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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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.
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.