When dieting for a figure or bodybuilding show many competitors drop their sodium intake 3 days before the show in order to hold less water under the skin and give them a leaner look. Hydration is a means of survival and like many of the bodies systems sodium/potassium work to keep the body in homeostasis at all times, as you will see.
Sodium is responsible for extracellular fluid and potassium is responsible for intracellular fluid. It would be fun to just eat potassium foods and drop sodium foods in order to have full muscle with little water under the skin but unfortunately the body doesn't let us do that.
When you drop sodium from your diet, your blood pressure decreases because of a decrease in blood volume and water is pushed out of your vascular system. If the water isn't in your blood vessels it means it's going to be around them and that means holding water subcutaneously (say goodbye to that definition).
With a drop in sodium or by over consuming potassium a hormone called aldosterone signals your kidneys to immediately conserve sodium. The problem is that water always follows solutes and if re absorption of sodium happens; water will follow, making you look flat. Check out this study result that shows how fast this can happen after you drop sodium:
|Normal Diet Low Sodium|
|1 Day||2 Days||6 Days|
10.4 (ng/100 ml)
Even though urinary sodium is almost half from day 1 one to day 2 serum sodium stays identical.
The bodies natural physiology is to keep 65% of fluid intracellular and 35% extracellular. What I have found to work best with athletes has been to aim to keep your sodium level and potassium level at a regular level (1-3g of sodium and 1.5-2g of potassium from food) and provide a slight drop in sodium the night before the show. It's a small move that makes a small difference but at least it's a safe move making sure you don't screw up all that hard earned work and weeks of dieting.