3 of 5 Changed "soda" to "cola" to disambiguate

Is carbonated water unhealthy?

I have a coworker who says that drinking soda leads to kidney problems. I did some Googling and while I found a lot of blogs and message board posts saying such, the only credible link I found was this paper, Carbonated beverages and chronic kidney disease, whose abstract reads:

Drinking 2 or more colas per day was associated with increased risk of chronic kidney disease. Results were the same for regular colas and artificially sweetened colas. Noncola carbonated beverages were not associated with chronic kidney disease. These preliminary results suggest that cola consumption may increase the risk of chronic kidney disease.

I can't read the entire paper, but the abstract makes it sound as if the risk lies more in soda consumption than carbonated water consumption.

My question is two-fold:

  1. Is carbonated water, regardless of its source, unhealthy? The study I cited seems to say no, but many websites - albeit some that are questionable - state otherwise. Are there other rigorous studies that find otherwise? Is the study I cited considered the official say on the matter?
  2. Why would carbonated water be linked to kidney problems? And if it's not carbonated water, but cola, what about cola makes it unhealthy with regards to kidney function?

Thanks