Stevia:

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The stevia plant contains a very sweet active compund (Steviol) which is not a sugar and has no calories, thereby it does not trigger any sort of processing or response by the body.

stevia's flavour is slightly more problematic and only works in certain combinations. In most recipes I use Stevia to strengthen the sweetness that is imparted by other sweetners, to reduce the amount of sugary sweetners used in a recipe, without compromising the sweetness level of the product.