Epidemiological studies have revealed an association between obesity and cognitive impairment in otherwise healthy subjects. Additionally, obesity is associated with significant reductions in both gray and white matter volumes, particularly within the frontal lobes.

Mechanisms underlying the associations between obesity and diabetes and cognitive dysfunction are unclear; however, one hypothesis proposes that adaptations in dopamine signaling secondary to diet, adiposity, and metabolic dysfunction underlie neurocognitive impairment in diabetes and obesity.

In MDD, cognitive impairment is recognized as a principal mediator of psychosocial impairment, although antidepressant medications have not generally been shown to improve measures of cognitive control and executive function. Vortioxetine is an exception in that it is a multimodal antidepressant that has been reported to improve performance in objective measures of cognition.

Over the years, the cognitive impairment of MDD has been the source of considerable debate: do cognitive deficits simply reflect a state marker of the depressive episode; or perhaps a manifestation of pre-existing susceptibility or vulnerability to MDD; or, perhaps “the scar hypothesis” has more merit, whereby cognitive dysfunction is predominantly a consequence of the first episode of MDD and is thereafter expected to persist or worsen with subsequent episodes.

More recently, results from a systematic review including 29 publications, representing 34 unique samples and 121,749 participants, suggested that the cognitive deficits predicting MDD likely represent deleterious effects of subclinical depression symptoms on performance, rather than premorbid risk factors for disorder. While the debate over state vs trait continues, evidence supports the concept that the association between obesity and MDD at the observational level may be explained, at least in part, by shared genetic factors.

In parallel, there has been growing interest in the differences between obese and lean subjects in the activation of certain brain regions or networks, spurred on by fMRI techniques that measure brain activity by detecting changes associated with blood oxygenation level-dependent (BOLD) signals. Recently, Chao and colleagues applied such techniques to the study of obesity and psychiatric illness in a group of patients ranging in age from 22 to 58 years. Patients were assessed for symptoms of anxiety and depression using the Hospital Anxiety and Depression Scale (HADS):

Multimodality imaging models were used to assess brain functional connectivity differences between normal weight, control, and obese participants. Investigators identified a disruption in the default mode network (DMN) and dysfunction in reward-based learning and reward processing (including differences in the anterior cingulate cortex, the orbitofrontal cortex, and the amygdala) in patients with morbid obesity when compared to normal weight individuals.

The authors suggested that a larger sample size study with specific eating behavior phenotype subgroups and longitudinal design in the future might help delineate the dysfunction brain circuits in obesity in detail and clarify the causal relationships.

Adiposity is clearly only a piece of the broader picture of MDD and cognition. Moderately vigorous physical activity has shown a positive influence on cognitive control that persists following the adjustment of significant covariates and adiposity. Nonetheless, the overlap between obesity and MDD as relates to cognitive function is intriguing and promises to spur advances across disciplines.