Histamine exists in a multitude of foods and displays an emerging role within food intolerances. Our aim was to identify the activity of porcine diamine oxidase (DAO) required for the in vitro degradation of histamine amounts that are found in typical meals containing histamine (75 mg, equaled 150 mg/L). Furthermore, we investigated an actual dietary supplement that is commercially available for histamine intolerant individuals for its histamine reduction capability. Kinetic investigations of porcine DAO showed a substrate inhibition by histamine concentrations greater than 56 mg/L (0.5 mM). The stability of free porcine DAO was tested in a fed state simulated intestinal fluid and exhibited a half-life period of around 19 min. A total of 50 nanokatal (nkat) free porcine DAO, which equaled the amount of enzyme isolated from around 100 g pig kidney, were necessary for the in vitro reduction of around 90% of the histamine. The dietary supplement that contains a pig kidney extract did not show DAO activity. Instead, the used histamine (0.75 mg) was apparently reduced due to the adsorption of histamine onto a capsule component by 18.9 ± 2.3% within 5 hr. Although the capsule preparation retained its overall structure and shape for at least 90 min in simulated gastric fluid, the apparent histamine reduction was significantly reduced to 12.1 ± 2.3% (P ≤ 0.05). In conclusion, an alternative to the pig kidney DAO or an improved capsule preparation is needed to ensure an adequate supplementation for histamine-intolerant humans. PRACTICAL APPLICATION: Histamine intolerance is an emerging issue in our society and the intolerance-related physiological symptoms are currently not reliably treatable due to a lack of scientific investigation. A commercially available dietary supplement for histamine intolerance does not fulfil the requirements for a satisfactory histamine reduction in intolerant humans. The activity of the histamine degrading enzyme diamine oxidase, required for a satisfactory histamine degradation, is by far higher than the theoretical amount apparently given in the dietary supplement. With this knowledge, it is obvious that improved food supplements must be developed to help histamine intolerant humans.