May 11, 2026 – While they share similar depressive and cognitive symptoms, the biological underpinnings of bipolar disorder and major depressive disorder are distinct. A novel study appearing in Biological Psychiatry: Cognitive Neuroscience and Neuroimaging , published by Elsevier, is the first to identify clinically relevant pathways linking metabolic dysfunction, brain structure, and cognition in mood disorders, with stronger and more specific effects observed in bipolar disorder. It highlights the potential of targeting metabolic pathways to improve cognitive symptoms in bipolar disorder.
Major depressive disorder and bipolar disorder are disabling psychiatric conditions, significantly impairing mood regulation and biological rhythms. Even when a person's mood is stable, persistent challenges with memory and focus make it hard to function in everyday life. Increasing evidence suggests a strong link between mood disorders and metabolic dysfunction. Conditions such as obesity, diabetes, and insulin resistance are associated with a higher risk of depression, and vice versa.
"Mood disorders are highly heterogeneous, which often delays diagnosis and effective treatment, highlighting the need for more targeted approaches," explains lead investigator Elena Mazza, PhD, Psychiatry and Clinical Psychobiology, Division of Neuroscience, IRCCS Ospedale San Raffaele, Milan, Italy. "In a cohort of 78 patients with major depressive disorder and 81 with bipolar disorder, we investigated how insulin resistance and related hormones are associated with brain structure and clinical outcomes, with a particular focus on cognitive function, given the critical role of insulin in neuronal communication, learning, and memory."
Researchers combined metabolic biomarkers, structural brain imaging, and cognitive assessments to examine how metabolic dysfunction relates to brain structure and cognition. They applied a multivariate statistical approach to investigate these relationships and to assess whether they differed between diagnoses.
Patients with bipolar disorder were found to exhibit a more severe metabolic profile, characterized by insulin resistance and leptin dysregulation, likely reflecting a more severe illness course, as greater illness burden—specifically a higher number of mood and manic episodes—was associated with worse metabolic dysfunction and lifetime burden of illness.
Investigators found that metabolic alterations like insulin resistance are associated with cognitive deficits, potentially through their impact on gray matter volume in key brain regions involved in cognitive functioning and are linked to poorer performance in memory, attention, and executive function. Notably, these associations were observed only in bipolar disorder, suggesting that insulin and leptin resistance may play a key role in linking metabolic dysfunction to cognitive impairment by promoting inflammatory and neurotoxic processes that affect brain structure, particularly in regions supporting cognition.
Editor-in-Chief of Biological Psychiatry: Cognitive Neuroscience and Neuroimaging Cameron S. Carter, MD, University of California Irvine School of Medicine, comments, "Interestingly, the effects of metabolic dysfunction on clinical and neural outcomes were primarily observed in bipolar disorder. These findings could be consistent with a neuroprogressive model of bipolar disorder, in which repeated episodes may drive not only clinical worsening but also cumulative metabolic and neurobiological changes. This highlights the importance of early and effective intervention to prevent both clinical deterioration and its associated biological consequences."
The study's findings point towards a previously unknown, clinically meaningful pathway linking metabolic dysfunction to cognitive impairment in bipolar disorder through its impact on brain structure.
"Beyond traditional antidepressant treatments, interventions aimed at enhancing insulin sensitivity—such as insulin-sensitizing agents or intranasal insulin—have already shown promising cognitive benefits," notes lead author Laura Raffaelli, PhD candidate, Psychiatry and Clinical Psychobiology, Division of Neuroscience, IRCCS Ospedale San Raffaele, and Vita-Salute San Raffaele University, Milan, Italy. "More recently, GLP-1 receptor agonists, currently used for metabolic conditions, have gained attention for their potential positive effects on both mood and cognition, representing a promising avenue for future therapeutic development."
Dr. Mazza concludes, "Our findings highlight that metabolic health is not just a peripheral concern, but a key factor influencing brain structure and cognitive functioning in mood disorders. The results of our study help explain why cognitive symptoms often persist even when mood symptoms improve, underscoring the complex interplay between brain and metabolic health. By clarifying these mechanisms, our study opens the door to more personalized treatment strategies that integrate metabolic and psychiatric care."
