SummaryObjectivesTraumatic brain injury (TBI) is a major cause of long‐term disability with variable recovery. Preclinical studies suggest that vitamin D status influences the recovery after TBI. However, there is no published clinical data on links between vitamin D status and TBI outcomes. The aim was to determine the (i) prevalence of vitamin D deficiency/insufficiency, and associations of vitamin D status with (ii) demographic factors and TBI severity, and with (iii) cognitive function, symptoms and quality of life, in adults after TBI.DesignRetrospective audit of patients seen between July 2009 and March 2015. Serum vitamin D (25‐hydroxy‐cholecalciferol) was categorized as deficient (<40 nmol/l), insufficient (40–70 nmol/l) or replete (>70 nmol/l).PatientsA total of 353 adults seen in tertiary hospital clinic (75·4% lighter skinned, 74·8% male, age median 35·1 year, range 26·6–48·3 year), 0·3–56·5 months after TBI (74·5% moderate–severe).MeasurementsSerum vitamin D concentrations; Addenbrooke's Cognitive Examination (ACE‐R), Beck Depression Inventory‐II (BDI‐II), SF‐36 Quality of Life, Pittsburgh Sleep Quality Index.ResultsIn total, 46·5% of patients after TBI had vitamin D deficiency and 80·2% insufficiency/deficiency. Patients with vitamin D deficiency had lower ACE‐R scores than those of vitamin D replete (mean effect size ± SEM 4·5 ± 2·1, P = 0·034), and higher BDI‐II scores than those of vitamin D insufficient (4·5 ± 1·6, P = 0·003), correcting for age, gender, time since TBI and TBI severity. There was no association between vitamin D status and markers of TBI severity, sleep or quality of life.ConclusionVitamin D deficiency is common in patients after TBI and associated with impaired cognitive function and more severe depressive symptoms.

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