Bicarbonate is crucial to the pH buffering system. Enzymes work best at certain pHs. Carbonated drinks could be affecting pH levels in the gut and thereby altering actions of various enzymes in the gut. A small lift could result upon drinking a carbonated drink. That small lift multiplied a billion times could translate into a multi-billion dollar industry.
Seltzer, which is carbonated water, with absolutely no added ingredients may be a safe drink imparting a very small lift. Any added ingredients would ruin the seltzer. 98% of seltzer water has been ruined by added ingredients.