Gluten- and casein-free diet makes a meal of autism science

From the moment a child is diagnosed with autism, their family enters the unknown. Conference halls are lined with salespeople, letterboxes are stuffed with pamphlets, and life is transformed into a whirlwind tour of a fantastical array of therapies and potions that are positioned as the “cure all” for their child’s difficulties.

Diet modification is one of the most prominent alternative therapies. A diet free of gluten (found in wheat, rye and barley) and casein (the main protein in dairy products) attracts more attention than any other and is estimated to be given to approximately 40% of individuals with autism spectrum disorder (ASD).

In some cases, clinicians recommend diets; in others, they’re instigated by parents eager to find anything that may help their child. But there is currently little scientific evidence the gluten-free, casein-free (GCFC) diet has beneficial effects for children with autism.

The theory

The theory most commonly associated with the use of the GCFC diet relates to proposed differences in the functioning of the gut of individuals with autism.

Our bodies extract nutrition from food through the intestines, which is where small molecules cross the mucosal lining and enter our bloodstream. It is thought that some individuals with autism have a “leaky gut” – that is, their intestines are more permeable than normal – and this may allow molecules to enter the bloodstream that should not otherwise be there.

The GCFC diet is based on the theory that opioid peptides, formed from the incomplete breakdown of foods containing gluten and casein, may enter the bloodstream due to the increased intestinal permeability. From there they cross the blood–brain barrier and disrupt brain development and functioning.

On the face of it, this theory has some plausibility.

The gut has its own nervous system, called the enteric nervous system, which is how we know when we are hungry and when we need to go to the toilet. The enteric nervous system is closely related to the development of the central nervous system, including the brain.

Extensive research has also shown that a significant minority of individuals with autism – perhaps around 30% – experience considerable difficulties with gut functioning at some stage in their lives.

The assumptions here are that the gut difficulties experienced by individuals with autism are causally related to the differences in brain development. By using the GCFC diet, gut problems are reduced and autistic symptoms should improve.

This is a neat theory, but unfortunately there are some evidence gaps that need to be plugged.

The problems

The first problem relates to testing the underlying theory itself. If an inability to break down gluten and casein leads to an excess in opioid peptides, then we would expect to find high levels of these molecules in children with autism.

However, three studies have found no evidence of excessively high levels of opioid peptides in the urine of children with autism. Other studies have questioned the existence of a “leaky gut” altogether.

The second problem is the results of studies that have tested the effectiveness of GCFC diets with children with autism. A 2014 systematic review found major methodological limitations of studies in this area. These included lack of a control group, poor definitions of inclusion criteria, and very small sample sizes.

The studies that found a positive effect of GCFC diets on the behaviours of children with autism had the most significant flaws. Conversely, the studies that were considered to be most methodologically rigorous tended to find no benefits from the use of GCFC diets.

The third problem relates to the question of “what’s the harm?” This argument goes: “Despite there being no evidence for the effectiveness of GCFC diets, nor the theory underpinning it, what’s the harm in families trying this on the odd chance that it may be beneficial for their child?”

Unfortunately, some question marks remain over the safety of GCFC diets. Several studies have reported that a GCFC diet is associated with reduced bone density in both children with and without autism.

Others have suggested it’s unethical to recommend a diet that has significant logistical and financial implications for families who are already stretched to capacity.

The future

Currently, there is not sufficient scientific evidence that GCFC diets can be beneficial for children with ASD.

Furthermore, given the uncertain safety profile, only when there is a clear intolerance or allergy to the foods that a GCFC diet eliminates would it be prudent to recommend the diet.

The key determiner to changing or strengthening this conclusion is the findings from well-designed treatment trials. It is completely understandable for parents to have a burning desire to do anything that may help their child. It is because of this that we, as scientists, must do better in providing solid evidence to guide parental and clinical decision-making.

The quality of autism intervention science has increased significantly over the past decade, and it is critical that alternative therapies are also tested as part of this agenda.