Vitamin D (Calciferol)
Fat-soluble secosteroid more accurately classified as a prohormone than a true vitamin: synthesised in keratinocytes from 7-dehydrocholesterol under UV-B (290–315 nm) or obtained as D₃ (animal foods) or D₂ (fungi). Two successive cytochrome P450 hydroxylations — hepatic CYP2R1 produces calcidiol (25(OH)D), renal CYP27B1 produces calcitriol — generate the active hormone. Calcitriol binds VDR at Kd ~0.1 nM; VDR/RXR heterodimer binds DR3-type VDREs in promoters of >1,000 gene loci. Classical skeletal roles in calcium/phosphate homeostasis are unambiguous. Non-skeletal evidence (cancer, CVD) is contested: VITAL (n=25,871) found no benefit for CVD or cancer incidence in a well-nourished US population; Autier 2017 argues low 25(OH)D is largely a consequence, not cause, of poor health (reverse causation).
Overview
Vitamin D is a fat-soluble secosteroid — structurally related to steroid hormones but distinguished by a photochemically broken B-ring in the four-ring steroid backbone (hence seco-steroid). Unlike true vitamins that cannot be synthesised endogenously, vitamin D is more accurately a prohormone: its biologically active form, calcitriol, acts as a classical nuclear receptor ligand governing gene expression in virtually every tissue type.
Two primary dietary forms are recognised: Vitamin D₃ (cholecalciferol), the mammalian form synthesised in skin from 7-dehydrocholesterol and found in animal-derived foods, is approximately twice as effective as D₂ at raising serum 25(OH)D. Vitamin D₂ (ergocalciferol) is produced by UV-irradiation of ergosterol in fungi and is used in some supplements and fortified foods. The skin synthesis pathway has a built-in photoregulatory mechanism: excess UV-B converts pre-vitamin D₃ to tachysterol and lumisterol (inactive photoproducts), preventing toxicity from sun exposure alone.
Global vitamin D deficiency affects an estimated 1 billion people. Key risk factors: high latitude, limited outdoor exposure, dark skin (melanin absorbs UV-B — requires 3–5× more sun time vs. pale skin to synthesise equivalent D₃), advancing age (reduced 7-DHC in epidermis; reduced renal CYP27B1 activity), obesity (fat-soluble D₃ sequestered in adipose tissue, lowering serum 25(OH)D per unit synthesised), malabsorption (Crohn's, coeliac, post-bariatric surgery), and chronic kidney disease (impaired renal 1α-hydroxylation). The clinical serum marker is 25(OH)D (calcidiol), with a half-life of ~2–3 weeks, reflecting cumulative sun exposure and dietary/supplemental intake.
Mechanism of Action
Two-Step Metabolic Activation
7-Dehydrocholesterol (keratinocytes / dermal fibroblasts)
|
| UV-B photons (290-315 nm) -> photolysis of B-ring
v
Pre-vitamin D3 -> [thermal isomerisation] -> Cholecalciferol (D3)
| (excess UV-B -> tachysterol + lumisterol: built-in toxicity limit)
| Binds vitamin D-binding protein (DBP) -> circulation
v
─── LIVER ───────────────────────────────────────────────────────
CYP2R1 (primary; microsomal) / CYP27A1 (minor; mitochondrial)
| 25-hydroxylation (largely constitutive; substrate-driven)
v
25(OH)D3 / Calcidiol [t1/2 ~2-3 wk; clinical status biomarker]
| DBP-bound (>99%); only free fraction filtered at glomerulus
v
─── KIDNEY (proximal tubular cells) ──────────────────────────────
CYP27B1 (1alpha-hydroxylase)
Upregulated by: PTH (cAMP-PKA), low phosphate, low Ca2+, IGF-1
Inhibited by: FGF-23 (FGFR1-Klotho), calcitriol (neg feedback), high Ca2+
| 1alpha-hydroxylation (rate-limiting, tightly regulated)
v
1,25(OH)2D3 / Calcitriol [Kd ~0.1 nM to VDR; t1/2 ~4-6 h]
|
| CYP24A1 (24-hydroxylase, VDR-induced) -> calcitroic acid (bile excretion)
v
VDR / RXR heterodimer -> DR3 VDREs (AGGTCAnnnAGGTCA)
|
v >1,000 target genes:
|-- TRPV6, calbindin-D9k (Ca2+ absorption: duodenum)
|-- TRPV5, calbindin-D28k (Ca2+ reabsorption: DCT)
|-- RANKL (osteoblasts -> osteoclastogenesis)
|-- PTH suppression (parathyroid gland)
|-- Cathelicidin/LL-37 (macrophages: antimicrobial)
`-- CYP24A1 (self-inactivation feedback)
- UV-B skin synthesis: 7-DHC in keratinocytes absorbs UV-B → pre-vitamin D₃ → thermal isomerisation to cholecalciferol → enters circulation bound to DBP.
