From Routine Supplementation to Targeted Repletion: Vitamin D Deficiency in UVB-Constrained, Pollution-Exposed, and...

Background/Objectives: Vitamin D deficiency remains common in UVB-constrained, pollution-exposed, sedentary, and ageing populations, yet recent guidelines discourage routine screening and empiric high-dose supplementation in generally healthy adults. This review clarifies the clinical distinction between untargeted population supplementation and monitored repletion of documented deficiency in high-risk individuals. Methods: We conducted a focused narrative review of PubMed/MEDLINE, Scopus, Web of Science, and the Cochrane Library. The focused evidence search supporting the narrative synthesis was completed in June 2026. References published subsequently were included during manuscript preparation only when relevant for contextual purposes and were not incorporated retrospectively into the evidence-selection process underlying the review's clinical conclusions. Evidence was interpreted according to baseline and achieved 25-hydroxyvitamin D [25(OH)D], dosing schedule, route, body composition, endpoint, and population risk. No formal risk-of-bias assessment, certainty grading, or systematic meta-analytic synthesis was performed. Key Findings: The strongest rationale for vitamin D repletion remains musculoskeletal, particularly for osteomalacia, muscle function, osteoporosis care, and fracture-related vulnerability in deficient or high-risk patients. Management should separate generally healthy adults from patients with an indication for testing or treatment. Historical and regional expert frameworks describe time-limited higher-dose repletion for documented deficiency, but these regimens should not be interpreted as a current universal Endocrine Society treatment standard. Short-term trials using moderate cumulative oral doses suggest broadly comparable biochemical repletion with daily, weekly, or monthly schedules; equivalence has not been established for long-term clinical outcomes, frail populations, or large intermittent doses. Extra-skeletal associations remain less trial-supported. Conclusions: Vitamin D deficiency should be managed using clinically indicated assessment, explicit repletion and maintenance regimens, and selective biochemical reassessment. Targets above conventional skeletal adequacy thresholds remain investigational and should not be applied as universal treatment goals.