Doctors have long looked to the brain to understand children’s behaviour, yet fresh evidence is quietly pointing much lower down.
New research from Slovakia suggests that the trillions of microbes living in a child’s gut could share surprising patterns across very different mental and developmental conditions, reshaping how clinicians think about autism, anorexia and ADHD.
A hidden connection between three very different disorders
Autism, attention deficit hyperactivity disorder (ADHD) and anorexia nervosa rarely sit in the same clinical box. One is a neurodevelopmental condition, one a behavioural disorder, one a severe eating disorder most often framed in psychological terms.
Yet a team at Comenius University in Bratislava reports that these diagnoses may overlap in a place few parents expect: the bowels.
The study, published in the journal Neuroscience, examined stool samples from 117 children. Some had autism, some ADHD, some anorexia, and others were neurotypical controls.
The researchers found that all three groups of affected children showed signs of gut dysbiosis — a measurable imbalance in intestinal bacteria — compared with healthy peers.
Reduced microbial diversity stood out in children with autism and ADHD. In general, a more varied gut ecosystem is associated with better resilience, stronger immunity and more stable metabolism. Here, that variety had narrowed.
Across all three conditions, the balance between two broad bacterial phyla, Bacteroidetes and Firmicutes, was clearly shifted. Similar shifts have previously cropped up in chronic inflammatory diseases, from obesity to inflammatory bowel disease.
Anorexia’s unexpected microbial fingerprint
The most striking pattern emerged in girls with anorexia nervosa. Their stools contained markedly higher levels of Desulfovibrio, a group of bacteria that thrive in low-nutrient, sulphate-rich environments.
At first glance, that might sound like a simple reflection of extreme dietary restriction. But the authors argue that the picture is more complex.
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The unusual abundance of sulphate-reducing bacteria in anorexic patients suggests that malnutrition alone does not fully explain this microbial signature.
The same children also showed extra amounts of Cyanobacteria and Verrucomicrobiota, two lesser-known groups increasingly implicated in metabolic and inflammatory processes. Together, these shifts point to a specific intestinal ecosystem forming alongside — and possibly reinforcing — restrictive eating patterns.
Which bacteria are changing – and why that matters
When the scientists zoomed in on individual genera, several recurrent trends appeared.
- Escherichia levels were elevated in both autism and ADHD groups.
- Desulfovibrio was more frequent in ADHD and anorexia.
- Cyanobacteria and Verrucomicrobiota appeared in higher amounts in anorexia.
- Faecalibacterium, a beneficial genus, was depleted across affected children.
- Bifidobacterium and broader Actinobacteriota were specifically reduced in autism.
Escherichia species include familiar residents such as certain strains of E. coli. In normal numbers they cohabit without trouble. When they multiply or move beyond their usual niche, they can trigger gut irritation, infections and immune activation.
Faecalibacterium sits at the opposite end of the spectrum. It helps produce butyrate, a short-chain fatty acid that nourishes intestinal cells and curbs inflammation. Low levels of this genus have been linked to depression, inflammatory bowel disease and colorectal cancer.
Many of the depleted bacteria are those that help digest fibre, produce vitamins and keep low-grade inflammation in check.
These bacterial shifts might partly stem from the children’s habits. Autistic children, for example, are more likely to follow rigid food routines, avoid certain textures or colours and eat a narrower range of foods. A review by the UK’s National Autistic Society has highlighted how such patterns can skew fibre intake and nutrient balance.
The Slovak team caution that gut changes are unlikely to be only an effect of fussy or restrictive eating. Their data suggest a feedback loop: the microbiome shapes appetite, hormones and brain signals, which then shape behaviour, which then reshapes the microbiome.
A shared biological substrate across mental and developmental disorders
The gut–brain axis, once a fringe concept, is now one of the hottest areas in neuroscience. Signals pass in both directions along the vagus nerve, through hormones released into the bloodstream and via immune cells primed in the intestine.
