Here’s a great Q&A about flat vs sparking water from the experts at www.Self.com:
Q: Is there a health-related downside to drinking lots of sparkling water or seltzer?
A: For the most part, you're safe with bubbly water. Fizzy H2O has been popular for hundreds of years and remains a great no-calorie refresher. The effervescent sips are simply plain water infused with carbon dioxide, which is bad for the atmosphere but safe for us to ingest. Many dieters enjoy these drinks as an alternative to regular water, and they are just as hydrating. Another plus: Seltzer doesn't cause bone-density loss (colas may), and although some people experience bloating and gas due to the carbonation, they're not a health concern. The only downside to soda water is that it's slightly acidic, and scientists speculate that it might erode tooth enamel. So to protect your pearly whites, choose regular water, which has no acid but can be fortified with good-for-your-teeth fluoride. Finally, skip club soda. It contains sodium and might not quench your thirst.