Funding. This manuscript is based on work performed with funding from the Office for Dietary Supplements commissioned through the Agency for Healthcare Research and Quality (Contract No. 290-2007-10055-I). None other than the authors had any role in study design, collection, analysis, or interpretation of data, in the writing of the report, or the decision to submit the paper for publication. The authors are independent from the funders.

Competing interests. We declare that none of the authors has a conflict of interest in this submission. All authors have completed the Unified Competing Interest form at (available on request from the corresponding author) and declare that no author has support from companies for the submitted work. No other relationships or activities that could appear to have influenced the submitted work.