*3.6. Anamnestic Intolerance Prior to OD and Observed Sensitivity during Food Reintroduction*

In an initial anamnesis, the children were asked about their eating habits. In addition to "likes" and "dislikes," they were also asked about suspected sensitivity as a possible trigger of ADHD. Of the 16 subjects, eleven (68.8%) did not match the information in the anamnesis questionnaire with the sensitivities found later. Of the 16 subjects, there is a match between a favorite food (eggs) and a sensitivity only in one participant (6.25%). One presented (6.25%) a correspondence of a dislike (fish) with an incompatible food. In three of the 16 subjects (18.75%) the suspicion was confirmed. The questionable foods are milk (2) and soy (1). In all three participants, physical symptoms in addition to ADHD symptoms occurred after consuming these foods.

#### **4. Discussion**

A main aspect of the study is to show that OD is a useful method to find out if there are food related changes of ADHD symptoms in children. Furthermore, our study is the first to investigate the individual behavioral responses on different foods related to ADHD symptoms.

Pelsser recommends that a change in diet should be considered in all children with ADHD, as Pelsser's study showed a significant effect in children with ADHD and ODD of the elimination diet. This requires medical supervision and parental cooperation when following this restrictive elimination diet [14]. In a previous study we could also show the positive effects of OD in children with ADHD [22].

Before starting the diet, it is not obvious if children react to food, and when they do so, what kind of foodstuff result in an increase of ADHD symptoms. Children included in this study did react after 4 weeks of diet. The participants showed individual food sensitivity concerning type of food, intensity or pattern of reactivity. In all patients, ADHD symptoms were intensified by various foods during the food challenge after OD.

After detecting food related ADHD symptoms by OD, it is important to find out, by reintroduction, to what kind of food the children are sensitive. In our small sample we could demonstrate 27 different foods which increased the symptoms of ADHD in our participants.

Comparing this study with previous studies on the OD by Egger et al., a concordance of frequency of intolerance more often regarding milk products can be shown. Cow's milk sensitivity occurred in 64% of the cases in Egger and Carter, in 60% of the cases in Hiedl, and in our current study in 68% of the cases. From this, one can conclude that the most common food intolerance that leads to an increase in ADHD symptoms is cow's milk. However, not all of the food sensitive children did react to milk, so there should never be a general recommendation to avoid milk in the context of ADHD. In all four studies, wheat and grains in general could be detected as common provoking foods. Egg was also more often on the list of intolerant foods in all studies. [11,13,14,16,37,38], but only individual dietary recommendations should be given in the context of the individually detected food sensitivity.

The study also shows that almost every participant experienced an increase of far more than two points in ACS after the food challenge. The amount and type of reaction were individually different. The strongest reaction was seen in one subject after taking paprika. There was an immediate increase in ADHD symptoms by 25 points as measured by ACS.

It could not be shown that a connection between certain foods and behavior can already be established during the pre-diet phase by evaluating the food and behavior diary. The pre-diet phase should, however, continue to be at the forefront when carrying out an OD to observe usual eating habits and to train the protocoling. It is also useful to have a pre-diet control protocol in order to test whether the change in daily life when focusing on nutrition and behavior of the child does influence ADHD symptoms or not.

An OD with subsequent reintroduction can be the most useful diagnostic tool to identify individual food sensitivity in connection with ADHD. Because every participant reacted very individually to different foods, there must be an individual dietary recommendations for each individual child.

Several possible different pathomechanisms are noted to cause an increase in symptoms. Most children (70%) reacted to milk products. Of these, 73% did not react to dairy products which were free of lactose. This leads to a hypothesis that the pathomechanism of lactose intolerance could be connected to ADHD symptoms. In a previous study from

Edreffy et al. [39], the differences in oligosaccharide metabolism between ADHD and healthy controls were described. This study supports the hypothesis that carbohydrate metabolism differs in ADHD subjects compared to control. Alabaf et al. [40] investigated physical health in children with a neurodevelopmental disorder on the basis of the database of The Children and Adolescent Twin Study in Sweden (CATSS). Their results showed that children 9–12 years of age diagnosed with ADHD suffered more than twice as often from lactose intolerance as the age-matched total population. The occurrence of celiac disease was also described to be higher in patients with ADHD. The prevalence of celiac disease increased significantly in patients with ADHD and comorbidity such as autism spectrum disorder and learning disorder.

We found ADHD related symptoms worsening in children after the consumption of different grains. This could indicate probable digestive problems in the context of different grains. Niederhofer et al. [41] reported an overrepresentation of celiac disease, identified by measuring the celiac specific antibodies anti-gliadine and anti-endomysium in patients with ADHD. Ten out of 67 patients with ADHD were diagnosed with celiac disease. A significant improvement of ADHD symptoms under gluten-free diet was observed by patients and parents. In our study, we also found remission of ADHD symptoms in grain sensitive children after three days of grain-elimination during the food reintroduction.

