What Zero Calories Really Means
Though labeled as zero-calorie, most diet sodas contain trace amounts of calories from artificial sweeteners and other additives. This is legal under FDA regulations, which allow anything with fewer than 5 calories per serving to be labeled as calorie-free.
The Truth About Labels
While the number of calories is small, consuming large amounts of diet soda may still impact your daily calorie count.
Conclusion
Diet soda may not be completely calorie-free, but the calories are minimal unless consumed in large quantities.