Histamine intolerance is a presumed set of adverse reactions to ingested histamine in food believed to be associated with flawed activity of DAO and HNMT enzymes. This set of reactions include cutaneous reactions (such as itching, flushing and edema), gastrointestinal symptoms (such as abdominal pain and diarrhea), respiratory symptoms (such as runny nose and nasal congestion), and neurological symptoms (such as dizziness and headache). However, this link between DAO and HNMT enzymes and adverse reactions to ingested histamine in food is not shared by mainstream science due to insufficient evidence. The exact underlying mechanisms by which deficiency in these enzymes can cause these adverse reactions are not fully understood but are hypothesized to involve genetic factors. Despite extensive research, there are no definitive, objective measures or indicators that could unambiguously define histamine intolerance as a distinct medical condition.