. ---
**Author:** ![Chief\_Pedant](https://avatars.discourse-cdn.com/v4/letter/c/a6a055/32.png) [@Chief\_Pedant](https://boards.straightdope.com/u/Chief_Pedant)\ **Post date:** [January 22, 2011, 12:21pm UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/6 "2011-01-22T12:21:59Z")
The colon does not have the ability to extract free water and leave behind solutes to a sufficient degree. It is, essentially, a semipermeable membrane. As such hypertonic solutions would not be effective in adding net free water to a dehydrated individual if administered as enemas. As far as using a rectal route as a means to filter out other contaminants such as mechanical debris or pathogens, an oral route would be much preferable. The upper gut is designed to take in contaminated food and fluid much better than is the colon. Just because the colon is full of bacteria does not make it an appropriate receptacle for dirty things, physiologically speaking. ---
**Author:** ![Chief\_Pedant](https://avatars.discourse-cdn.com/v4/letter/c/a6a055/32.png) [@Chief\_Pedant](https://boards.straightdope.com/u/Chief_Pedant)\ **Post date:** [January 22, 2011, 12:25pm UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/7 "2011-01-22T12:25:08Z")
The colon does not have the ability to extract free water and leave behind solutes to a sufficient degree. It is, essentially, a semipermeable membrane. As such hypertonic solutions would not be effective in adding net free water to a dehydrated individual if administered as enemas. As far as using a rectal route as a means to filter out other contaminants such as mechanical debris or pathogens, an oral route would be much preferable. The upper gut is designed to take in contaminated food and fluid much better than is the colon. Just because the colon is full of bacteria does not make it an appropriate receptacle for dirty things, physiologically speaking. As to the survival of folks who have apparently tried this, I make two comments. First, I don’t see where the relative tonicity of the fluid was measured. Second, there was no control to see whether or not per os would have been just as effective. And, of course, there is the _post hoc ergo propter hoc_ fallacy. After all, many people in years past survived blood-letting. That does not mean they survived because of it… ---
**Author:** ![orcenio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/orcenio/32/1036_2.png) [@orcenio](https://boards.straightdope.com/u/orcenio)\ **Post date:** [January 22, 2011, 12:41pm UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/8 "2011-01-22T12:41:54Z")
> [@cochrane](#): > > Whether or not this is actually effective, Bear Grylls did this in one of his Man vs Wild episodes in a raft in the middle of the ocean. He did not use sea water, [but rancid water](http://www.youtube.com/watch?v=TtIG4TuVnvg). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 22, 2011, 7:30pm UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/9 "2011-01-22T19:30:32Z")
The colon is where most of the water we take in gets absorbed, anyway, regardless of which end the water entered through. If it were capable of filtering water administered from below, then it would be equally capable of filtering water administered from above. ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [January 22, 2011, 8:40pm UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/10 "2011-01-22T20:40:19Z")
> [@Chronos](#): > > The colon is where most of the water we take in gets absorbed, anyway, regardless of which end the water entered through. If it were capable of filtering water administered from below, then it would be equally capable of filtering water administered from above. Unless the saltiness of the water caused the person to wretch. Of course, that the body wretched it back out might be an indication that the human body has evolved to naturally determine that salt water is a net negative. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [January 22, 2011, 9:47pm UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/11 "2011-01-22T21:47:52Z")
Nearly the same discussion in [this thread starting with post #8](http://boards.straightdope.com/sdmb/showthread.php?t=398065&highlight=colon). It includes the following: > [@Rectal absorption of alcohol?](https://boards.straightdope.com/t/rectal-absorption-of-alcohol/382182/12): > > Sorry, a big **NO!!** for this one. And I am a doctor. > > A seawater enema would pull water out of the body via the colon. And accelerate dehydration. > > In fact, a standard recipe for salt water enemas is 1 tsp. of salt in 1 quart of fresh water. When given, it doesn’t overload the kidneys so much as a pure fresh water enema would, because _much less water is absorbed due to that very small amount of salt!_ Double the concentration of salt in that recipe, and it will begin to pull fluids out of the body. A quart of seawater has close to 10 times that much salt in it! > [@Sage\_Rat](#): > > Unless the saltiness of the water caused the person to wretch. It’s “retch.” ---
