First, the amounts are usually tiny; for example, you’d have to drink almost five 12-ounce cans of Diet Coke Plus to get your RDA of vitamins B3 (niacin), B6, and B12.   Second, certain water-soluble vitamins break down over time in an acidic environment like soda, says DerMarderosian, particularly if left unrefrigerated. Drinks like Vitamin Water may avoid this, but they usually are full of sugar.   A better idea: Get your vitamins from a well-balanced diet.