Why So Many Mental Health and Brain-Gut Disorders Overlap

Many people who struggle with a chronic disease, in particular such as compromised mental or gut health do not fit neatly into just one diagnostic box. A person with bipolar disorder may experience symptoms that resemble schizophrenia. Others may deal with attention problems, compulsive behaviors, or substance use at the same time. Someone diagnosed with depression may also have anxiety. Despite the categorization into many different diagnoses in the DSM-5 (Diagnostic and Statistical Manual of Mental Disorders) psychiatrists have long observed this overlap, called comorbidity.

Comorbidities are also common in chronic disorders of gut brain interactions (DGBIs) where patients with IBS also can meet symptom criteria for functional dyspepsia but also in individuals with anxiety or depression, who often suffer from chronic functional gut disorders. However, until recently, we did not fully understand why these overlaps happen.

A major new genetic study co-led by KS Kendler from Virginia Commonwealth University and JW Smoller from Harvard Medical School and a large team of international investigators, published recently in the prestigious journal Nature set out to answer this question by looking deep into the DNA, our genetic architecture. The research team asked a simple but powerful question: Do different psychiatric disorders share the same genetic roots? Their answer is clear and striking, YES — much more than we previously realized.

A Massive Study of the Genetics of Mental Illness

To tackle this question, the scientists analyzed genetic data from over one million people, including individuals diagnosed with 14 different psychiatric disorders. These included schizophrenia, bipolar disorder, major depression, anxiety disorders, post-traumatic stress disorder (PTSD), attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder, obsessive–compulsive disorder (OCD), anorexia nervosa, and several substance-use disorders (such as alcohol, nicotine, cannabis, and opioid dependence).

Rather than studying each disorder in isolation, the researchers compared them side by side. They examined so called common genetic variants — small differences in DNA that many people share — and asked how often the same variants increased risk across multiple disorders.

Five Major Genetic “Families” of Psychiatric Disorders and a General Vulnerability Factor

One of the most important discoveries of this study is that the genetic risk for these 14 disorders can be grouped into five major clusters, or genetic factors. Each factor represents a shared biological vulnerability that cuts across traditional diagnoses. These factors are a Schizophrenia–Bipolar Factor, a Neurodevelopmental Factor, a Compulsive Disorders Factor, a Substance Disorders Factor, and two other factors which are particularly relevant for chronic GI disorders.

One of these factors include an Internalizing Disorders Factor, characterizing a group of disorders that includes major depression, anxiety disorders, and PTSD. These conditions are often characterized by emotional suffering — sadness, fear, worry, and increased stress sensitivity. Genetically, they are tightly connected, with very few genes unique to just one disorder.

The other factor of particular interest is what the researchers called the p-factor, characterizing a group with a general vulnerability or reduced resilience to mental illness. This factor reflects a general vulnerability to mental illness across many conditions, especially those involving emotional distress. The p-factor was strongly linked to traits such as:

• High stress sensitivity
• Neuroticism
• Loneliness
• Self-harm and suicide risk

This suggests that some people are genetically more sensitive to stress and emotional adversity overall, which can then show up as depression, anxiety, PTSD, or other disorders depending on life experiences and environment.

In our own research, we found that two of these traits making up the Internalizing Disorders Factor and the p-factor, neuroticism and increased stress sensitivity/reactivity are also more prevalent in a subgroup of patients with chronic gastrointestinal disorders, IBS and ulcerative colitis and a previous genetic study identified an overlap of genetic risk factors or vulnerability genes between IBS and anxiety. Consistent with the concept of the brain-gut microbiome system some of the same vulnerability genes that influence brain function have also been identified in the “little brain” of the gut, the enteric nervous system. Patients in these subgroups with increased stress sensitivity have more severe and persistent symptoms, and are more likely to experience flares of their chronic disease over time.

What’s Happening in the Brain?

To understand what these shared genes actually do, the researchers examined where and when these genes are active in the brain.

Many shared genetic risk genes are especially active before birth and in early childhood, when the brain is forming its basic structure. This is consistent with an extensive body of literature and clinical experience that has shown that vulnerability to psychiatric (and chronic functional gastrointestinal) illness often begins very early in life — even if symptoms do not appear until adolescence or adulthood. The study also found that different disorder clusters involve different brain cell types. Internalizing disorder genes were more closely tied to oligodendrocytes, a type of support cell that helps insulate brain circuits and maintain efficient communication, whereas the general p-factor involved broad biological processes such as gene regulation and brain development, rather than highly specific circuits.

Why This Changes How We Think About Diagnosis and Treatments

Traditional psychiatric diagnoses are based on symptoms: how a person feels, thinks, and behaves. While this approach is useful clinically, it does not always reflect what is happening biologically.

This study shows that many disorders we label as separate in psychiatry (and gastroenterology) actually share the same genetic foundations. In some cases — such as depression and anxiety — the genetic differences between diagnoses are surprisingly small.

This does not mean diagnoses are meaningless. Instead, it suggests that current categories may be too rigid and that a more biology-based classification system could better reflect how mental illness truly works and why the same treatments may be effective in patients with different diagnoses. For example:

• Antidepressants can help anxiety, PTSD, and sometimes even chronic pain.
• Behavioral therapy approaches such as cognitive behavioral therapy are useful across many diagnoses, but effectiveness varies between diseases with different genetic risk factors.
• Lifestyle interventions that reduce stress, improve sleep, and strengthen brain resilience benefit multiple conditions at once, but may work particularly well in disorders involving factor 2.

In the future, treatments may be designed to target shared biological pathways, rather than one diagnosis at a time.

Important Limitations to Keep in Mind

The researchers pointed out several limitations or their impressive study:

• Most genetic data came from people of European ancestry, meaning results may not fully apply to all populations.
• Genetics explains vulnerability, not destiny. Environment (in particular in utero and in early life), trauma, social support, and lifestyle interacting with these genetic risk factors play a major role as has previously been pointed out by an extensive body of scientific evidence. One may even speculate that gut microbiome to brain signaling in early life may interact with the genetic vulnerability genes.
• Based on these well-known environment-gene interactions, shared genes do not mean identical experiences — each person’s mental health journey is unique.

This landmark study provides one of the clearest pictures yet of the shared genetic architecture of mental illness with implications for other chronic disorders including pain and disorders altered gut-brain interactions. It shows that psychiatric disorders are deeply interconnected, rooted in overlapping biology, and shaped early in brain development.

Rather than seeing clinical diagnoses as isolated labels, this research encourages us to think in terms of networks, systems, and shared vulnerability — a shift that could ultimately lead to more compassionate, personalized, and effective care not only of mental health conditions, but of chronic diseases in general.