Can I drink through my bottom?

We put this to Miles Parkes, Gastroenterologist, and Ari Ercole, Intensivist, both from Addenbrooke's 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 caecum 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 transport of solute, that's to say of sodium and chloride ions, and so on. It's 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, it's 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 wouldn't 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 it's 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!