**Author:** ![cochrane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cochrane/32/10441_2.png) [@cochrane](https://boards.straightdope.com/u/cochrane)\ **Post date:** [January 22, 2011, 9:57pm UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/12 "2011-01-22T21:57:15Z")
> [@orcenio](#): > > He did not use sea water, [but rancid water](http://www.youtube.com/watch?v=TtIG4TuVnvg). You’re right, I do remember now. At the time I posted, I forgot to make the distinction. _Mea culpa_. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [January 23, 2011, 5:31am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/13 "2011-01-23T05:31:03Z")
> [@Chronos](#): > > The colon is where most of the water we take in gets absorbed, anyway, regardless of which end the water entered through. That simply isn’t correct in any way at all. Most water gets absorbed in the stomach and the duodenum, a bit in the rest of the small intestine, and very, very little in the colon. ---
**Author:** ![MisterBik](https://avatars.discourse-cdn.com/v4/letter/m/8e8cbc/32.png) [@MisterBik](https://boards.straightdope.com/u/MisterBik)\ **Post date:** [January 23, 2011, 9:44am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/14 "2011-01-23T09:44:48Z")
Thanks very much for the words folks 🙂 I’m glad I didn’t accidentally get stranded at sea without checking this first 😉 ---
**Author:** ![don\_t\_ask](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@don\_t\_ask](https://boards.straightdope.com/u/don_t_ask)\ **Post date:** [January 23, 2011, 11:26am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/15 "2011-01-23T11:26:08Z")
In regard to Cecil’s article I am of the impression from Dougal Robertson’s own account _Survive the Savage Sea_ that only 2 enemas were administered. The first, as indicated, to make use of an unpalatable mix of mostly rain water, turtle blood and a little sea water. They simply wanted to not waste it even though it was undrinkable. No reference is made to the bowel filtering in any way, they simply hoped to absorb the water through the bowel walls. Each had an enema of one to two pints which they would not have been able to drink anyway due to their shrunken stomachs. On the second occasion the enemas were administered to those who hadn’t had a bowel motion despite eating solids. they were for the purpose of preventing an obstruction. Robin’s health may have been aided by the fact that, as he was the largest in the group and the only non family member, Robertson gave him larger rations as it would be natural for Robin to be looking for signs of discrimination. His own family were often angry about this. ---
**Author:** ![Waenara](https://avatars.discourse-cdn.com/v4/letter/w/50afbb/32.png) [@Waenara](https://boards.straightdope.com/u/Waenara)\ **Post date:** [January 24, 2011, 9:33am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/16 "2011-01-24T09:33:10Z")
> [@Blake](#): > > That simply isn’t correct in any way at all. Most water gets absorbed in the stomach and the duodenum, a bit in the rest of the small intestine, and very, very little in the colon. ?? I’ve taken several anatomy and physiology classes in university (currently I’m a third-year nursing student), and my understanding is that the primary function of the large intestine is to absorb water. If the colon doesn’t absorb all the water it’s supposed to, diarrhea occurs. Edited to add: I googled and found the following information. It’s true that the stomach and small intestine secrete a large amount of fluid and absorb fluid as well. But it isn’t true that the colon absorbs “very, very little” water. It looks like it absorbs about a litre per day. From this [site](http://www.siumed.edu/mrc/research/nutrient/gi42sg.html): > [@](#): > > An important function of both small intestine and colon is the absorption of water and electrolytes. Approximately 2000 ml of food and drink is ingested daily, and the volume of gastrointestinal secretions (salivary, gastric, biliary, pancreatic and intestinal) is about 8,000 ml daily; therefore, approximately 10 liters of fluid enters the intestine each day. Of the 8 liters secreted, about 1-1.5 liters enter as saliva, 2-3 liters are secreted by the stomach, about 2 liters enter as bile and pancreatic secretion (about 1 liter each), and about 2 liters are secreted by the small intestine. (Please note that these figures are approximate, not absolute. Volumes may vary, depending on experimental method and conditions.) Of the 10 liters which enters the gut each day, only about 1 liter passes into the colon, about 90% having been absorbed across the small intestinal epithelium. Only about 150 ml is lost in the feces daily, with the remainder being absorbed by the colon. It should be obvious that any derangement in intestinal fluid absorption would profoundly influence the balance of fluid and electrolytes in the body, and that the normal functioning of the intestines plays a significant role in regulating water and electrolyte balance. ---
