Reb-A (Stevia Extract) is an all-natural sweetening ingredient extracted from the stevia leaf that is used in powder or liquid form to heighten sweetening intensity and improve taste.
The stevia leaf contains a number of very similar compounds or steviol glycosides that are all sweet. The best tasting of these is Rebaudioside A. In its highly purified form, it has sweetness between 250 to 400 times that of an equal weight of sugar.
For Rebaudioside A, the R1 position is a single glucose molecule, the R2 position is two glucose molecules. If Rebaudioside A only had the same sweetness as sugar it would have much more calorific content than sugar. However, nature has produced a molecule that is hundreds of times sweeter than sugar and therefore a very small amount is needed to sweeten a product meaning it contributes essentially zero calories.