Vitamin D3 (Cholecalciferol)

What Is Vitamin D3 (Cholecalciferol)?

Vitamin D3 (cholecalciferol) is a fat-soluble secosteroid that functions as a hormone precursor, essential for intestinal calcium and phosphate absorption and normal bone mineralisation. It is synthesised in skin on exposure to UVB sunlight and is also obtained from a few foods (oily fish, egg yolk, fortified foods) and supplements. It is converted in the liver to 25-hydroxyvitamin D (the status marker) and then in the kidney to the active hormone calcitriol. Deficiency causes rickets in children and osteomalacia in adults, and it is one of the most widely supplemented nutrients, often for bone, immune, and mood claims.

Evidence

Vitamin D’s role in calcium homeostasis and bone health, and the deficiency diseases rickets and osteomalacia, are well established, and reference intakes are set by NIH’s Office of Dietary Supplements: an RDA of 15 mcg (600 IU)/day for adults up to age 70 and 20 mcg (800 IU)/day for adults over 70 ^{1Reference 1National Institutes of HealthReviewVitamin D — Health Professional Fact Sheet — review/referenceView study →}. The popular expectation that routine vitamin D supplementation prevents fractures in the general (non-deficient) population is not well supported: the large VITAL randomised trial found that vitamin D3 (2,000 IU/day) did not lower the risk of total, non-vertebral, or hip fractures versus placebo in generally healthy midlife and older adults not selected for deficiency or low bone mass ^{2Reference 2LeBoff MS et al. · 2022RCTSupplemental vitamin D and incident fractures in midlife and older adults (VITAL) — randomised controlled trialView study →}. This does not overturn the value of correcting genuine deficiency, where supplementation clearly matters. Broader claims for immune function and mood are physiologically plausible and often studied, but I did not retrieve and verify specific outcome trials for those in this quick pass; they should be confirmed in a full pass.

Dosage & Safety

The RDA is 600–800 IU/day (15–20 mcg) for adults ^{1Reference 1National Institutes of HealthReviewVitamin D — Health Professional Fact Sheet — review/referenceView study →}. Many products and clinical repletion protocols use higher doses; these are informational and not a personal recommendation. NIH sets a tolerable upper intake level of 4,000 IU/day (100 mcg) for adults ^{1Reference 1National Institutes of HealthReviewVitamin D — Health Professional Fact Sheet — review/referenceView study →}. Because vitamin D is fat-soluble and stored, sustained excessive intake (well above the UL) can cause hypercalcaemia with nausea, kidney effects, and other toxicity — this is a genuine, if uncommon, risk that distinguishes it from water-soluble vitamins ^{1Reference 1National Institutes of HealthReviewVitamin D — Health Professional Fact Sheet — review/referenceView study →}. Specific drug interactions and pregnancy-specific cautions were not fully assessed in this pass.

References

1. National Institutes of Health, Office of Dietary Supplements. Vitamin D — Health Professional Fact Sheet — review/reference. NIH ODS. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
2. LeBoff MS, et al. (2022). Supplemental vitamin D and incident fractures in midlife and older adults (VITAL) — randomised controlled trial. N Engl J Med. https://pubmed.ncbi.nlm.nih.gov/35939577/