Abstract 

Diamine oxidase (DAO) supplements have gained increasing attention in recent years due to their potential to support DAO deficiency, histamine intolerance and related symptoms. However, choosing a reliable and trustworthy DAO supplement can be a challenging task for patients and Healthcare practitioners. One of the main concerns is the lack of regulatory oversight on dietary supplements, which may result in misleading or incomplete labelling, incorrect dosages, and inadequate quality control. Such situations may lead to patients consuming supplements with insufficient amounts of active enzyme or with questionable purity, potentially resulting in undesired health outcomes. Thus, we tested the DAO activity of a variety of already in-the-market supplements and compared them with one another and against the specified activity, if any, by the manufacturer. Our results show a great discrepancy in most of the products and significant differences in DAO activity between different manufacturers.