The shrub Stevia rebaudiana Bertoni, commonly known as Stevia, was first mentioned by the Spanish physician and botanist Pedro Jaime Esteve (1500-1556) who found it in the north-east of the territory now called Paraguay.
Guarani Indians of this area as in southern Brazil are using "ka'a he'ê" ("sweet leaf"), as it is called in Guaraní, since hundreds of years as a sweetener in yerba mate, and several tribes reported the use of this plant in the control of fertility of women, applying concentrated Stevia infusions for prolonged periods.
It is precisely this contraceptive property that is discussed since the 70s until today in the scientific literature. The reason is simple: Who wants to consume a sweetener that suddenly makes you barren?
Stevia leaf contains a complex mixture of glycosides (compounds where one or more sugar molecules are bound to a non-carbohydrate moiety). These compounds give the leaves an intensely sweet taste, about 30-45 times sweeter than sucrose, the sweet stuff of refined sugar. To date, ten different chemical compounds (chemically, all steviol glycosides) were isolated which are responsible for the sweet taste of the plant: stevioside, rebaudioside A, B, C, D, E and F, dulcoside A, rubusoside and steviolbioside. The highest concentration of the sweetening effect comes from Stevioside and rebaudioside A, responsible for the extract of Stevia being 250-300 times sweeter than sucrose with almost cero calories (about 0.2 calories per gram).
Both sweet steviol glycosides are chemically diterpenic glycosides, substances composed of two molecules of different types of sugar and a molecule called steviol. Steviol serves as "backbone" of the chemical structure and is structurally similar to the plant hormones gibberellin and kaurene. Several studies show that these glycosides are – at least partially - metabolized in the body releasing the sugar molecules and steviol.
Is It Safe to Use Stevia instead of sugar?
It is precisely this compound steviol that for many years called attention to toxicologists. In studies with bacteria and in cell-cultures it was demonstrated that this compound is genotoxic (i.e. is capable of changing the genetic information). However, more recent studies with mice, rats and hamsters, indicated that it requires relatively high concentrations of steviol to cause any considerable damage to the DNA, the molecule of life containing all our genetic information.
Browsing toxicological databases, there are hundreds of publications discussing potential adverse health effects of stevia extract, but the results are not very consistent. In particular, the effects on fertility and the potential carcinogenicity of Steviosides were subject of controversy in the scientific world. It was a study published in 1968 by Professor Joseph Kuc Purdue University in Indiana, USA, which initiated a controversial discussion about stevia and fertility. Prof. Kuc detected a clear contraceptive effect on female rats that were administered high doses of stevia. The fertility rates of the rats dropped by up to 79 percent.
While the outcome of this study was not confirmed by other scientific groups, a study published in 1999 by Prof. Melis of the University of São Paulo also reported a reduction of sperm quantity in male rats after applying high doses of Stevia glycosides. Concerns of carcinogenicity or mutagenicity were not confirmed in the vast majority of the toxicological studies.
Although adverse health effects of Stevia never really have been tested in humans directly, the authorities in the United States, Canada and the European Union considered Stevia extracts not to be safe in the application as a tabletop sweetener due to the lack of long-term toxicological studies. In contrast, authorities in other countries like Japan, China, Australia, New Zealand, Brazil and Mexico have a different point of view and accepted the use of extracts of Stevia as a natural sweetener. In several other countries, in particular in Latin American and Asia, Stevia and its extracts are available with and unverified regulatory status. In Japan, Stevia extracts are already commercially available since 1971 as tabletop sweetener and there are no reports about health problems associated with this product.
In the U.S., the Food and Drug Administration on (FDA) approved the use of Stevia extracts as "nutritional supplement" but not as tabletop sweetener. Only the glycoside Rebaudioside A in its pure form is considered as "Generally Recognized Safe Substance" (GRAS), since December 2008. In contrast, Stevioside, the other main compound of Stevia extracts, was not recognized as GRAS by the FDA.
Both, in Canada and the European Union (EU), the use of Stevia as a tabletop sweetener was prohibited based on the fact that there was insufficient evidence to prove its safety. But now this situation likely is going to change. In April 2010, the European Food Safety Authority (EFSA) performed a new evaluation of the available toxicological information. As a result of this review, Stevioside and Stevia extracts in general are now considered safe when used as a tabletop sweetener - at least under certain conditions.
EFSA established an acceptable daily intake (ADI) of 4 mg per kilogram of body weight of steviosides, the same ADI recommended by the World Health Organization according to a WHO document published in 2008. In common words, an adult weighing 70 kg can consume each day 280 mg of Stevia extract without running any health risk. As Stevia extract is about 250 times sweeter than table sugar, an adult can replace daily 70 grams of refined sugar with Stevia extract. This is equivalent to about 4-5 tablespoons or about 20 teaspoons of sugar. As children have a lower body weight, the dose should be reduced in proportion to their weight.
It is interesting to compare these data with Aspartame, the globally most used synthetic tabletop sweetener. Food safety authorities worldwide have set acceptable daily intake (ADI) values for aspartame at 40 mg/kg of body weight based on a 1980 Joint FAO/WHO Expert Committee on Food Additives recommendation (Food and Agriculture Organization of the United Nations). This means that – strictly based on toxicological available information – Stevia is considered about 10 times more "toxic" than Aspartame.
Even though Stevia sweetener is a product isolated from a plant and not product of a classical chemical process, being critical is never misplaced, because "natural" does not necessarily mean risk free. As a conclusion, Stevia extracts can be considered safe if not consumed in large quantities. The common idea that this "natural" product is safer than other commercially available tabletop sweetener is not supported by the available toxicological information.
The author is chemist specialized in the evaluation of environmental and health impacts of chemical substances. He also has two websites, related to Healthy Weight Loss Techniques and Career Development.
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