**Author:** ![EvilTOJ](https://avatars.discourse-cdn.com/v4/letter/e/ee59a6/32.png) [@EvilTOJ](https://boards.straightdope.com/u/EvilTOJ)\ **Post date:** [January 24, 2011, 10:42am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/17 "2011-01-24T10:42:21Z")
> [@Nametag](#): > > Water purification by reverse osmosis, which appears to be the mechanism here, requires physical pressure to counteract the osmotic pressure, which for seawater is roughly 26 atmospheres. I don’t think that kind of pressure can be generated in the rectum, much less the colon. Looks like _someone_ has never won a Taco Bell eating contest. ---
**Author:** ![bouv](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@bouv](https://boards.straightdope.com/u/bouv)\ **Post date:** [January 24, 2011, 3:30pm UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/18 "2011-01-24T15:30:51Z")
> [@cochrane](#): > > Whether or not this is actually effective, Bear Grylls did this in one of his Man vs Wild episodes in a raft in the middle of the ocean. I think that Man vs Wild is a very good show on survival, if you just remember to actually do the opposite of everything Bear Grylls does. ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [January 25, 2011, 2:10am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/19 "2011-01-25T02:10:31Z")
> [@Blake](#): > > That simply isn’t correct in any way at all. Most water gets absorbed in the stomach and the duodenum, a bit in the rest of the small intestine, and very, very little in the colon. Sorry, not true. You’ve got it backwards. The colon is primarily responsible for absorbing fluids. Very little absorption of anything takes place in the stomach; it’s for breaking foods down. The small bowel is where the nutrients are absorbed, along with some fluid absorption. I’ve got patients with ileostomies (meaning they have no colon, but excrete directly from the small bowel); their stools are very liquidy; they have to drink a lot of fluids to absorb enough to stay hydrated. And the OP has already been adequately answered by previous posters. No, you can’t filter saltwater (or toxins) through your rectum. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [January 25, 2011, 9:58am UTC](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547/20 "2011-01-25T09:58:55Z")
> [@Qadgop\_the\_Mercotan](#): > > Sorry, not true. You’ve got it backwards. The colon is primarily responsible for absorbing fluids. Very little absorption of anything takes place in the stomach; it’s for breaking foods down. The small bowel is where the nutrients are absorbed, along with some fluid absorption. > > I’ve got patients with ileostomies (meaning they have no colon, but excrete directly from the small bowel); their stools are very liquidy; they have to drink a lot of fluids to absorb enough to stay hydrated. No, I’m sorry, you are wrong. This has actually been described as the “fact” that is most commonly misunderstood by medical students, so it’s not a surprise. The colon does indeed regulate water balance in the faeces by either secreting or absorbing water. However that is its sole role as it pertains to water: to regulate faecal consistency. If the faeces are excessively moist then it will absorb water, if they are dry it will secrete water. However the amount of water involved in very minor. There is certainly no justification in the claim that “The colon is primarily responsible for absorbing fluids” You can reason this out for yourself if you think about it for a few seconds. In a healthy person, ingested material takes several \*hours \*to reach the colon. When you are thirsty and drink, how long does it take before the thirst is quenched? Is it 6 hours, or is it closer to 6 minutes? Heck, if you drink excessively following dehydration, how long is it before you urinate? Minutes or hours? Where you have gone wrong is conflating “the bowel regulates water balance in the faeces” with “the bowel is where most water absorption occurs”. Of course the two are almost completely unrelated. [Next page](https://boards.straightdope.com/t/could-you-really-filter-saltwater-via-your-rectum/568547.md?page=2)