to contribute little to the overall methyla­ tion in vivo (Vahter and Gustafsson 1980) because iAsV and iAsIII are rapidly absorbed in the small intestine (Vahter 1983), espe­ cially when As is ingested in a soluble matrix (e.g., drinking water). However, soil­bound and/or dietary­ bound As may follow a dif­ ferent digestion scenario in the gut, and a large fraction may end up in the colon lumen, where it is subjected to the resident micro­ bial community. The finding of MMMTAV and the highly toxic MMAIII as metabolites from human colon microorganisms indicates that presystemic methylation will not lead to detoxification. In addition, in vitro stud­ ies with Caco­2 human epithelial colorectal adenocarcinoma cells suggest that the absorp­ tion of methylated arsenicals (DMAV, 10.0%; TMAO, 10.9%) is more efficient than that of iAsIII (5.8%) and iAsV (1.6%) (Laparra et al. 2005, 2007). Intestinal absorption of methy­ lated thioarsenicals should be examined in future research. Regarding the variability between indi­ viduals regarding presystemic As metabolism, we investigated the gut microbiota from only one human. Interindividual variability in gut microbial composition is very high; thus, we expect gut microbiota from different individ­ uals to display distinct As metabolic profiles. Such interindividual variation in metabolism by human gut microbiota was previously reported for ingested phytoestrogens (Bolca et al. 2007) and, interestingly, also for the metalloid bismuth (Michalke et al. 2008). Therefore, variability in gut microbial As metabolism should be given the same atten­ tion as the genetic variations that may gov­ ern interindividual differences in As response (Hernandez and Marcos 2008). Conclusion The present study shows that presystemic metabolism of soil­derived As may be rel ­ evant in the human body when significant amounts of As become available to colon microorganisms. 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