According to the very basics of human physiology, excess of Na in blood stream will cause water molecules to leave cells and bind to sodium, which causes dehydration of cells and higher blood pressure, to which kidney should respond. That's why we feel thirsty after salty foods - to take more water into the system so eventually cells will go back to balance.

Prolonged exess of sodium which is maintained by habitual consumption of packaged foods causes imbalance and unnecessary burden to the system, so it works much less effective with obese people.

Water retention in various places of the body is also caused by exess of salt, which is a side-effect of a chronic imbalance.