- Hepatic 25-hydroxylation (CYP2R1): Constitutive, substrate-driven; produces calcidiol (25(OH)D) — the clinical status marker with t½ ~2–3 wk.
- Renal 1α-hydroxylation (CYP27B1): Rate-limiting; tightly regulated; upregulated by PTH and low mineral status; produces calcitriol (1,25(OH)₂D₃).
- VDR binding: Calcitriol diffuses into cells, binds VDR with Kd ~0.1 nM; VDR/RXR heterodimer binds DR3-type VDREs (direct repeats, 3-nt spacer).
- Transcriptional regulation: >1,000 target genes including TRPV6 (Ca²⁺ absorption), RANKL (bone remodelling), cathelicidin (innate immunity), PTH suppression, and CYP24A1 (autoregulatory inactivation).
Pleiotropic Actions via VDR
Calcium & Bone Homeostasis
VDR → ↑TRPV6 + calbindin-D9k → ↑intestinal Ca²⁺ absorption (10–15% → 30–40%); ↑TRPV5 in DCT → ↑renal Ca²⁺ reabsorption; ↑RANKL on osteoblasts → osteoclastogenesis for bone remodelling. Deficiency → rickets (children) / osteomalacia (adults) / secondary hyperparathyroidism.
Innate Immunity
Macrophages and DCs express CYP27B1 — local calcitriol production autocrine/paracrine. VDR → ↑cathelicidin/LL-37 (broad antimicrobial peptide) and ↑β-defensin 2. Critical in TB susceptibility (VDR-cathelicidin axis). Most clearly beneficial in frank deficiency.
Adaptive Immune Modulation
↓DC IL-12 → ↓Th1 polarisation; ↓IL-23/IL-17 → ↓Th17 activity; ↑Foxp3⁺ Treg differentiation via VDR in naïve CD4⁺ cells → ↑IL-10, ↑TGF-β → immune tolerance. Relevant in MS, T1D, IBD models — but RCTs of supplementation largely negative for clinical outcomes.
Non-Genomic Rapid Signalling
Membrane VDR / PDIA3 → rapid (seconds–minutes) ↑intracellular Ca²⁺ (PLC → IP₃), ↑PKC, ↑MAPK → rapid insulin secretion from pancreatic β-cells; vascular smooth muscle tone regulation independent of nuclear VDR.
Dietary Sources & RDA
| Source | Serving | Vitamin D Content | % RDA (600 IU/day) |
|---|---|---|---|
| Wild-caught salmon | 3 oz (85 g) | 600–1,000 IU D₃ | 100–167% |
| Cod liver oil | 1 tsp (5 mL) | ~400 IU D₃ | 67% |
| Canned tuna | 3 oz | 150–200 IU D₃ | 25–33% |
| UV-irradiated mushrooms | 3 oz | 400–1,000 IU D₂ | 67–167% (variable; rapidly degrades) |
| Fortified milk | 8 oz (240 mL) | 100–120 IU D₃ | 17–20% |
| Egg yolk | 1 large | 40–60 IU D₃ | ~8% |
| Fortified orange juice | 8 oz | 100 IU D₃ | 17% |
RDA (US): 600 IU/day (adults <70); 800 IU/day (≥70). Tolerable Upper Intake Level: 4,000 IU/day (US) / 100 µg/day (EU) — conservative. Most dietary patterns fall far short of needs at high latitude or with limited sun — supplementation is routinely required in at-risk groups.
