Dietary & Supplement Calcium Intake
Calcium reaches the gut lumen from two different sources.
- 1000–1200 mg: RDA, adults (Varies by age and sex.)
- ~300 mg: Dairy serving (One cup of milk.)
- 500–600 mg: Common supplement dose (Per calcium carbonate tablet.)
- ~500 mg: Max efficient single dose (Efficiency drops above this.)
Food-bound calcium
Dairy, greens, and fortified foods release calcium during digestion.
Supplement forms
Carbonate needs stomach acid; citrate absorbs without food.
Luminal free Ca²⁺
Both sources yield free ionic calcium in the lumen.
Vitamin D Activation — Skin, Liver, Kidney
Vitamin D must be hydroxylated twice before it can act.
- D3: Skin synthesis (Made from UVB sunlight.)
- 25(OH)D: Liver hydroxylation (Storage form, measured in blood.)
- 1,25(OH)₂D: Kidney hydroxylation (Calcitriol, the active hormone.)
- >30 ng/mL: Sufficient serum level (Threshold for adequate status.)
Cutaneous synthesis
UVB converts skin cholesterol precursors into vitamin D3.
Hepatic 25-hydroxylation
The liver makes 25(OH)D, the main circulating marker.
Renal 1α-hydroxylation
Kidneys convert it to calcitriol, the true hormone.
Calcitriol-Driven Active Transcellular Transport
Calcitriol switches on a three-protein relay across the gut cell.
- TRPV6: Entry channel (Apical calcium entry channel.)
- Calbindin-D9k: Cytoplasmic shuttle (Carries Ca²⁺ across the cell.)
- PMCA1b: Basolateral pump (Pumps calcium into the bloodstream.)
- ~50–60%: Max active fraction (At low intake, high vitamin D.)
TRPV6 gating
Calcitriol upregulates TRPV6 expression at the apical membrane.
Calbindin shuttling
Calbindin buffers and ferries calcium across the cytoplasm.
PMCA1b extrusion
ATP-driven pumps push calcium out toward capillaries.
Passive Paracellular Absorption at High Intake
When intake is high, calcium also slips between epithelial cells.
- Tight junctions: Pathway (Gaps between epithelial cells.)
- Minimal: Vitamin D dependence (Works regardless of vitamin D.)
- ~10–15%: Typical fraction (Roughly constant across intakes.)
- >2500 mg: Dominant when (Active transport becomes saturated.)
Concentration-driven diffusion
A steep luminal gradient pushes calcium through the gaps.
Non-saturable kinetics
Unlike TRPV6, this route scales linearly with dose.
Clinical relevance
It explains why split dosing improves supplement efficiency.
Calcium Delivery to the Bone Mineralization Front
Absorbed calcium finally reaches osteoblasts building new bone crystal.
- Hydroxyapatite: Mineral formed (Ca₁₀(PO₄)₆(OH)₂ crystal lattice.)
- ~4 months: Remodeling cycle (Full cycle in adult bone.)
- ~99%: Skeletal calcium share (Of total body calcium.)
- ~500 mg: Daily bone turnover (Deposited and resorbed daily.)
Blood-to-bone transit
Blood carries absorbed calcium to active osteoblast surfaces.
Osteoid mineralization
Calcium and phosphate crystallize onto the collagen matrix.
Deficiency consequences
Chronic shortfall favors bone resorption over new mineral.