Autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) are early onset neurodevelopmental disorders that affect behavior, mood, and cognition. Dietary interventions that affect the microbiome may alleviate behavioral symptoms associated with these disorders, but evidence is inconclusive and further study is needed.
Autism spectrum disorder (ASD) is characterized by deficits in social behavior and communication, while the symptoms of attention deficit hyperactivity disorder (ADHD) include hyperactivity, impulsivity, and inattention. The two disorders are often comorbid, and they share a common genetic etiology. Although the etiology is complex and poorly understood, research suggests that genetic and environmental factors interact to influence the development and course of both disorders.
As nutrition influences neurodevelopment, diet could convey risk for, or protection against, the development and the ensuing pathology of ASD and ADHD. Dietary interventions have focused on the elimination of foods thought to aggravate neurobehavioral symptoms. For ASD, elimination diets have focused on eliminating gluten, a protein found in grains like wheat, and casein, a peptide found in milk. In ADHD, artificial food additives and oligoantigenic foods, or foods that are commonly associated with allergic reactions including turkey, pears, rice, lettuce, and water, have been the focus of intervention. As part of a typical elimination diet, foods are eliminated and then reintroduced to see whether they affect neurobehavioral symptoms.
A group of Dutch researchers recently conducted a literature review to examine the efficacy of elimination diets commonly used to treat symptoms of ASD and ADHD. The review was published in the journal European Child & Adolescent Psychiatry. They found that the efficacy of gluten- and casein-free (GFCF) diets for ASD is inconclusive, given a lack of well-designed studies with adequate sample sizes. The diets may benefit individuals with ASD who report gastrointestinal disturbance, but there is little evidence of positive effects on ASD symptoms. Further, if adopted over the long-term, these diets could put children at risk of nutritional deficiencies.
Evidence for the efficacy of ADHD elimination diets was slightly more encouraging, but remains inconclusive. The researchers found small effects for the efficacy of the elimination of food additives, but studies included children with other diagnoses besides ADHD and children with suspected sensitivities who might be more likely to respond positively to such diets. Similarly, when studies with stronger designs were reviewed, there was little evidence for the efficacy of the elimination of oligoantigenic foods. Like GFCF diets, antigenic diets could deprive children of important nutrients, but there is little direct evidence of these risks.
Further research is needed to investigate the effect of diet on neurobehavioral symptoms, and the potential mechanisms through which diet might influence symptoms. Accumulating evidence suggests that the microbiome, the environment in which gut microbes develop and reside, influences central nervous system function through various bodily pathways. Along with the central nervous system, this environment develops at birth during sensitive periods of development. Optimal development of brain and neural function, which is altered in ASD and ADHD, may depend on the health of the microbiome. The microbiome has been implicated in physical and mental health and well-being, and diet affects the microbial composition of this environment.
Some studies have shown that microbial composition differs in individuals with ASD in terms of both concentration and microbial type. Further, conditions that alter the composition may improve some ASD symptoms. The Dutch authors who published this article cite unpublished research that suggests compositional differences in the microbiome in individuals with ADHD as well, but this evidence is preliminary and further study is needed.
Clinicians have a responsibility to base treatment recommendations on existing evidence, which is currently weak for the efficacy of GFCF diets to treat ASD symptoms. Parents should be advised regarding the evidence base and the risks of GFCF diets. Although further study is needed to assess the efficacy of food additive elimination diets to treat ADHD, restricting consumption of additives by limiting processed foods is generally considered a sound strategy, as they offer no nutritional benefit. Oligoantigenic diets require strict supervision and evidence for their efficacy is currently lacking.
In general, well-designed randomized clinical trials that follow participants over long periods of time are needed to assess the efficacy of dietary interventions for children with ASD and ADHD. Future research should include reintroduction and food challenge phases. Researchers should also focus on identifying subsets of children who respond to these treatments as well as what distinguishes slow versus fast responders.
Written By: Suzanne M. Robertson, Ph.D