The Stevioside molecule - rotatable in 3 dimensions

Stevia is a generic name for a plant originating from South America, and sweetening products derived from it. Stevia rebaudiana is a green plant classified in the Asterales ("the sunflower family").
Its leaves contain a number of sweet compounds which can be quite easily extracted and purified without chemical modification. As such, these compounds can be considered as natural sugar substitutes.

The two most common components of Stevia leaf extract are known as stevioside and rebaudoside A, and these are said to be 250-300 times as sweet as sugar (sucrose).
These compounds are glycosides, i.e. compounds containing sugars, mostly glucose. These are bonded to a non-sugar (aglycone) section of the molecule called steviol , a diterpene (composed of four isoprene units) similar to retinol and phytol.
Stevioside has 2 glucose units at one end of the molecule and 1 glucose unit at the other, whereas rebaudioside A has one more glucose.

Despite containing glucose, it is not digested like dietary polysaccharides. It is often described as a zero calorie sweetener, with none of the physiological effects of sugars like glucose on the body.

The leaves of the Stevia plant have been used as sweetening agents for hundreds of years by the people of Brazil and Paraguay. The active ingredients - Stevia glycosides - have been developed for use as sweeteners in Japan for more than 40 years - as they were seen as an alternative to artificial sweeteners saccharin and cyclamate, which were under suspicion of causing cancer. Stevia glycosides have faced a number of regulatory issues in the USA but are now considered as safe, and they have recently been approved in the EU.

Coca-Cola 'life' contains a mixture of sucrose and steviol glycosides ("natural sources") resulting in a slight reduction of the calorific value of this variant compared to the original [sugar-sweetened] version.