Summary: While Major Depressive Disorder (MDD) and Bipolar Disorder (BD) share overlapping symptoms, new research identifies distinct biological pathways linking metabolic dysfunction, brain structure, and cognition. The study finds that insulin resistance and leptin dysregulation are more strongly and specifically associated with structural brain changes and cognitive impairment in people with Bipolar Disorder than in those with Major Depressive Disorder.
These results suggest that addressing metabolic factors—particularly insulin and leptin resistance—may be an important strategy for treating persistent cognitive difficulties that often remain after mood symptoms improve.
Key research findings
- Disease-specific pathways: Metabolic alterations, especially insulin resistance and leptin imbalance, were linked to cognitive deficits in Bipolar Disorder but not in Major Depressive Disorder.
- Worse metabolic profile in BD: Patients with Bipolar Disorder showed a more severe pattern of metabolic dysfunction than those with MDD. Greater illness burden—measured by the number of mood and manic episodes—correlated with worse metabolic health.
- Impact on brain structure: Metabolic dysfunction was associated with reduced gray matter volume in brain regions critical for cognition and emotional regulation. These structural changes help explain declines in memory, attention, and executive function.
- Persistent cognitive symptoms: Metabolic influences on brain structure and neural signaling can persist independently of active mood episodes, accounting for why cognitive problems frequently continue after mood stabilizes.
- Support for a neuroprogressive model: The findings are consistent with a model in which repeated mood episodes produce cumulative metabolic and neurobiological changes, highlighting the value of early intervention to limit long-term decline.
Source: Elsevier
A study published in Biological Psychiatry: Cognitive Neuroscience and Neuroimaging (Elsevier) is the first to map clinically meaningful pathways that connect metabolic biomarkers, brain gray matter volume, and cognitive performance in mood disorders. The relationships were notably stronger and more specific in Bipolar Disorder, pointing to metabolic pathways as potential therapeutic targets to improve cognition in BD.
Mood disorders such as MDD and BD are disabling conditions that affect mood regulation and biological rhythms. Even when mood stabilizes, many patients struggle with ongoing deficits in memory, concentration, and higher-order thinking, impairing daily functioning. Growing evidence links mood disorders with metabolic disturbances—conditions like obesity, diabetes, and insulin resistance are more prevalent among people with mood disorders and may contribute to cognitive decline.
Lead investigator Elena Mazza, PhD (Psychiatry and Clinical Psychobiology, Division of Neuroscience, IRCCS Ospedale San Raffaele, Milan) emphasizes the clinical heterogeneity of mood disorders and the need for targeted approaches. In this study, researchers examined 78 inpatients diagnosed with Major Depressive Disorder and 81 inpatients with Bipolar Disorder, assessing insulin resistance and related hormones alongside structural brain imaging and cognitive testing. Given insulin’s important role in neuronal communication, learning, and memory, the team focused on how metabolic dysfunction might affect cognition via brain structure.
Using a multivariate statistical approach, the investigators combined metabolic biomarkers, T1-weighted MRI-derived gray matter volumes, and neuropsychological measures (Brief Assessment of Cognition in Schizophrenia) to explore diagnosis-specific pathways. In the overall sample, a latent metabolic construct negatively predicted both gray matter volume and cognitive performance, with gray matter positively influencing cognition. Crucially, stratified analyses revealed that the metabolism-to-cognition relationship was significant only in Bipolar Disorder. In BD, insulin resistance markers and leptin were the strongest contributors to the metabolic construct and were linked to lower gray matter volume and poorer cognitive outcomes. No comparable structural paths were detected in the MDD group.
The brain regions most affected included areas central to cognitive processing and emotional regulation—regions known to express high densities of insulin and leptin receptors. This supports a mechanistic model in which insulin and leptin resistance promote inflammatory and neurotoxic processes that alter brain structure and impair memory, attention, and executive function.
