Reverse osmosis removes all of the minerals from the water - that's what makes it distilled.
As far as sodium depletion goes - you can, during your contest prep, make the last 1/2 gallon to 1 gallon of water you drink distilled water to help leach out all minerals and make your skin super thin. Still, this is dangerous and you should really avoid it.
This was orginally posted on another board by Thunder:
Sodium is one of the most delicately controlled minerals in your body and you're not going to be able to voluntarily disrupt that balance for very long, before your body adapts and makes the necessary adjustments.
More sodium in = More sodium out.
By having a higher intake of sodium, you're simply setting your 'bar' higher. That is, what is considered 'normal' to your body is now higher. But the end of day balance will remain unchanged. Think about the Na/K balance b/w intracellular and extracellular compartments. You cannot change this more than transiently.
Elevated sodium levels keep aldosterone down. Aldosterone, when elevated, inhibits sodium loss and promotes reuptake of sodium in the kidneys. We don't want that. Provided one is drinking adequate fluids, the low levels of aldosterone will allow 'excess' sodium to be continually flushed.
What affects 'water retention' is changes in sodium intake. What is important is relative changes, and not absolute quantities.
If one day you drastically increase sodium intake above and beyond what is considered normal to your body, you're going to get some water retention. Sodium and water hydrogen bond with one another, so where sodium goes, water will follow.
Now, if you had increased sodium intake, and kept it at that higher intake, yes, you'd initially hold some water. However, once your body adapted, it would then eliminate the excess sodium (through blunted aldosterone levels), again to maintain the delicate balance. As already mention, where sodium goes, water follows, so that excess water you gained, will fall off you, even if you kept your sodium intake high.
Increases in blood volume (as a result of higher sodium intake) and the resultant improvement in nutrient delivery and waste removal is another benefit.
Low sodium when dieting is a monster mistake. It will eventually make you hold water (think aldosterone and Anti-Diuretic Hormone). For the competitive athlete, sodium should not be cut. This is one particular reason some bodybuilders can never completely fill out. Not enough salt. Throw the salt back in and whammo ... full and vascular, just like the day after the show.
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I don't completly agree with Thunder, I don't think body builders should practice high-sodium diets because of other health risks, but the basic ideas are there. It's realitive changes in the amount of sodium you consume that matter. If you are looking to sodium deplete, sodium load first for a few days while drinking mad amounts of water and your aldosterone levels will drop, cut the sodium a lot and water in half each day for 3 days and you'll pee off all your salt and water off and be dry dry dry.