BDNF and depressive symptom measurement: In RCTs with baseline 25-hydroxyvitamin D [25(OH)D] < 30 ng/mL, vitamin D supplementation ≥ 2000 IU/daily or 50,000 IU/weekly for ≥12/8 weeks increased circulating BDNF by ≈ +7% and reduced Beck Depression Inventory II (BDI-II) scores by 1.7–7.6 points [39,40].