(crossposted from NS forums) Feel free to discuss, rip, etc.
I hear a lot of people talk about salt-associated water weight from binges. However, what I suspect is actually happening is water associated with glycogen. It's well known that about 4 times the water weight of glycogen is associated with it.
What I think really happens in this (and pretty much any) diet, is that we go into deficit by adherance. As we go into deficit, the glycogen stores in the muscles are naturally depleted. This is the initial "water woosh" people talk about. This has nothing to do with salt, per se.
When people come out of deficit, the glycogen stores are replenished. Along with associated water.
Therefore the "water gain" people get is not from salt in restaurant food. It would happen as easily from completely unsalted food.
The other implication is that while dieting, your weight is a little bit "false" in a sense. Since you are running with that glycogen-water missing. The implication is that when you finish your diet, you should maybe go a couple pounds below goal (or taper off deficit slowly) to prevent jumping back up over goal. [However, a counter-acting effect is the depressed metabolism from dieting...such that when you go off goal, you will actually be able to tolerate slightly MORE food than simple calorie math from loss rate would calculate.]
In theory, carb intake and timing could affect the glycogen water balance. But effectively, I think it is deficit that matters. Only if you have a precise need to shed water weight, should you worry about carb balance or timing (and that only for a temporary effect like a show or a weighing for sports).
Of course there may be some other effects going on (relative dehydration like having a hot bath, perhaps salt, perhaps female hormone cycle). But I suspect glycogen-water balance is driving the bus. And we never seem to talk about it here...
Of course operationally, it's all irrelevant except at the very beggining and very end of the diet. Sure weight may be just a little bit "falsely" depressed by the water woosh. But if one stays in deficit, one will just keep losing fat. And then the 2-5 pounds of water weight become immaterial. The important effect is the massive change in body composition coming down from 230 to 160 (for example). Not that one got a little false woosh at the begining, that comes back at the end.
Personally, I can see this effect from changes in my maintenance diet. If I maintain at 2500 (no exercise) and then drop calories to 2000 and add 500 cals of exercise...that is a 1000 calorie deficit. In a day or two, I get a water woosh. 3 pounds down, just like that, despite less than a pound of calorie weight added. And the converse!
Although glycogen-water is associated with "low carb diets" and NS is not "low carb" on a percent nutrient basis, it's low carb compared to what is needed to maintain a fat person's weight! I suspect it is deficit more than carb balance as the important factor in deciding how the body depletes energy stores. If you are in fat burning mode (i.e. dieting, i.e. starving), your body will deplete muscle glycogen first. Then hit fat by conversion in the liver.
There is a big emphasis in the diet world on finding new things and getting involved in intricate things like intermittent fasting and extreme food compositions (e.g. ketosis). But the main effects of glycogen depletion and water woosh will take place with any strong calorie deficit and are perhaps better understood in that simple framework.
(good overview, emphasis on low carb aspect)