What vitamin D actually does in the body
Vitamin D is a hormone precursor that runs calcium absorption, bone mineralization, and immune signaling. What it does, what the big trials found, and the dose that works.
What people ask us next.
- What does vitamin D do in the body?
- It acts as a hormone precursor. Converted to calcitriol, it drives calcium absorption from the gut and mineralizes bone, its core job, and binds receptors in immune and other tissues for broader effects (NIH ODS). Deficiency causes rickets and osteomalacia. The large trials (VITAL 2019) show the clearest benefit is correcting a measured deficiency rather than dosing already-sufficient people.
- What is vitamin D good for?
- Bone health is the best-established benefit, via calcium absorption, especially when combined with calcium in deficient or older people. It also modestly supports immune function and respiratory-infection resistance, again largest in those deficient at baseline (Martineau 2017). It is not a proven broad preventive for cancer or heart disease in replete people (Manson 2019).
- How much vitamin D should I take per day?
- The RDA is 600 IU/day for adults under 70 and 800 IU/day over 70 (IOM 2011). Common maintenance supplementation is 1000-2000 IU/day, the dose VITAL used. The tolerable upper intake level is 4000 IU/day; higher doses should be tied to a measured deficiency and clinician oversight, not taken open-ended.
- Is D3 better than D2?
- Yes, for raising and sustaining blood levels. Tripkovic et al. 2012 found D3 (cholecalciferol) raises serum 25-OH-D more effectively than D2 (ergocalciferol) at equivalent doses. D2 is mainly relevant as a prescription high-dose or vegan option. For maintaining status, choose D3. Our vitamin D ranking compares verified D3 products by $/1000 IU.
- Do I need a blood test before supplementing?
- It is the only way to dose precisely. The marker is serum 25-hydroxyvitamin D; a common sufficiency target is 30-50 ng/mL, and below 20 ng/mL is deficient. A maintenance dose of 1000-2000 IU/day is low-risk without testing, but anyone using higher doses or correcting a suspected deficiency should test, then retest in 8-12 weeks.
- Can you take too much vitamin D?
- Yes. Toxicity comes from supplements, not sun, and presents as hypercalcemia. It generally requires sustained intake well above the 4000 IU/day upper limit, often 10000 IU/day for months (NIH ODS). Staying at or below the UL without a measured deficiency keeps the risk near zero.
Every claim, cited.
- [01]NIH Office of Dietary Supplements. Vitamin D Fact Sheet for Health Professionals. Prohormone metabolism, calcium absorption, RDA and UL.
- [02]Institute of Medicine. 2011. Dietary Reference Intakes for Calcium and Vitamin D. National Academies Press. RDA 600-800 IU/day; calcium absorption.
- [03]Manson JE, Cook NR, Lee IM, et al. 2019. Vitamin D supplements and prevention of cancer and cardiovascular disease (VITAL). N Engl J Med 380(1):33-44. 2000 IU/day, no reduction in primary endpoints in replete population.
- [04]Martineau AR, Jolliffe DA, Hooper RL, et al. 2017. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis. BMJ 356:i6583. Modest protection, largest in deficient daily-dosed subjects.
- [05]Tripkovic L, Lambert H, Hart K, et al. 2012. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D: a systematic review and meta-analysis. Am J Clin Nutr 95(6):1357-1364. D3 superior to D2.
The Larderlab Team builds evidence-led frameworks for eating, lifting, and stocking a kitchen. We cite every claim. We publish the spreadsheet when possible. We buy what we review at retail price. When new data lands, we revise with a dated note.
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