Autism and
the Gut–Brain Axis

1. Digestion in Autism Doesn't Matter — Or Does It?

"Another note from school. Things were better for a while, and now they're worse again — the same cycle. I asked the doctor what we could do, how to help him and myself. The answer I almost always hear is the same: 'It's autism. It can't be treated. You have to accept it.'" – says one of the mothers I work with. I hear similar conversations almost every week.

What about diet and lifestyle? They affect everyone's health, wellbeing, concentration, and emotions — this doesn't stop being true just because a child has received an ASD diagnosis. Changing a child's diet isn't easy, though: children with ASD often struggle with food selectivity and heightened sensitivity to taste, smell, or texture, which requires patience and a proper plan of action. Diet matters for neurotypical and neurodivergent children alike — and understanding why requires understanding how the gut–brain axis works.

Digestive problems are rarely the first thing we think of when observing a child's difficult behavior. Many children on the spectrum communicate verbally in a limited way or not at all, and some experience impaired interoception — the ability to sense signals from their own body. A child may therefore fail to recognize abdominal pain, or be unable to name it.

Parents often notice isolated symptoms — bloating, burping, foul-smelling stools, constipation, diarrhea, or a distended abdomen — but rarely connect them to the child's behavior. Many have struggled with similar complaints themselves for years and consider them normal, or are told "that's just how the child is" — so the problem never gets addressed. This is precisely where we often lose the chance to notice one of the most important pieces of the puzzle.

Research shows that gastrointestinal problems occur significantly more often in children on the autism spectrum than in neurotypical peers, and their severity often correlates with difficulties in social functioning and behavior. Scientists describe this relationship as the gut–brain axis — a bidirectional communication system linking the digestive tract, the gut microbiota, and the central nervous system.

If this mechanism exists, it's worth asking: why does assessing digestive function so rarely become one of the first steps taken after an ASD diagnosis? This is the gap NNMCCD aims to fill — not replacing existing therapies, but bringing structure to the diagnostic-therapeutic process so that assessing digestive function becomes a natural part of comprehensive care for the child, as we show later in this article.

2. Why Can the Gut Influence Brain Function?

Only a decade or so ago, the gut was seen mainly as an organ for digestion. Today we know it plays a far more complex role — it is one of the body's most important communication channels, in constant contact with the brain through several pathways at once, whose combined action explains why disorders of the digestive tract can affect the nervous system.

Vagus Nerve – The Fastest Information Pathway

The first communication channel is the vagus nerve — a direct connection between the digestive tract and the brain. Through this pathway, signals from the gut rapidly influence the regulation of stress, tension, mood, and autonomic nervous system activity; its proper functioning plays an important role in maintaining the body's overall balance^[11].

Immune System – The Body's Largest Immune Organ

The second communication channel is the immune system — the gut is the body's largest immune organ, where it continuously decides what constitutes a threat. When the intestinal barrier weakens, pro-inflammatory cytokines and other molecules that activate the immune response enter the bloodstream, affecting not only the digestive tract but also neurodevelopmental processes and brain function^[11][13].

In some children with ASD, an impaired immune response may additionally be explained by folate receptor antibodies — we discuss their role in more detail in a separate article on leucovorin therapy.

Microbiota Metabolites – The Gut's Chemical Language

The third communication channel consists of gut microbiota metabolites — primarily short-chain fatty acids such as butyrate, propionate, and acetate. These small molecules support intestinal barrier integrity, participate in regulating the immune response, and can modulate inflammatory processes within the central nervous system^[12]. What matters, then, is not only which bacteria are present, but what they produce — which depends on digestive quality and diet. This is why probiotic therapy alone rarely provides a complete solution.

These three mechanisms don't operate independently — they complement one another. When intestinal barrier function is disrupted, the consequences can extend to the immune system, detoxification processes, and brain function.

3. What Does the Research Show?

The mechanisms described above are no longer merely a hypothesis — in recent years, numerous studies have allowed us to view the gut–brain axis from a clinical perspective. Three questions arise: how often do gut problems occur in children with ASD, how does their microbiota differ, and is there evidence that changes in the gut actually affect brain function and behavior?

Gastrointestinal Problems in Children with ASD Are Not the Exception

One of the best-documented facts is simply how often digestive complaints occur. A landmark meta-analysis by McElhanon and colleagues found that, compared with neurotypical children, children with ASD have^[8]:

More recent meta-analyses confirm the same pattern^[7], and questionnaire-based studies show that the more severe the gastrointestinal symptoms, the more often difficulties appear in social communication, restrictive behaviors, sleep problems, and anxiety^[9]. This doesn't yet indicate a causal relationship, but it clearly shows that digestive function is not a marginal issue for children on the spectrum.

What's Different in the Gut Itself?

If symptoms occur so often, do the guts of children with ASD actually differ biologically from those of neurotypical children? A growing body of research suggests they do — microbiota analyses show different proportions of major bacterial groups^[10]. The 2023 study by He and colleagues found increased levels of Clostridium bacteria and more frequent Candida fungi in children on the spectrum compared with a control group^[4]. These changes aren't a single cause of autism, but part of a broader phenomenon known as gut dysbiosis — an imbalance in the microorganisms inhabiting the digestive tract.

Can the Microbiota Really Influence Behavior?

This is the question parents ask most often: are the observed differences merely correlation, or do they actually affect a child's functioning? One of the strongest arguments for such a link comes from research on fecal microbiota transplantation (FMT) — when microbiota from individuals with ASD was transferred to laboratory animals, the animals showed behavioral changes resembling traits seen in people on the spectrum^[16]. This doesn't mean FMT is a first-choice treatment, but it shows that the microbiota can actively help shape the gut–brain axis, rather than merely being a passive bystander to disease.