Clinical Evidence
| Trial / Evidence | Design | Key Result | GRADE |
|---|---|---|---|
| DIPART Meta-analysis (2010) Hip fracture prevention |
IPD meta-analysis; n=68,500; 7 trials | Vitamin D₃ + calcium → 16% ↓ hip fracture risk (HR 0.84; 95% CI 0.74–0.96) in institutionalised elderly. Vitamin D alone: less consistent benefit. | Moderate |
| VITAL Trial (2019) Cancer & CVD prevention |
RCT; n=25,871; 5.3 yr follow-up; 2,000 IU D₃/day + omega-3 | No significant reduction in cancer incidence (HR 0.96; p=0.59) or CVD events (HR 0.97; p=0.74). Cancer-related mortality: HR 0.83 (p=0.06 — marginal, not prespecified primary). Baseline 25(OH)D median ~30 ng/mL (already sufficient). | Moderate (negative) |
| Martineau Meta-analysis (2017, BMJ) Acute respiratory infections |
IPD meta-analysis; 25 RCTs; n=11,321 | Daily/weekly dosing → OR 0.81 (95% CI 0.72–0.91) for any respiratory infection. Severely deficient (<25 nmol/L): OR 0.30 — 70% risk reduction. Bolus dosing: no significant benefit. | Moderate |
| Autier et al. (2017, Lancet DE) Non-skeletal outcomes |
Systematic review of meta-analyses & RCTs | Strong observational associations between low 25(OH)D and diverse disease outcomes likely reflect reverse causation (sick/inactive people → less sun → lower D) rather than causation. RCTs consistently negative for non-skeletal benefits in non-deficient populations. | Low–Insufficient (non-skeletal) |
Evidence hierarchy: Vitamin D supplementation has unambiguous benefit for skeletal outcomes (rickets prevention, fracture reduction with calcium in elderly) and in frank deficiency (<20 ng/mL). For non-skeletal outcomes — cancer incidence, CVD, T2DM, autoimmunity — observational associations are strong but RCTs are largely negative, consistent with reverse causation. VITAL is the best-powered non-skeletal RCT and was clearly negative for primary endpoints. The most compelling benefit signal remains in severely deficient populations for infectious disease outcomes.
Deficiency & Excess
| Status | Signs & Symptoms | Lab Marker | Threshold |
|---|---|---|---|
| Deficiency | Children: rickets (bowed legs, craniotabes, rachitic rosary). Adults: osteomalacia (bone pain, proximal myopathy), secondary hyperparathyroidism, ↑fracture risk, ↓immune defence, fatigue | Serum 25(OH)D | <20 ng/mL (<50 nmol/L) — Endocrine Society / NIH ODS |
| Insufficiency | Subclinical: mild fatigue, myalgia, secondary hyperparathyroidism driving bone loss, impaired immune regulation | Serum 25(OH)D | 20–29 ng/mL (50–75 nmol/L) |
| Sufficiency | Normal calcium homeostasis, bone mineralisation, immune function | Serum 25(OH)D | 30–60 ng/mL (75–150 nmol/L) |
| Toxicity (hypervitaminosis D) | Hypercalcaemia: nausea, vomiting, polyuria, polydipsia, confusion, weakness, nephrolithiasis. Prolonged severe excess: metastatic calcification (vascular, renal). Almost exclusively from excessive supplementation — sun alone cannot cause toxicity (photoregulatory conversion). | Serum 25(OH)D; serum Ca²⁺; urinary Ca²⁺ | >150 ng/mL (>375 nmol/L); typically requires sustained >10,000 IU/day. CYP24A1 inactivating mutations (Williams syndrome) cause hypersensitivity at lower doses. |
Connections
References
- Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266-81. doi:10.1056/NEJMra070553 · PubMed 17634462
- Manson JE, Cook NR, Lee IM, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease (VITAL). N Engl J Med. 2019;380(1):33-44. doi:10.1056/NEJMoa1809944 · PubMed 30415629
- Autier P, Mullie P, Macacu A, et al. Effect of vitamin D supplementation on non-skeletal disorders. Lancet Diabetes Endocrinol. 2017;5(12):986-1004. doi:10.1016/S2213-8587(17)30357-1 · PubMed 29102597
- Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. doi:10.1136/bmj.i6583
- DIPART Group. Patient level pooled analysis of 68,500 patients from seven major vitamin D fracture trials. BMJ. 2010;340:b5463. doi:10.1136/bmj.b5463
- Berg JM, Tymoczko JL, Stryer L. Biochemistry. 9th ed. W.H. Freeman; 2019.