**1. Introduction**

With a worldwide prevalence of 5.3% among children and adolescents, Attention-Deficit/Hyperactivity Disorder (ADHD) is the most common behavioral disorder [1,2]. ADHD occurs across cultures in about 5% of children and about 2.5% of adults. In children there is a gender related ratio of 2:1 (male: female), [3]. The mechanisms triggering ADHD have not yet been fully identified. Genetic predisposition and pre-, peri- and postnatal environmental influences play a decisive role as do multiple interacting factors [4–6]. Nutrition also plays a role in the development of ADHD [7]. Previous study results have supported the theory that ADHD is an expression of a genetically determined neurodevelopmental disorder. Depending on the degree of severity, the guidelines recommend different treatment options, including parent training, behavioral therapy, pharmacotherapy, and dietary interventions [8–10].

As early as 1922, Shannon noted an increase in restlessness and sleep disorder in children in association with food allergies. After eliminating foods such as tomatoes, eggs

**Citation:** Yorgidis, E.; Beiner, L.; Blazynski, N.; Schneider-Momm, K.; Clement, H.-W.; Rauh, R.; Schulz, E.; Clement, C.; Fleischhaker, C. Individual Behavioral Reactions in the Context of Food Sensitivities in Children with Attention-Deficit/ Hyperactivity Disorder before and after an Oligoantigenic Diet. *Nutrients* **2021**, *13*, 2598. https://doi.org/ 10.3390/nu13082598

Academic Editor: Roser Granero

Received: 23 June 2021 Accepted: 26 July 2021 Published: 28 July 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and grains, there was reduction or even disappearance of ADHD symptoms [10]. In 1983, Egger et al. were the first to carry out the Oligoantigenic Diet (OD)—as a dietary diagnostic method to identify food allergies in the field of allergology—in the context of ADHD. Foodstuffs during diet were consciously reduced to very few hypoallergenic foods. The choice of food was initially kept to a minimum. Approved foods were hypoallergenic, mainly including foods which rarely caused adverse reactions. Throughout the OD the ADHD patients showed significant improvements: of 76 participants in this study, 62 children reduced their symptoms. Furthermore, 21 patients no longer met diagnostic criteria for ADHD. Most children responded to two to seven different foods. Re-exposure to the foods caused reappearance or intensified symptoms of ADHD [11]. A study by Pelsser et al. (2009) also showed a reduction in ADHD symptoms after an OD. Here 60% of the participants showed a reduction in symptoms of at least 50% measured with the ADHD Rating Scale [12].

Since then, further studies have shown that nutrition is a strong mediator and/or moderator of ADHD symptoms [8–22]. Dietary interventions in ADHD including elimination diets have shown highly significant effects [13–15,17–22] with effect sizes up to Cohen's *d* = 5.0 in unblinded studies [7].

Severe dietary interventions such as restricted elimination diets do have a clear impact on daily life and therefore cannot be kept under blinded conditions. In order to minimize the various biases which influence the assessment of ADHD symptoms, Dölp et al. [22] used blinded video ratings to evaluate their primary outcome diagnostic tool, ARS, in the context of dietary intervention. The results showed hardly any difference between blinded and unblinded ratings. Dölp et al. found that OD can lead to symptom reductions in food sensitive children and adolescents with ADHD. After 4 weeks of diet, approximately 60% of the patients showed significant improvement in their condition in ARS [22].

The objective of the present study is to answer the following questions: is it possible to identify foods that intensify typical ADHD symptoms in children by applying OD? Do the participants show different reactions to the same food? Can individual food sensitivities already be identified in a pre-diet phase? Can strong leads to later diagnosed food sensitivities be seen already in the anamnesis?
