This is a reprint of this subject from another forum.....
Why do I need sodium in my PWO shake?
From the Dextrose, Maltodextrin & Sodium article (see a pattern?):
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Earlier in the article I discussed the sodium/glucose co transport mechanism. This concept falls under the heading of secondary active transport. Primary active transport takes place via a pumping system. You see, each of your cells contain proteins which break down ATP, into ADP + P + Energy, and use the products to power the pump. The Sodium/Potassium Atpase, pumps three sodiums out of the cell, and only two potassiums into it. This makes sodium’s concentration higher on the outside of the cell. Additionally the inside of the cell is more negatively charged then the outside. Sodium is a positively charged ion and attracted to the negative area. It has been pumped against its electrochemical gradient (concentration is greater outside of the cell and more negative). Thus, Na+ (sodium) will now move back into the cell.
There are proteins within a cell membrane, which act to transport glucose. However, the binding site for glucose has a low affinity for it, unless sodium is bound to it. Due to the electrochemical gradient, sodium enters a binding site specific for it on the protein, and when it does so, the protein changes its shape (allosteric reaction), so that sodium can now bind, and be transported into the cell. This is called co transport because two substances are transported into the cell together, and secondary active transport because it takes advantage of the concentration gradient set up by the primary mechanism. By taking in the proper amount of sodium, you increase the concentration gradient outside of the cell, and therefore increase sodiums ability to bind to transport proteins. In doing so you not only increase glucose absorption, but as pointed out, you also further increase water uptake across the lumenal membrane of the intestine as well. {Poster’s note: this has already been calculated for you in the online PWO calculator.}
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Summary for the lazy types: It transports the nutrients to your cells and you need to replenish what you’ve lost from sweating during a workout.