"Brain-gut axis" and "microbiome" have also been shown to be related to mental disorders [42,43]. Though the microbiome in humans shows a high interpersonal variation, its composition is influenced by geography, culture, and diet [44,45]. The type of nutrition plays a decisive role in the composition of the microbiome. Despite the evidence that abnormal development of the intestinal microbiome has long-term implications on host health, the causal contributions of abnormal intestinal microbiome variations to disease states have yet to be elucidated [45].

One can therefore consider whether there is a complicated mechanism behind ADHD symptoms in relation to food intake. The microbiome can be regarded as a trigger or amplifier of ADHD symptoms [43]. Kumperscak et al. found improvements in children´s behavior after three months probiotic treatment with "*Lactobacillus rhamnosus*" [46]. A randomized controlled trial published in 2016 showed a significant improvement in ADHD symptoms in children with autism after supplementation of "Lactobacillus Plantarum PS128" [47]. These results reinforce the hypothesis of microbial influence on ADHD symptoms.

Pelsser investigated the immunological response comparing allergic and non-allergic reactions in food intolerances and food allergies in children with ADHD. IgE is implicated in typical food allergies. If reactions to foods are not mediated by IgE, the assessment of IgG levels might be useful, when considering the aim of establishing a relation between foods and ADHD. According to this theory, eating foods that induce high IgG levels would lead to a substantial behavioral relapse whereas eating foodstuff that induces low IgG levels would not. The results did not support this hypothesis [14].

This suggests that food sensitivities in ADHD are not necessarily allergic reactions. However, a cell-mediated allergic reaction has not been investigated and therefore cannot be excluded. A pilot study from Dieterich et al. [48] could identify inflammatory processes in the gut reflected in inflammation-related intolerance reactions to foods, without showing a systemic inflammation in blood parameters. Food intolerances relate to high interferon IFN-γ concentrations in different gut regions. Gut mediated reactions to food intolerances showed differences to inflammation parameters in food allergy. In different gut regions, they found an increase of IFN-γ which might result from an unspecific immune response to an intestinal dysbiosis in the intestine and to a release of micro-biotic peptides as described by Farin et al. [49]. This might explain our results concerning the individual reactions to intolerant foods in food reintroduction. We found different intensities of reactions and individual time-courses to recovery. This might be related to a highly individual gut microbiome and subsequently its metabolites, which can stimulate local inflammatory processes in the gut.

This supports the theory that the composition of gut microbiome plays an important role in this context.

Whatever mechanisms a dietary intervention relies on, Stevenson et al. [50] pointed out in their research review form 2014 that a restricted elimination diet might be beneficial for ADHD symptoms in children and adults.

#### *Limitations and Future Directions*

The evaluation of the daily behavior could, besides dietary intervention, also be impacted by various factors such as social interactions or physical health.

For the assessment of ADHD symptoms, they must occur in at least two contexts (usually at home and at school). Unfortunately, teacher´s ratings could not be collected in total. This is a clear limitation of the study.

The study was open, non-randomized, without a control group and without blinding the diet. Focusing on behavior and eating habits while implementing serious changes in daily life might lead to a remarkable bias in parent's ratings. To corroborate these preliminary findings, an extension of the study with a larger number of subjects would be important to confirm the effects already observed.

Children had to control their eating behavior by themselves. Whether children always provided their information on food consumption truthfully is not guaranteed. Although the importance of correct information for the diary was clearly explained, there certainly were undocumented dietary violations.

#### **5. Conclusions**

The European treatment guidelines on ADHD recommend that restricted elimination diets may be beneficial for children with ADHD and with a history of adverse reactions to food. However, our data indicate that there are many ADHD children without a history of adverse reaction to food that may profit from OD and subsequent identification of highly individual food sensitivity.

In summary of the available results, Oligoantigenic Diet seems to be a useful tool to identify food sensitive ADHD patients. Subsequently, detected individual food sensitivities leading to individualized dietary recommendations are useful as an additional option to the existing multimodal therapy concept.

**Author Contributions:** E.Y., L.B., N.B., C.C., H.-W.C., C.F. contributed to the literature search, E.Y., L.B., N.B., K.S.-M., H.-W.C., R.R., E.S., C.C., C.F. to the methodology, E.Y., L.B., N.B., H.-W.C., R.R., E.S., C.C., C.F. the interpretation of the data and E.Y., L.B., N.B., H.-W.C., R.R., E.S., C.C., C.F. to the writing of the manuscript. All authors had full access to the data. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** The study was approved by the local ethics committee (application number 111/14) in accordance with the World Medical Association's Declaration of Helsinki.

**Informed Consent Statement:** Informed consent and assent were obtained for all participants involved in the study.

**Data Availability Statement:** The data are not publicly available according to description of confidentiality and data sharing procedures described in the study's informed consent and assent documents.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

