hmm, http://journals.cambridge.org/action/displayFulltext?type=6&fid=901744&jid=&volumeId=&issueId=01&aid=901740&fulltextType=RA&fileId=S0029665104000060

'Conclusions

While the evidence is not overwhelming, it suggests that Cr is an essential trace element in mammals, including man, and affects carbohydrate and lipid metabolism through the action of insulin. The daily requirement for human subjects is small, i.e. approximately 30 μg, such that it is difficult for healthy individuals to develop Cr deficiency. Thus, the use of Cr supplements is probably unnecessary for the general public. However, the use of certain Cr supplements, such as Cr(pic)3, is probably harmful. While nutritional supplement levels of Cr do not appear to have beneficial effects, pharmacological quantities of Cr may increase insulin sensitivity in both healthy subjects and subjects with type 2 diabetes. The biomolecule chromodulin may be part of an insulin-potentiating pathway and may explain the requirement for Cr.'