Report Why is carbonated water bad for us? in Post-op Gastric Bypass Posted June 21, 2011 I've read the literature on how diet soda can trigger weight gain in some patients, so that's definitely out, but what's so bad about carbonated water? We have free Talking Rain sparkling water at work and I've been very tempted to try some, especially on hot days. Is the carbonation itself bad for us? Can sparkling water lead to weight gain or other adverse effects for gastric bypass patients?