So I was thinking about how I need sugar, or maybe acid, in my drink to really make me feel like my thirst is quenched (just drinking lots and lots of water doesn't seem to do it). And perhaps that's due to highly successful manipulation by soda companies, because soda (carbonation + sugar + acid) seems best at quenching thirst.
Anyways, I'd heard that drinking soda actually makes you more dehydrated. Is that true? The can of Sprite in front of me has 38 g of sugar in 355 mL of solution. At a molar mass of 180 g/mol (for glucose/fructose), this comes out to 0.59 M, or 590 mOsm/L. A saline solution has 9 g of NaCl (58.4 g/mol) per 1 L of solution, i.e. 0.154 M. And it dissociates completely, so it has an osmolarity of 310 mOsm/L.
So it's true! Well, maybe not. According to Wikipedia, urine has an average osmolarity of 500-800 mOsm/L without really trying (i.e. with a restricted fluid intake, healthy individuals produce higher concentrations). So maybe you're still hydrating when you drink 590 mOsm/L Sprite (I'm not counting the citric acid or carbonates, but my gut feel is that those are negligible compared to the sugar). The scary part is that it's just about half the concentration of 3.5% salinity sea water (35 g/L NaCl = 0.6 M = 1200 mOsm/L).
Update: SC points out a pretty fundamental flaw in my reasoning: sugar is special. There is active transport and all sorts of metabolism and other reactions that happen, and your kidneys try very hard not to even get any into the glomerular filtrate. So we can probably ignore the sugar, which most likely makes soda hypotonic. It'd be different if it were a 590 mOsm/L salt solution.
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