Vitamin D (25-OH)

25-hydroxyvitamin D (25-OH-D, calcidiol) is the major circulating form of vitamin D and the standard clinical biomarker for assessing vitamin D status, produced in the liver by hydroxylation of cholecalciferol (D3) or ergocalciferol (D2). It reflects both dietary intake and skin synthesis from UVB exposure; its half-life of roughly 2–3 weeks makes it a reliable index of longer-term status compared with the active hormonal form 1,25-(OH)₂D. Deficiency (commonly defined as <50 nmol/L or <20 ng/mL) is associated with impaired bone mineralization, falls, increased susceptibility to respiratory infection, and observationally with higher all-cause and cardiovascular mortality, although randomized trials have shown mixed results for supplementation on hard outcomes. Sufficiency thresholds remain debated, but most guidelines recommend >50 nmol/L and many longevity-oriented practitioners target 75–100 nmol/L.