RE: carbohydrates and water balance

Restricting carbohydrates will lead to water loss (This is often seen in the
first few weeks of a carbohydrate restriction diet for weight loss.)
Refeeding carbohydrates after restriction will lead to relative water
retention. (Many people on a carbohydrate restriction diet for weight gain
will gain 3-4 pounds after having carbohydrates.  I have assumed this is
water weight.)

I don't know the mechanism of this phenomenon.