Serum 25-Hydroxyvitamin D Deficiency Is Independently Associated with Cognitive Impairment, Depressive Symptoms, and Functional Dependency in Hospitalised Older Adults: A Cross-Sectional Study from Central Romania

Background: Vitamin D deficiency is highly prevalent in older adults and has been increasingly recognised as a potential contributor to cognitive decline, depressive symptomatology, and functional impairment. However, the clinical significance of specific 25-hydroxyvitamin D thresholds in relation to this multidomain geriatric phenotype remains incompletely characterised. Methods: We conducted a cross-sectional study of 1438 consecutive patients aged 65 years or older admitted for comprehensive geriatric assessment at a tertiary centre in Cluj-Napoca, Romania, between January 2023 and November 2025. Serum 25-hydroxyvitamin D 25(OH)D was categorised as deficient (<20 ng/mL), insufficient (20–30 ng/mL), or sufficient (≥30 ng/mL). Cognitive function was assessed using the Montreal Cognitive Assessment (MoCA) and Mini-Mental State Examination (MMSE), depressive symptoms using the Geriatric Depression Scale (GDS-30 and GDS-SF), and functional status using Activities of Daily Living (ADL) and Instrumental Activities of Daily Living (IADL). Multivariable linear regression analyses were adjusted for age, body mass index, serum albumin, and estimated glomerular filtration rate (eGFR). Results: Suboptimal vitamin D status was highly prevalent in this geriatric cohort, with 43.3% of participants meeting criteria for frank deficiency (<20 ng/mL). Lower 25(OH)D concentrations were significantly associated with worse cognitive performance, greater depressive symptom burden, and higher functional dependency. Serum 25(OH)D correlated positively with MoCA and MMSE scores and inversely with ADL, IADL, and GDS scores. In adjusted models, vitamin D remained independently associated with MoCA, IADL, and GDS. Stratified analyses suggested that the main clinical deterioration occurred below 20 ng/mL, while the 20–30 ng/mL range behaved as an intermediate phenotype closer to sufficiency than to frank deficiency. Conclusions: In this large cohort of hospitalised older adults, serum 25(OH)D deficiency below 20 ng/mL was independently associated with poorer cognition, more depressive symptoms, and greater functional impairment. These findings support routine vitamin D assessment in geriatric practice and suggest that the <20 ng/mL threshold identifies a clinically relevant high-risk phenotype.