The Double-Deficient Older Inpatient: Vitamin D and Vitamin B12 Co-Deficiency Identifies the Greatest Cognitive and Affective Burden

Background/Objectives: Vitamin D and vitamin B12 deficiencies are both widespread among older adults, yet their combined status is rarely examined within the same cohort and whether the two exposures interact or simply accumulate remains unresolved. This study examined the association between combined vitamin D and B12 status and cognitive, affective, and frailty outcomes in hospitalised older adults, and formally tested for an interaction between the two deficiencies. Methods: This cross-sectional study included 1359 consecutive patients aged 65 years or older admitted for comprehensive geriatric assessment in Cluj-Napoca, Romania, between January 2023 and April 2026, with concurrent serum 25-hydroxyvitamin D and vitamin B12 measurements. Patients were classified as having no deficiency, isolated vitamin D deficiency (<20 ng/mL), isolated vitamin B12 deficiency (<200 pg/mL), or co-deficiency, and outcomes were compared by multivariable regression with a formal interaction term. Results: Co-deficiency affected 96 patients (7.1%). Isolated vitamin D deficiency was independently associated with poorer cognition and greater frailty, while isolated B12 deficiency showed no association with any outcome. Co-deficient patients carried the heaviest cognitive burden of all groups and showed a signal of elevated depressive symptoms that was significant at the primary B12 threshold but attenuated at an alternative threshold, indicating this finding should be interpreted cautiously; no interaction term reached significance. Conclusions: Vitamin D deficiency, alone or combined with B12 deficiency, consistently tracked with poorer cognition and frailty, and co-deficiency marked the subgroup at greatest cognitive and affective risk through an additive rather than synergistic mechanism. These findings support routine vitamin D assessment in geriatric practice, with concurrent B12 deficiency flagging patients who warrant closer attention.