Stevioside mutagenicity tests

Compounds used as or added to food must be absolutely safe. This means that not only the added parent compound, but also its possible metabolites must be safe, for the possibility exists that compounds that themselves are not harmful can be taken up by the human body and be metabolised into products that may have some harmful effects. Therefore, the parent compounds as well as their metabolites should be thoroughly tested in toxicological studies. In one such study, steviol, the aglycone of stevioside, showed a weak mutagenic activity [1] and, although in later experiments these results could not be reproduced [2], this has led to a whole controversy in scientific literature. 

Only the bacteria from the caecum or colon were able to degrade stevioside into steviol (caecum of mice, rats and hamsters colon of man). The bacteria from the human colon also formed steviol epoxid in vitro, that was again metabolised to steviol. However, in vivo this epoxid formation probably will not occur due to the anaerobic conditions of the human colon. It was correctly concluded that steviol is the only possible metabolite [3]. Anyway, steviol epoxid has been tested in mutagenicity studies and showed to be inactive [1]. 

In 1985 it was published [1] that stevioside was completely safe but that metabolically activated steviol was mutagenic in a "Forward Mutation Test". Steviol had to be applied together with the microsomal fraction of liver of animals treated with carcinogenic compounds (polychlorinated biphenyl or phenobarbital plus 5,6-benzoflavone). This publication has led to a confusing discussion between advocates and opponents of the use of Stevia or stevioside. To unravel the problem we must consider the opinion of authoritative international organisations such as OECD (Organisation for Economic Co-Operation and Development) and ICH (International Council of Harmonisation). To accept new substances as food three different mutagenesis tests are accepted and required by the OECD as well as by the ICH. These can be seen in Table 1. 

Studies on the effect of stevioside on fertility found that it produced no abnormal mating performance or fertUity, and produced no teratogenic effects. Various tests have not shown mutagenic or genotoxic activity however, mutagenic activity of metabolized ste-viol has been reported. ... 

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