All 22 symptoms, including GI, cardiovascular, respiratory and skin complaints improved significantly (Table 1) during the oral supplementation of DAO from visit 1 to visit 3 (Wilcoxon p < 0.0001; Dunn p < 0.001). During the follow-up period from visit 3 to visit 5, the symptoms sum score increased (Wilcoxon p = 0.0008; Dunn p < 0.01), but, at visit 5, the symptoms sum score was still significantly lower than at visit 1 (Wilcoxon p = 0.0004) (Fig. 1).
Evaluated GI symptoms, including abdominal pain, intestinal colic, bloating, diarrhea, constipation, nausea, belching, emesis, postprandial fullness and dysmenorrhea, significantly improved from visit 1 to visit 3 (Dunn p < 0.001; Wilcoxon p < 0.0001). During the follow-up period from visit 3 to visit 5, the symptoms sum score increased (Dunn p < 0.05; Wilcoxon p = 0.0044), but, at visit 5, the symptoms sum score was still significantly lower than visit 1 (Wilcoxon p = 0.0026). A significant reduction of symptoms with DAO ingestion was demonstrated using a sub-score, for the 5 most common and diagnostic GI symptoms of HIT: bloating, diarrhea, abdominal pain, belching and fullness. (Dunn p < 0.001; Wilcoxon p < 0.0001). During the 4 weeks of follow-up, a slight re-occurrence of symptoms was indicated (Wilcoxon p = 0.0086). Although, at visit 5, these symptoms were still significantly lower, compared to visit 1 (Wilcoxon p = 0.0059) (Fig. 2).
Other symptoms of histamine intolerance
Cardiovascular symptoms, including headache, vertigo, palpitations and collapse, significantly improved with DAO ingestion (Wilcoxon p < 0.0001). During the follow-up period from visit 3 to visit 5, the symptoms sum scores slightly increased (Wilcoxon p = 0.028), but, at visit 5, the symptoms were still significantly lower, when compared to visit 1 (Wilcoxon p = 0.0033). Respiratory symptoms, including rhinorrhea, nose congestion, sneezing and asthma, significantly improved from visit 1 to visit 3 (Dunn p < 0.01; Wilcoxon p = 0.0142). During the follow-up period from visit 3 to visit 5, the symptoms sum score increased (Dunn p < 0.05; Wilcoxon p = 0.007) and reached the sum score of visit 1 (Wilcoxon p = 0.9099) (Fig. 3). The symptoms of the skin—pruritus, urticaria, flush and swollen, reddened eyelids—significantly improved with DAO ingestion (Dunn p < 0.01; Wilcoxon p = 0.001). During the follow-up period from visit 3 to visit 5, the symptoms sum scores slightly increased (Wilcoxon p = 0.0236), but, at visit 5, the symptoms sum score, when compared to visit 1, was significantly lower (Wilcoxon p = 0.0364).
The symptom intensity, for every symptom a severity score from 0 (no symptoms) to 5 (very intense), indicated by patients was reduced for all symptoms—GI, cardiovascular, respiratory and skin complaints—due to the ingestion of DAO, as shown in Fig. 3. After the follow up period, none of the scores for symptom intensity returned to the intensity score of visit 1 (Fig. 4).
During DAO ingestion, 60.7% of patients showed slightly increased serum DAO values (Wilcoxon p = 0.2918), and DAO decreased again during the follow-up period (Wilcoxon p = 0.346) (Fig. 5). Histamine values, in plasma, remained unchanged during the study period.
