When you gain five pounds in one day people always say "water retention from sodium" and undigested food. To see if this is plausible, I want to get a sense for the scale of the effect. Suppose you consumed an additional 2300 mg of sodium (one US RDA, also the amount of sodium in one 11-ounce bag of Doritos, so it's a common real-world scenario for me). How many pounds of water would this cause the average person to retain?

closed as too broad by JohnP, FredrikD, Kneel-Before-ZOD, user2861, Eric Oct 10 '14 at 18:17

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  • I suspect there are way, way too many confounding factors to answer this in vivo. What is your current electrolyte balance (not just sodium, but all of them)? How hydrated were you? How quickly is that sodium digested, and in what form? How hot is it outside? How much will you sweat? I think you get the idea... – G__ Jul 9 '14 at 16:24
  • All of these things would be simple to hold constant in lab rats. Doesn't anyone care enough to try? – Noumenon Jul 9 '14 at 22:53

While I agree with Greg, I did some digging and came up with this information: An extra 400 milligrams of sodium in your body results in a 2-pound weight increase. Now, to me, this statistic is questionable, as the author does not site a source, and 2 pounds seems like a lot for that amount, but I cannot find any other discussion which offers a number, so take from it what you will.

If you are really curious, conduct a test yourself, where you maintain a constant diet for a couple days taking frequent weight measurements, then add some sodium to the diet and take more measurements. While this might not be exact, it will be adjusted to your body type, and will give you the best idea of how your body responds to additional sodium. Make sure to hydrate well before starting the experiment, as all the sources I came across say fulfilling your hydration needs results in a loss of the excess water weight, and so this would allow you a clean slate from which to gather results.

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    The rationale they give for the weight increase is "Your body works to maintain a sodium/water balance that resembles sea water", which actually sounds reasonable. But sea water salinity is 35 g/kg, which works out to 16 grams of NaCl per pound, which is 6400 mg of Na per pound. Maybe that's the real answer? – Noumenon Jul 9 '14 at 22:58
  • That seems much more in line with what I would think it would be from experience (I know I've never gained 12 pounds from a bag of doritos) – meanderingmoose Jul 10 '14 at 2:18
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    @Noumenon - No. Saline solution is .9%, which is approximately the osmolarity of blood. This is about 9 grams per liter, not 35 as in seawater. – JohnP Jul 10 '14 at 14:44
  • @JohnP Based on what the article said, his calculations were correct, he just had incorrect information. I think the moral of the story here is not to trust any assertions by the linked website I linked. – meanderingmoose Jul 10 '14 at 14:54
  • If it's 9 grams per liter instead of 35, that just means 1600 mg of Na per pound instead of 6400, which would mean the answer to my original question would be "1.5 pounds maximum, assuming all of the sodium was retained in the body instead of being flushed out." – Noumenon Jul 11 '14 at 4:54

Once again, people mistake correlation for causation. Your body is very capable of adjusting, and you won't "retain" water for any longer than it takes the body to process the extra sodium out of the body. It's a very transitory process.

If you would like to read some literature, here are two studies, one on acute ingestion of sodium and phosphate, and one on how acute ingestion affects thirst. These suggest that while acute ingestion causes blood Na levels to rise, it has little to no effect on thirst, and that it gets processed out by the kidneys in very short order.

So if you have a heavy sodium intake day, you may retain some water over the next 8-24 hours as the body processes and establishes equilibrium (This is done by receptors in the hypothalamus, by the way), but unless you have a disease state of some sort, the body restores order in a short amount of time.

It is the chronic, not acute, ingestion of excess Na that causes so many problems as far as edema, health, etc.

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