Sodium bicarbonate likely improves high-intensity exercise performance by buffering the production of hydrogen ions, preventing a drop in pH in the blood or the muscles.[1][2][3][4][5][6][7]
The gastrointestinal side effects caused by high doses of sodium bicarbonate (bloating, nausea, vomiting, etc.) are probably caused by a reaction between bicarbonate and stomach acid, which releases CO2 in the stomach.[1][2]