Stevia is a natural sweetener made from the leaves of a South American herb, Stevia rebaudiana Bertoni, commonly known as sweetleaf or sugarleaf.

It's a rather controversial herb, in that it has been approved as a food additive in several countries, including Brazil, China, and Japan, but has been banned in Europe, Hongkong, and Singapore.

In America, we've gone at it both ways: Stevia can be sold as a dietary supplement, but it cannot be promoted as a sweetener or used as a food additive.

The FDA categorizes stevia as "an unsafe food additive" because "there is no regulation in effect that provides for the safe use of stevia, nor is there a sufficient basis to conclude that stevia is generally recognized as safe among qualified experts for its intended use in food."

In fact, says the FDA, "literature reports have raised safety concerns about the use of stevia, including concerns about control of blood sugar and effects upon the reproductive, cardiovascular, and renal systems."

Stevia has been labeled as an "unsafe food additive" since 1991, when its import was restricted at the request of an anonymous party. The complainant has never been identified, but many stevia enthusiasts believe that the sweetener industry was behind the complaint. Stevia remained banned until 1995, when the 1994 Dietary Supplement Health and Education Act forced the FDA to allow stevia as a dietary supplement, although not as a food additive.

There have been some investigations into the safety of stevia, most of which are criticized as poorly done by stevia enthusiasts. According to one study, male rats fed high doses of stevioside (the component of stevia that makes it sweet) for 22 months ended up with reduced sperm production.

When female hamsters were fed large amounts of a derivative of stevioside called steviol, they had fewer and smaller offspring. In the laboratory, steviol can be converted into a compound that may promote cancer by causing mutations in DNA; however, whether that conversion happens in humans is questionable. Finally, very large amounts of stevioside reportedly can interfere with the absorption of carbohydrates in animals and disrupt the conversion of food into energy within cells.

Stevia proponents brush aside these studies as flawed, preferring to point to recent research by the World Health Organization (WHO), which concluded in 2006 that stevioside is not genotoxic, that the genotoxicity of steviol in the test tube doesn't occur in living creatures, and that evidence of carcinogenic activity does not exist.

During thirty years of stevia use in Japan, no adverse reactions have surfaced. On the other hand, the Japanese are apparently moderate in their stevia consumption, whereas there is some concern by the powers that be that we gluttonous Americans would pile on the stevia to potentially toxic levels. It does appear, however, that if you use stevia leaves sparingly, you should be okay.

Sources: FDA, August 2007; Medline Plus; Wikipedia; Center for Science in the Public Interest