Vitamin D deficiency affects an estimated 1 billion people worldwide and 35–40% of Americans — a prevalence that reflects our largely indoor, sunscreen-using, melanin-rich (in darker-skinned individuals, melanin reduces UV-B conversion), and often obese population (adipose tissue sequesters vitamin D). Despite decades of research, optimal 25-hydroxyvitamin D [25(OH)D] levels and supplementation strategies remain contentious, with high-quality RCT data frequently disappointing observational promise.
Testing: Who Needs It, What to Measure
Serum 25(OH)D is the correct test — it reflects combined dietary and cutaneous synthesis and has a half-life of 2–3 weeks. 1,25-dihydroxyvitamin D (calcitriol) is the active form but tightly regulated by PTH and calcium; testing it rarely helps in most clinical scenarios except suspected granulomatous disease. Endocrine Society definitions: deficient < 20 ng/mL, insufficient 20–29 ng/mL, sufficient ≥ 30 ng/mL. The National Academy of Medicine considers ≥ 20 ng/mL adequate for bone health in most. Routine screening in asymptomatic healthy adults is not recommended by USPSTF (2021) — testing is appropriate in high-risk groups: malabsorption syndromes, osteoporosis, chronic kidney disease, dark skin + limited sun exposure, obesity, medications affecting vitamin D metabolism (glucocorticoids, anticonvulsants). Our laboratory supplies section includes serum collection tubes and specimen processing supplies for routine vitamin D screening panels.
Supplementation: What Actually Works
For bone health: supplementation at 800–1000 IU/day in adults ≥65 reduces fracture risk in deficient individuals — the VITAL trial (2000 IU/day, n=25,871) found no reduction in cancer or CVD events but suggested benefit in those with low baseline vitamin D. For immune function: strong mechanistic and some clinical evidence supports adequate vitamin D for innate immunity — deficiency increases respiratory infection risk, and supplementation reduces ARTIs in deficient patients (BMJ meta-analysis 2017). For cardiovascular and diabetes outcomes: large RCTs (VITAL, D-HEALTH) have been largely disappointing despite compelling observational data — likely confounded by metabolic factors. For depression: VITAL found no benefit in depression prevention. Practical protocol: deficiency (< 20 ng/mL) — treat with 50,000 IU vitamin D2 weekly × 8 weeks then 1500–2000 IU daily maintenance; target ≥ 30 ng/mL. Vitamin D3 (cholecalciferol) is preferred over D2 for superior bioavailability. K2 co-supplementation has theoretical benefit for calcium routing but lacks high-quality RCT data. Our nutrition catalog includes clinical-grade vitamin D and micronutrient products for patient care settings.
