Bipolar disorder is a chronic mental disorder related to cognitive deficits. Low serum vitamin D levels are significantly associated with compromised cognition in neuropsychiatric disorders. Although patients with bipolar disorder frequently exhibit hypovitaminosis D, the association between vitamin D and cognition in bipolar disorder, and their neuroaxonal integrity, is unclear.

To investigate the interaction effects between vitamin D and neurofilament light chain (NfL) levels on cognitive domains in bipolar disorder.

Serum vitamin D and NfL levels were determined in 100 euthymic patients with bipolar disorder in a cross-sectional study. Cognitive function was measured with the Brief Assessment of Cognition in Affective Disorders. We stratified by age groups and used general linear models to identify associations between vitamin D and NfL levels and their interaction effects on cognitive domains.

The mean vitamin D and NfL levels were 16.46 ng/nL and 11.10 pg/mL, respectively; 72% of patients were vitamin D deficient. In the older group, more frequent hospital admissions and lower physical activity were identified in the group with versus without vitamin D deficiency. The age-modified interaction effect of vitamin D and NfL was associated with composite neurocognitive scores and verbal fluency in both age groups, and with processing speed domain in the younger group.

We observed a high vitamin D deficiency prevalence in bipolar disorder. We identified the interaction of vitamin D and NfL on cognitive domains, and the effect was modified by age. Longitudinal or randomised controlled studies enrolling patients with various illness durations and mood statuses are required to validate our findings.

Factors affecting quality of life (QOL) ratings for people with dementia (PWD) have been well studied, but few studies have examined the effect of residence on PWD- and caregiver-rated QOL for PWDs. We designed this study to determine the factors related to PWD- and caregiver-rated QOL in dementia as well as factors related to the discrepancy in these ratings.

For this cross-sectional study, we analyzed data from a convenience sample of 106 PWD-family caregiver dyads (58 community-dwelling dyads and 48 nursing-home dyads). PWDs’ data included socio-demographic variables, QOL (assessed by the Quality of Life-Alzheimer's Disease [QOL-AD] scale), cognition, dementia severity, depression, comorbidities, and quality of the dyadic relationship. Caregivers’ data included socio-demographic variables, depressive symptoms, and mutuality of the dyadic relationship.

QOL-AD scores were lower when rated by caregivers than by PWDs. The key factors positively related to both PWD- and caregiver-rated QOL for PWDs were fewer PWD depressive symptoms and higher quality of the PWD-caregiver relationship. The key factors related to the discrepancy in PWD- and caregiver-rated QOL in dementia were PWD residence in a nursing home and lower dementia severity, as well as higher caregiver depression, being an adult child caregiver, and lower caregiver-PWD mutuality.

Caregiver-rated QOL for PWDs and the discrepancy in rated QOL were significantly associated with PWD residence. If caregiver-rated QOL is needed, the effect of residence of PWD should be taken into consideration. Caregivers’ depressive status and mutuality with PWD must be also carefully assessed.