According to the DSM5

According to the DSM5 (APA, 2013), ADHD is characterized by a persistent pattern of inattention and/or hyperactivity-impulsivity that interferes with functioning or development, with six or more of the symptoms being persisted for at least six months. The rationale of FFD as the ADHD intervention is …..
FFD is a restricted elimination diet consisting of a limited number of foods. The FFD approach involves three steps. First, during the elimination phase, the child would follow a very restricted diet of low-allergen foods that are unlikely to produce adverse responses (including rice, meat, fruits, and water) for a short term (typically 2-5 weeks). If ADHD symptoms improve, the child enters the next phase (reintroduction or challenge phase), during which foods suspected of causing adverse effects are reintroduced in a controlled way to identify any specific food triggering ADHD symptoms. Finally, the therapy consists of avoiding those certain foods that trigger ADHD.
FFD can be helpful as shown by the evidence from previous literature findings for the effectiveness of FFD as an ADHD intervention. As early as 1980s, Egger (1985) used a markedly restrictive FFD for 4 weeks and openly challenged the responders with artificial colours, preservatives, and foods. A double-blind, placebo-controlled test was then conducted to reintroduce the foods that had been identified to provoke symptoms. Marked improvement was found in 82% of the children with hyperkinetic syndrome. Kaplan et al (1989), in their crossover study of a diet excluding artificial colours, preservatives, chocolate, caffeine, and any other implicated substances reported by the families, also found significant improvement in the behaviors of more than half of their preschool-aged hyperactive boys. Similarly, Carter (1993) showed that parents of more than half of the children who completed the open FFD trial reported significant improvement in their children’s behavior, whereas Boris (1994) demonstrated a significant benefit of eliminating reactive foods in children with ADHD.

However, it should be noted that among the four studies, three of them (Boris, Carter, Egger) involved selection of ADHD children with certain food sensitivities. Therefore, FFD may have little effect for a general population of ADHD children, but confer a significant benefit for ADHD children with suspected food sensitivities. A more recent evidence provided by Pessler (2011) on an unselected group of ADHD children, showed that the restricted elimination diet had a significant beneficial effect on ADHD symptoms in 64% of the children, and reintroducing foods led to a significant behavioral relapse in clinical responders.
Pessler 2017 conducted a review that interpreted the FFD results in the context of previous research by including the results of the first FFD meta-analysis by Benson 2007, as well as of Sonuga 2013. Based on their systematic review of meta-analyses of double-blind placebo-controlled trials evaluating the effect of FFD intervention on ADHD, Pessler 2017 suggested that the effect sizes of a FFD are medium to large, justifying the implementation of FFD intervention in ADHD children.

While all these studies showed that FFD seems to hold some promises with respect to reducing ADHD symptoms, it may be important for me to bring your attention to consider several issues before you decide to trial a FFD as the ADHD intervention for your child.
First, several FFD studies were undertaken some time ago. Therefore, it is unclear whether the findings would still apply when diets and available food items have changed so remarkably in the intervening time period. Second, a methodological flaw might probably exist in most of the FFD studies, namely the problem concerning whether the raters were truly blinded. While the outcome measures are based on parents’ and teachers’ reports of behavior rating scales, keeping such raters blind in a FFD is challenging given their roles in administering the diet and supervising whether the child is following the diet. Parents initiating the FFD intervention might have strong expectations of the FFD benefits, hence parental expectations cannot be fully ruled out as a possible cause of the behavioural improvements, since knowing the intervention might have placebo effects on the outcome. Moreover, the behavioural improvements might also be caused by increased attention for the child (Pessler 2011). Therefore, expectation bias originating from the raters may have potentially contributed to the positive outcomes in FFD studies.
Furthermore, since ADHD is a heterogeneous disorder, its multiple causes would possibly differ between individuals (Faraone SV, Asherson P, Banaschewski et al). Considering that the interpersonal variability and individual responsiveness to a diet may probably be high 102, subgroup meta-analyses of the beneficial effect of FFD on ADHD in groups of children do not provide information of whether a FFD intervention would be beneficial for an individual patient.

Seeing that the efforts to significantly limit the number of foods a child can eat may lead to conflicts that create problems in their own right, one issue is that a marked change in diet might create conflict between the child and the parent. It is important to recognize that FFD can be difficult to implement and sustain. Introducing restricted diets in ways that focus on securing the cooperation and agreement of the child is an important goal. Realistically, maintaining a child on a restricted diet over long periods would be virtually impossible without such cooperation and sustaining a child’s cooperation over time is likely to prove challenging. FFD requires strong adherence from the child as the diet needs to be strictly followed. If the child sticks to the FFD at home but eats restricted foods at any other time, such as at school, daycare, or a friend’s house, the results might end up inconclusive.

Meanwhile, it is important to consider how well the child can accept the diet Moreover, the child might feel stigmatized and isolated since they have to obey strict rules and regulations about diet habits that would likely differ from their peers (future). The child might also feel unhappy with being forbidden from having the food their friends have. Therefore, following a FFD might possibly have adverse effects on the child’s self-esteem and social status. Therefore, the child’s mental health needs to be constantly monitored (Stevenson).

While FFD appears to have a consistently positive effect in the short-term in some ADHD children, it is, however, not meant as a long-term treatment, but a diagnostic tool to identify diet-sensitive children. The actual treatment is the individually tailored diet designed after repeated challenges have identified which food items should be avoided. While it is achievable, the entire reintroduction process may take at least one year (Pessler 2011) and is considered burdensome (Pessler 2009, Millichhap 2012). Seeing that FFD is a complex and intensive process that can easily last for 18 months, this long-term approach requires highly motivated parents and children.

Since FFD usually involves only a limited number of foods, a restricted FFD for a long period can become nutritionally inadequate, especially in growing children. Hence, its implementation should be properly supervised by a qualified professional such as a dietician to avoid nutritional deficiency. Whilst undergoing the dietary restrictions in FFD, the child’s nutritional status, growth and general health have to be regularly monitored (NICE, 2008). In view of the demand of a multidisciplinary team approach to FFD, this may limit the availability of this intervention for ADHD.

Last but not least, one issue to consider is the unknown long-term effectiveness of FFD as an ADHD intervention. Symptoms changes were monitored over relatively short periods of time, no studies followed the participants longer than 9 weeks, and no studies have assessed the effect of the final individual diet. Furthermore, children might outgrow the sensitivity to the incriminating foods when they are avoided for a long period of time. Since the long-term outcomes of FFD have not been evaluated, the questions of whether treatment effects of FFD are maintained over time, and whether the ADHD symptoms would return to the former level of severity if the FFD is terminated, remain unanswered. Other considerations, such as what specific foods your child typically reacts to, how restrictive the final diets need to be, and whether the nutritional value of this diet is satisfactory, all need to be taken into accounts before implementing FFD as the ADHD intervention to help your child.
Considering the above reasons, I would probably not advise the FFD approach as a realistic recommendation of ADHD intervention for your child. However, that is just my two cents. You are more than welcome to discuss any of this in more detail with me. Please feel free to come into the office to share your thoughts with me or if you would like to know more about other alternatives. I understand your concerns and I would be more than pleased to help you and your child as much as I can.