Calcium (Elemental Ca)

Direct Answer

Calcium is the most abundant mineral in the human body (1.0–1.2 kg), with 99% sequestered in hydroxyapatite bone lattice and 1% tightly regulated at 8.5–10.5 mg/dL serum for cardiac contractility, nerve signaling, and blood coagulation. Adult RDA is 1,000–1,200 mg/day.

Source: NIH ODS / EFSA · Last reviewed Aug 28, 2026

Specs

Nutrient Type	Essential Macromineral
Adult RDA / RDI	1,000 mg / day (Men 19–70, Women 19–50) / 1,200 mg (Women 51+, Men 71+)
Tolerable Upper Limit (UL)	2,500 mg / day (Adults 19–50) / 2,000 mg / day (Adults 51+) (Avoid nephrolithiasis / vascular calcification)
Chemical Forms	Calcium Carbonate (40% elemental Ca / requires stomach acid), Calcium Citrate (21% elemental Ca / acid-independent), Hydroxyapatite
Primary Biological Function	Hydroxyapatite bone/dental structure [Ca10(PO4)6(OH)2], Troponin-C actin-myosin muscle contraction, Factor IV blood clotting cascade, Exocytosis neurotransmitter release
Deficiency Condition	Osteoporosis, Osteopenia, Rickets (children), Tetany (Trousseau's and Chvostek's signs of hypocalcemia)
Top Whole Food Sources	Parmigiano-Reggiano, Sharp Cheddar, Whole Milk, Sardines with bones, Kale, Tofu (Calcium-set), Fortified Plant Milk
Parathyroid Hormone (PTH) Homeostasis	Serum Ca2+ drop triggers PTH secretion, stimulating renal 1-alpha-hydroxylase to produce active Calcitriol (1,25-OH-D3) and osteoclast bone resorption

The Calcium Absorption Window

Intestinal calcium absorption saturates at ~500 mg elemental calcium per meal via active TRPV6 transport. Splitting supplemental intake into two daily doses maximizes systemic bioavailability.