Vicky Manning

I've actually done some studying on this.
Regardless of gastric surgery or not soda's even diet soda can lead to weight gain indirectly.

The super sweet taste awakens the desire for sweets. And MAY lead to other indulgences. The sweet taste can over power your taste buds and make otherwise perfectly normal sweet tasting things to be less sweet, which MAY lead to adding sweetener and may loop around and make cause #1 more plausible. CAFFEINE can be a diuretic and cause you to dump more Water than you take in, which MAY lead to further soda intake - etc. If you're not getting enough water anyhow it may actually cause dehydration. While a can once in a while may be a treat the fact may be that since you already have needed surgical help to loose weight you may not be strong willed enough and allow something as "innocent" as a sweet diet soda to put you back into the bad eating habits that caused your obesity in the first place. WATER is the best liquid. Personally, I still have a tough time choking down tap water but I force myself to stay hydrated. Personally I don't want a (former) favorite treat to lead me back down a wrong path.
WARNING: IANAD - the opinions and theories expressed here are my own.