Gut bacteria in toddlers predict their mental health years later

Emotions don’t appear out of nowhere. They grow, shift, and take shape as the brain matures. But what if the earliest influences on emotional health aren’t found in the brain at all?

UCLA Health scientists now believe that the foundation of emotional well-being might begin in the gut.

Their new study reveals how the bacteria living in a toddler’s intestines may shape emotional circuits in the brain – long before school even starts.

Early gut and emotions

The research team examined data from Singapore’s long-term GUSTO project, which tracks children from birth.

Stool samples were collected when participants were two years old, revealing which microbes dominated each child’s gut. At age six, those same children underwent brain scans, and by seven and a half, caregivers reported emotional and behavioral symptoms.

Children with a greater presence of bacteria from the Clostridiales order and Lachnospiraceae family showed higher risks of internalizing symptoms – behaviors linked to sadness, anxiety, and withdrawal. The connection wasn’t direct.

Gut microbes seemed to influence how emotion-related brain networks communicated, particularly regions that manage fear, reward, and stress regulation.

How the gut affects emotions

When researchers analyzed brain scans, they found two key connectivity patterns. One involved networks connecting the amygdala, orbitofrontal cortex, and salience regions – areas essential for processing and regulating emotions.

The second involved connections across sensory and cognitive systems, influencing how emotional information is interpreted.

Specific microbes appeared to strengthen or weaken these brain links. For example, Eubacterium and Terrisporobacterwere tied to stronger emotional regulation networks.

In contrast, Fusicatenibacter and Catenibacterium were linked to fewer emotional difficulties.

These associations suggest that certain microbes might support resilience, while others could increase vulnerability to stress-related symptoms later in life.

Diverse gut, stable emotions

Another bacterial group stood out – those dominated by Veillonella and Intestinibacter. These microbes indirectly affected emotional outcomes through their influence on brain connectivity.

Children with more diverse microbiomes, measured by Faith’s phylogenetic diversity index, showed stronger communication between emotion-related brain networks.

Dr. Bridget Callaghan is an associate professor of psychology and Bernice Wenzel and Wendell Jeffrey Term Endowed Chair in Developmental Psychology at UCLA.

“By linking early-life microbiome patterns with brain connectivity and later symptoms of anxiety and depression, our study provides early evidence that gut microbes could help shape mental health during the critical school-age years,” said Dr. Callaghan.

Stress-linked gut response

The study found that some microbes might be more sensitive to stress than others. Members of the Clostridiales and Lachnospiraceae families have already been connected to stress and depression in adults.

These bacterial groups seem to react to early life adversity, possibly influencing how emotional regulation systems form in the brain.

This might explain why some children develop anxiety or depression even without major trauma. The gut’s microbial environment could “train” emotional circuits to respond more strongly to stress.

In other words, how the body digests food and processes stress might come from the same biological source – the gut-brain loop.

Gut powers brain activity

The study didn’t stop at identifying microbes. It also explored what these microbes were doing.

The researchers found that metabolic pathways tied to nicotinamide and pentose phosphate processing were linked with emotion-related brain networks. These pathways regulate energy balance in the brain, keeping neurons active and stable.

When microbial energy systems shift, the brain’s emotional circuits might shift too. Subtle changes in the gut could alter how efficiently neurons communicate, potentially influencing emotional responses to stress, reward, or uncertainty.

The findings add a new layer to the gut-brain story: microbes may not just signal to the brain but also power it.

Dr. Callaghan said more research is needed to know which bacterial species drive these changes.

“We need to figure out what species within these larger groups are driving the findings. Once we have that information, there are relatively straightforward ways to change the microbiome, like probiotics or diet, that we could use to address issues,” she explained.

Gut shapes mental health

The implications reach far beyond biology. If early gut bacteria influence emotional development, mental health care could begin much earlier than current approaches allow.

Supporting gut health in early childhood – through nutrition, reduced stress, and possibly probiotics – might one day form part of emotional health strategies.

The idea is both practical and profound. Those microbes that quietly multiply in a toddler’s stomach could be fine-tuning the emotional rhythms of the growing brain.

Nurturing the gut in early life might not just build stronger bodies but also calmer, more resilient minds.

The study is published in the journal Nature Communications.

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