In this study, the researchers measured circulating appetite-regulation hormones. Two stood out: peptide YY (PYY) and leptin.
| Hormone | Main role | Pattern in the study |
|---|---|---|
| Peptide YY (PYY) | Signals satiety after meals, slows gut motility | Reduced in children with anorexia and ADHD |
| Leptin | Long-term regulator of appetite and body weight | Also reduced in anorexia and ADHD |
Both hormones help the brain judge when the body has had enough food. Reduced levels might skew hunger cues, contribute to disordered eating and affect energy levels, attention and mood.
The data suggest that an unstable microbiome may nudge hormone levels off course, subtly shifting behaviour and mental state over time.
For autism, anorexia and ADHD, this raises a provocative question: are clinicians sometimes treating brain-based symptoms while missing a shared biological terrain in the gut?
The researchers stop short of saying that intestinal microbes cause these disorders. Genetics, early life experiences, stress, social context and family history all remain major drivers. The findings instead point to a common vulnerability layer that might amplify symptoms or influence how severely they appear.
What this could mean for future treatments
If part of the problem lies in a disturbed intestinal ecosystem, then the toolkit for intervention could expand dramatically in the years ahead.
Experts already discuss several microbiome-focused strategies:
- Personalised nutrition plans that increase fibre and prebiotic foods to nourish Faecalibacterium and Bifidobacterium.
- Targeted probiotics containing specific strains shown to affect mood, attention or anxiety in early trials.
- Synbiotics, combining probiotics with the fibres they feed on.
- Carefully monitored faecal microbiota transplantation (FMT) in severe, treatment-resistant cases, although this remains experimental.
None of these would replace psychological therapy, behavioural work or medication. Instead, they could sit alongside those approaches, aiming to cool inflammation, restore microbial balance and stabilise appetite signals.
Key concepts readers often ask about
What exactly is “gut dysbiosis”?
Dysbiosis simply means that the intestinal ecosystem has shifted in an unhealthy direction. That can involve:
- Too few species overall.
- Growth of bacteria linked with inflammation.
- Loss of helpful groups that process fibre and produce protective compounds.
People with dysbiosis may not feel “sick” in a classic way. Instead they might live with subtle symptoms: bloating, constipation, fatigue, fluctuating mood, food cravings or brain fog.
How might the gut influence behaviour in daily life?
Consider a child with ADHD who eats a very restricted, low-fibre diet rich in ultra-processed snacks. Over time, fibre-loving bacteria decline. Inflammatory species gain ground. Low-grade gut inflammation nudges immune cells to release signalling molecules that reach the brain.
This child might feel more restless, less able to focus and more drawn to quick sugar hits. At the same time, altered PYY and leptin levels send confusing messages about hunger and fullness, making stable eating patterns harder to maintain.
Now imagine that same child starts a gradual shift towards whole foods, slightly more varied textures and perhaps a child-friendly probiotic under medical guidance. As microbial diversity increases, gut-produced short-chain fatty acids rise. These compounds can strengthen the intestinal barrier and may support calmer immune signalling. Over months, this could translate into small but noticeable changes in energy, concentration and mood.
Practical angles for families and clinicians
Parents sometimes ask whether changing diet alone can “fix” autism or anorexia. Current evidence does not support that idea, and such expectations can add guilt and pressure. The Slovak study instead suggests that diet and microbiome sit alongside genetics, environment and therapy as one more lever that might ease or worsen symptoms.
Clinicians are beginning to consider screening for gut problems in children with these diagnoses: chronic constipation, reflux, recurrent stomach pain or severe selectivity around food. Addressing these issues early may support both physical comfort and mental health.
Researchers are now planning larger, longitudinal studies to see whether microbial shifts appear before symptoms, change alongside them, or respond directly to interventions. Those answers will determine whether the microbiome becomes a central target in treatment or remains a useful, but secondary, marker of risk.