Diamine oxidase and histamine intolerance
Biogenic amines like histamine, putrescine, cadaverine, and agmatine are produced by bacterial decarboxylation in foods. Usually, ingesting of low amounts of biogenic amines does not affect general wellbeing. The amount of biogenic amines in food is affected by several factors, including: the manufacturing process, hygiene of raw materials, microbial composition, and the duration of fermentation (Doeun et al., 2017). If ingested food contains a high amount of biogenic amines and/or their degradation is inhibited or disturbed, histamine accumulates in the body (Reese et al., 2017). DAO is, with its ability to degrade biogenic amines, considered the main enzyme in GI and extracellular degradation of histamine (Elmore et al., 2007; Jones and Kearns, 2011). It is synthesized by apical enterocytes located in the intestinal villi and is released from the mucosa for digestion and into the blood circulation (Wollin et al., 1988). DAO from the white pea (Lathyrus sativus) reportedly modulates histamine toxicity in vitro. A combination of DAO and catalase protected against histamine toxicity and prevented H2O2-induced damage on the human intestinal Caco-2 cell line occurring during histamine’s oxidative deamination (Jumarie et al., 2017). An immune-regulatory influence of histamine within the gastrointestinal tract is known, but the effect of histamine on gut pathology, including inflammatory processes, is still poorly defined (Smolinska et al., 2014).
Diamine oxidase in serum
Recent findings speculated that low serum DAO values may be responsible for the symptoms of HIT (Kacik et al., 2018). Although, serum DAO values have not been established to reflect gastrointestinal DAO activity. However, the diagnosis of HIT, with the presence of two or more functional non-specific GI symptoms, may be supported with measurements of DAO values in serum (Reese et al., 2017). In this study, for diagnosis of HIT, besides functional nonspecific abdominal complaints, serum DAO values < 10 U/mL were used. Throughout the study, we have demonstrated slightly increasing serum DAO values, due to oral DAO supplementation.
A histamine-reduced diet was effectively shown to improve symptoms and potentially elevate serum DAO, after approximately 2 months. Depending on the compliance with a histamine-reduced diet, an improvement of HIT-related symptoms was demonstrated in nearly 80% of patients. Additionally, symptom improvement, combined with an increase of serum DAO values, occurred in more than 50% of patients (Lackner et al., 2019). Patients described GI symptoms to be the most prominent and severe of the HIT-related symptoms, along with a variability of symptom combinations (Doeun et al., 2017). It was reported that DAO capsules, taken in combination with histamine-containing tea, reduced symptoms of HIT (Komericki et al., 2011). Recently, oral DAO supplementation was reported as effective in patients with headaches. It significantly reduced the duration of migraine attacks (Izquierdo-Casas et al., 2019) and, in another study, caused symptom relief in patients with urticaria (Yacoub et al., 2018). We have demonstrated a significant reduction of all HIT-related symptoms with the oral supplementation of DAO. Before this study, 79% of the included patients complained of postprandial fullness and, 68% had bloating and abdominal pain. After 4 weeks of DAO ingestion, these symptoms were significantly reduced to 50% bloating, 43% postprandial fullness and 25% abdominal pain. Interestingly, we recognized that symptoms at the end of the study did not reach the same level of severity as at the beginning. We speculate that a slight recovery of the intestinal mucosa, due to the ingestion of DAO may be responsible. At visit 5, only the respiratory symptoms reached the same symptom sum scores as at the beginning of the study. This, we attributed to the fall season and its increased number of colds. However, a placebo effect, influencing the improvement of symptoms, in this study cannot be ruled out (Eisenbruch and Enck, 2015). Additional studies, including placebo-controlled randomized trials with high numbers of patients and long treatment periods, are needed to fully evaluate oral DAO supplementation.
Generally, a nutritionally adequate histamine-reduced diet can be developed, based on the individual’s symptomatology, by reducing the amount of ingested histamine and biogenic amines. A histamine-reduced diet may present a challenge to HIT patients, because the composition of biogenic amines and levels of histamine in food and drinks are frequently unknown. In general, at least roughly 20 percent of patients with food intolerance/malabsorption cannot comply a comprehensive diet plan (Wilder-Smith et al., 2017). Therefore, we conducted this study in HIT patients and found that oral DAO supplementation helps reduce symptoms. We speculate that this may be due to its ability to degrade ingested intestinal histamine.
In conclusion, we have demonstrated that oral supplementation of the enzyme DAO before meals could help improve HIT-related symptoms and symptom intensity in HIT.