Full mechanism, in text

The kidney activates it

1 · 25(OH)D enters proximal tubule

After bound to is filtered at the glomerulus, proximal tubule cells recapture the whole complex through the megalin and cubilin receptors. floats in blood bound to . As it passes through the kidney, some is filtered at the glomerulus and then recaptured by epithelial cells in the proximal tubule.

The recapture mechanism is elegant: the megalin + cubilin receptor complex on the apical membrane endocytoses the entire DBP-25(OH)D complex into the cell — the same cubilin recaptures the IF-B12 complex in the ileum.

Once inside the cell, 25(OH)D is released from DBP and caught in the mitochondrion by CYP27B1.

2 · CYP27B1 fires the 1α-OH

CYP27B1 adds another hydroxyl group at C1 of , producing calcitriol (1,25(OH)₂D), the active hormone that does the work.CYP27B1 (1α-hydroxylase) adds another hydroxyl group at the C1 position of → producing 1,25-dihydroxyvitamin D, 1,25(OH)₂D = calcitriol — the truly active steroid hormone that does the work.

By now vitamin D has been modified twice: at position 25 (liver) + at 1α (kidney). The complete structure now binds the nuclear vitamin D receptor () with high affinity and enters the nucleus to regulate gene transcription.

Its half-life is only 4-15 hours (vs weeks for 25(OH)D) — precisely because it is so potent it can't be allowed to drift through blood unchecked.

3 · PTH ↑ FGF23 ↓ Mg cofactor

CYP27B1 is tightly regulated: and low phosphate push it up, FGF23 and calcitriol itself hold it down, and it needs magnesium as a cofactor.CYP27B1 is one of the body's most exquisitely regulated enzymes:

Positive drivers:
· (parathyroid hormone) — rises when serum calcium is low → strongly upregulates CYP27B1 → more calcitriol → drives calcium absorption.
· Hypophosphatemia → directly stimulates CYP27B1.
· Growth hormone / → modest stimulation.

Inhibitors:
· FGF23 (fibroblast growth factor 23, secreted by osteocytes) — rises with high serum phosphate → inhibits CYP27B1 (preventing calcium and phosphate from rising together).
· Calcitriol itself provides feedback inhibition → a self-limiting loop.
· Hypercalcemia → CaSR-mediated inhibition.

Magnesium is a cofactor of CYP27B1 — even with adequate D supplementation, magnesium-deficient individuals stall at this step. This is the common hidden reason for " blood level is adequate but the patient feels no better."

4 · Binds VDR → enters nucleus

Calcitriol enters the cell and binds , pairs with RXR, enters the nucleus and binds VDREs on DNA, regulating about 1,000 genes.Calcitriol is a steroid hormone — it crosses the cell membrane and is caught in the cytoplasm by (the vitamin D receptor). VDR belongs to the nuclear hormone receptor superfamily (the same family as the estrogen receptor ER and glucocorticoid receptor GR).

The VDR-calcitriol complex heterodimerizes with RXR (retinoid X receptor) → enters the nucleus → binds VDRE (vitamin D response elements) on DNA → recruits coactivators / corepressors → regulates the expression of ~1,000 genes (about 3% of the genome).

Downstream effects:
· Intestinal TRPV6 + calbindin-D9k + PMCA1b (calcium absorption).
· Osteocalcin + (bone remodeling).
· Antimicrobial peptide LL-37 (immunity).
· Genes related to the muscle pathway.

Vitamin D is not just "a vitamin to supplement" — it is a steroid hormone. This is the chemical root of "when D is deficient, a little bit of everything is deficient."

Full mechanism, in text

1 · 25(OH)D enters proximal tubule

2 · CYP27B1 fires the 1α-OH

3 · PTH ↑ FGF23 ↓ Mg cofactor

4 · Binds VDR → enters nucleus

Kidney · activation to calcitriol

Full mechanism, in text

1 · 25(OH)D enters proximal tubule

2 · CYP27B1 fires the 1α-OH

3 · PTH ↑ FGF23 ↓ Mg cofactor

4 · Binds VDR → enters nucleus

Kidney · activation to calcitriol