Similar mechanisms — the vagus nerve, the immune system, microbiota metabolites — have also been described in ADHD, Rett syndrome, and other neurodevelopmental disorders^[11][13][14], reinforcing the hypothesis that this is not specific to autism alone, but a broader biological mechanism relevant to the nervous system as a whole.

Can the Microbiota Be Modified?

If the microbiota plays such an important role, can modifying it bring clinical benefits? Several randomized clinical trials and meta-analyses have evaluated the effect of probiotics and synbiotics on children with ASD^{[1][2][3][5][6]}. The direction of results is promising — some studies observed improvement in gastrointestinal symptoms and certain aspects of behavioral functioning, though the strength of evidence remains moderate, and treatment response varies between children.

The relationship between the microbiota and brain function isn't limited to autism — similar mechanisms have also been described in depression, anxiety disorders, and chronic stress, making the gut–brain axis one of the most dynamically developing areas of translational medicine today.

4. From Theory to Practice

This is often where a gap emerges between parents' experience and clinical practice. Parents ask about broader diagnostics — stool testing, microbiota assessment, or a deeper evaluation of digestive function — but are often told no guidelines exist, since scientific knowledge about the gut–brain axis is advancing faster than our ability to implement it in clinical practice — as shown by the FMT research mentioned above^[16].

Diagnostic tools that could help clarify individual functional disorders already exist, but most are available mainly in the private sector, involve additional costs, and require proper interpretation — testing alone doesn't yet translate into the right clinical decisions.

This is why we need standardized diagnostic-therapeutic pathways (clinical pathways), developed collaboratively by pediatricians, gastroenterologists, microbiologists, clinical dietitians, and gut–brain axis researchers. The proposed model starts with a basic assessment of digestive function and implementing safe, low-risk interventions with potentially high benefit — such as individually tailored dietary modification — with clinical response then guiding the direction of further, more advanced investigations.

A good example is microbiota transplantation — even this advanced method mainly changes the composition of the microbiota, without necessarily removing the causes of dysbiosis. It's like replanting a healthy plant into soil whose conditions remain unchanged: if the environment still doesn't support growth, the plant may not survive. Regardless of the method used, improving digestive function and an anti-inflammatory diet remain the foundation for further stages of therapy.

This is why NNMCCD members and patients receive concrete tools for the first steps of this journey — the ABC Guide, which helps implement key dietary and supplementation changes step by step. The greatest challenge for families is often not a lack of knowledge, but the consistent implementation of recommendations — which is why the next stage of NNMCCD's development is a mobile application and a network of trained specialists supporting parents.

5. From Clinical Practice

Clinical Dietitian Paulina Borkowska

Working with families of children with neurodevelopmental disorders, I often observe a similar pattern: parents arrive at the clinic feeling helpless, having already consulted many specialists and worked through long lists of recommendations — yet they still ask: "Is there anything we haven't checked yet?" The answer is often yes — not because earlier diagnostic work was wrong, but because it usually focuses on behavioral, speech, or psychological therapy, and less often includes a detailed assessment of digestive function, diet, or nutritional status.

Yet parents frequently describe everyday difficulties that extend far beyond the ASD diagnosis itself: constipation, diarrhea, abdominal pain, chronic bloating, a very limited range of accepted foods, sleep difficulties, mood variability, or increased self-injurious behavior. Each of these symptoms can have different causes, but all deserve careful analysis rather than purely symptomatic treatment.

In clinical practice, I repeatedly observe that addressing the basic elements of body function — improving diet quality, supporting digestion, correcting nutritional deficiencies, and treating co-occurring gastrointestinal problems — translates into improved quality of life for the whole family, and in some children, better concentration, sleep quality, or greater readiness to participate in therapy. It must be emphasized, however, that clinical observations, even when repeated, do not constitute scientific proof — which is why well-designed clinical trials are so necessary, to determine which children specific interventions actually benefit.

Every child is different. Autism spectrum disorders encompass an extremely diverse group of patients, so there is no single diet or approach suitable for everyone — what helps one child may be ineffective for another. This is why the future of medicine is increasingly moving toward personalized medicine, based on individual assessment of the patient, their symptoms, test results, and response to interventions — and this way of thinking underlies the NNMCCD model. Our goal is a structured, stepwise diagnostic-therapeutic pathway based on current scientific knowledge, clinical presentation, and the individual needs of the child and family.

Summary

In recent years, knowledge about the gut–brain axis has been developing remarkably fast. A growing body of research indicates that the functioning of the digestive tract, gut microbiota, and immune system may play an important role in the health of individuals with neurodevelopmental disorders.

This doesn't mean we already have all the answers, or that diet or supplementation can replace psychological, educational, speech, or medical care. It does mean that the digestive system deserves a place in the comprehensive assessment of a child, and that its function shouldn't be overlooked simply because the primary diagnosis is ASD.

NNMCCD was founded out of this need. Our mission is to build a bridge between advancing scientific knowledge and everyday clinical practice — educating families, developing evidence-based diagnostic-therapeutic pathways, and fostering collaboration among specialists from different fields. In parallel, we are building observational registries, conducting research, and pursuing research grants — our goal is not only to apply current scientific knowledge in clinical practice, but to actively contribute to generating it.

We believe that only the combination of rigorous research, clinical experience, and genuine partnership with families can produce solutions that are safe, effective, and practical to implement in everyday care.

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