Vitamin D3 (Cholecalciferol)
Essential Vitamins & Minerals · 8 specs
Direct Answer
Vitamin D3 is a secosteroid prohormone synthesized endogenously in skin via solar UVB irradiation of 7-dehydrocholesterol. Adult RDA is 600–800 IU (15–20µg), with a Tolerable Upper Limit of 4,000 IU (100µg)/day, regulating systemic calcium absorption and immune gene expression.
Source: Endocrine Society / NIH ODS · Last reviewed Aug 28, 2026
Specs
| Nutrient Type | Fat-Soluble Secosteroid Prohormone |
| Adult RDA / RDI | 600 – 800 IU / day (15 – 20 µg) |
| Tolerable Upper Limit (UL) | 4,000 IU / day (100 µg) (Endocrine Society: up to 10,000 IU safe) |
| Chemical Forms | Cholecalciferol (D3 - Animal/Lichen), Ergocalciferol (D2 - Fungi) |
| Primary Biological Function | Calcitriol receptor (VDR) activation, intestinal calcium/phosphorus absorption, antimicrobial peptide synthesis |
| Deficiency Condition | Rickets (children), Osteomalacia (adults), Immune dysregulation |
| Top Whole Food Sources | Sunlight UVB, Atlantic Salmon, Sardines, Egg Yolk, Fortified Milk |
| Target Serum 25(OH)D Level | 30 – 50 ng / mL (75 – 125 nmol / L) |
Hepatic 25-Hydroxylation to Calcitriol
Vitamin D3 is hydroxylated in the liver to 25(OH)D (calcifediol, the circulating biomarker) and subsequently in the kidneys via 1-alpha-hydroxylase into 1,25(OH)2D (calcitriol, the active steroid hormone).
Technical Comparisons
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Vitamin D3 (Cholecalciferol) vs Calcium (Elemental Ca)
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Vitamin D3 (Cholecalciferol) vs Magnesium (Elemental Mg)
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Vitamin D3 (Cholecalciferol) vs Potassium (Elemental K)
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Vitamin D3 (Cholecalciferol) vs Selenium (Elemental Se)
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Vitamin D3 (Cholecalciferol) vs Sodium (Elemental Na)
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Vitamin D3 (Cholecalciferol) vs Vitamin A (Retinol & Beta-Carotene)
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