Cameron S. Carter, MD (Editor-in-Chief, Biological Psychiatry: Cognitive Neuroscience and Neuroimaging) notes that these findings fit a neuroprogressive view of Bipolar Disorder: repeated episodes may drive clinical worsening alongside accumulated metabolic and neurobiological changes, underscoring the importance of early, effective treatment to prevent long-term decline.
Lead author Laura Raffaelli (PhD candidate, Psychiatry and Clinical Psychobiology, IRCCS Ospedale San Raffaele, Vita-Salute San Raffaele University) points out existing and emerging interventions that target insulin pathways. Insulin-sensitizing medications, intranasal insulin, and GLP-1 receptor agonists—drugs currently used for metabolic conditions—have shown promise for improving cognitive and mood outcomes and represent promising directions for future therapeutics in BD with metabolic comorbidities.
Dr. Mazza concludes that metabolic health should be considered an integral component of psychiatric care. The study clarifies why cognitive symptoms can persist despite mood improvement and supports integrating metabolic management into personalized treatment plans to better protect brain structure and cognitive function in mood disorders.
Key questions answered
Q: Why does metabolism affect memory?
A: Insulin is essential for neuronal communication and memory processes. Insulin or leptin resistance can trigger inflammatory and neurotoxic changes that physically alter brain regions supporting cognition.
Q: Could diabetes medications help treat Bipolar Disorder?
A: Potentially. Insulin-sensitizing agents, intranasal insulin, and GLP-1 receptor agonists are being studied for their potential to improve mood and cognitive function in bipolar disorder.
Q: Does having more mood episodes worsen metabolic problems?
A: Yes. The study found that a greater number of mood and manic episodes was associated with worse metabolic dysfunction, creating a cumulative biological burden over a patient’s lifetime.
Editorial notes
- Article edited by a Neuroscience News editor.
- The journal paper was reviewed in full by the editorial team.
- Additional context added by staff to clarify clinical implications.
About this research
Author: Eileen Leahy
Source: Elsevier
Contact: Eileen Leahy – Elsevier
Image: The image is credited to Neuroscience News
Original Research: Open access. “The anterior cingulate cortex modulates pupil-linked arousal” by Laura Raffaelli et al., Biological Psychiatry: Cognitive Neuroscience and Neuroimaging. DOI: 10.1016/j.bpsc.2026.02.003
Abstract
Background
Major Depressive Disorder and Bipolar Disorder are both associated with persistent cognitive deficits, but the biological mechanisms remain unclear. Metabolic dysfunction, notably insulin resistance, may contribute to structural brain changes and cognitive decline. Diagnosis-specific effects of metabolic disturbance on gray matter volume and cognition have not been fully explored.
Methods
The study included 81 inpatients with Bipolar Disorder (55 female, 26 male) and 78 inpatients with Major Depressive Disorder (45 female, 33 male). Participants completed neuropsychological testing (Brief Assessment of Cognition in Schizophrenia), underwent T1-weighted MRI for gray matter volume analysis, and provided blood samples for metabolic marker assessment. Partial least squares path modeling evaluated associations among metabolic biomarkers, gray matter volumes, and cognitive outcomes, stratified by diagnosis.
Results
Across the whole sample, a latent metabolic factor negatively predicted both gray matter volume and cognition, while gray matter positively predicted cognition. A significant diagnostic difference appeared for the metabolism-to-cognition relationship. In Bipolar Disorder, metabolism was significantly associated with reduced gray matter volumes and poorer cognition, whereas these structural associations were not significant in Major Depressive Disorder. Insulin resistance markers and leptin were the strongest contributors to the metabolic factor. Affected brain regions were central to cognitive and emotional regulation and bore a high density of insulin and leptin receptors.
Conclusions
The findings underscore the role of insulin resistance and leptin in shaping cognitive outcomes in mood disorders and highlight insulin-related pathways as promising therapeutic targets—particularly in Bipolar Disorder patients with metabolic comorbidities.