A structural mineral the body cannot make, drawn from the skeleton when intake falls short. Raising intake firms bone in year one only, modestly lowers blood pressure, reduces return of pre-cancerous bowel growths, and cuts dangerous high blood pressure in pregnancy where habitual intake is low. Whether it prevents broken bones is unsettled; harms cluster on supplements, not food. (Full Review)

Protocol

Standard total intake target

1,000–1,200 mg daily

1,000 mg for adults 19–50 and men to 70; 1,200 mg for women over 50 and men over 70. Counts food and supplements together, not supplements alone.

Split dosing

Max 500 mg per dose, with meals

Doses above 500 mg of elemental calcium saturate active absorption; splitting 1,000 mg into two 500 mg doses raises absorbed fraction and reduces gastrointestinal complaints.

Dose form

Carbonate 40% or citrate 21% elemental

Carbonate is cheapest; citrate absorbs about 22–27% better and works without gastric acid. Labels state elemental content.

Time to effect

Bone density

12 months

Changes take 12 months to register on a scan. The gain appears in year one and stops.

Blood pressure

8–12 weeks

Blood pressure shifts over 8–12 weeks. Small at the individual level but persistent.

Premenstrual symptoms

3 cycles

Premenstrual symptom change took three cycles in trial, across all four symptom factors.

Benefits

High: Increased bone mineral density; lower blood pressure; fewer recurrent colorectal adenomas; reduced pre-eclampsia and preterm birth risk

Medium: Reduced premenstrual symptom severity; lower kidney stone risk from food-based calcium

Low: Reduced fracture risk

Speculative: Suppression of the age-related parathyroid hormone rise

Contraindications

Primary hyperparathyroidism (serum calcium above 10.5 mg/dL)

Any documented hypercalcaemia

Chronic kidney disease stage 4–5 (eGFR below 30 mL/min/1.73 m²) or dialysis, unless prescribed as a phosphate binder

Recurrent calcium oxalate or calcium phosphate kidney stones with hypercalciuria (above 300 mg per 24 hours)

Granulomatous disease (sarcoidosis, tuberculosis, histoplasmosis)

Malignancy-associated hypercalcaemia (multiple myeloma, bone metastases)

Familial hypocalciuric hypercalcaemia

Digoxin and other cardiac glycosides (intravenous calcium)

Key Interactions

Thyroid hormone replacement (levothyroxine, liothyronine)

Bisphosphonates (alendronate, risedronate, ibandronate)

Tetracycline and fluoroquinolone antibiotics (doxycycline, minocycline, ciprofloxacin, levofloxacin)

Thiazide diuretics (hydrochlorothiazide, chlortalidone, indapamide)

Iron salts and zinc supplements

Proton pump inhibitors and H2 blockers (omeprazole, pantoprazole, famotidine)

Vitamin D, calcitriol analogues, high-dose vitamin A, and blood-pressure-lowering supplements (magnesium, potassium)

Corticosteroids and denosumab

Risk & Side Effects

High: Gastrointestinal symptoms

Medium: Kidney stones; incident coronary artery calcification; prostate cancer

Low: Myocardial infarction and coronary events; hypercalcaemia and milk-alkali syndrome; reduced iron absorption

Speculative: Lead contamination in natural-source products

Monitoring

Marker	Target	Why
Serum calcium (albumin-corrected)	9.2–9.8 mg/dL	Detects hypercalcaemia before symptoms
Ionised calcium	1.15–1.30 mmol/L	The physiologically active fraction, unaffected by protein binding
Intact parathyroid hormone (PTH)	15–35 pg/mL	Shows whether intake is adequate; a high value signals a shortfall
25-hydroxyvitamin D	40–60 ng/mL	Governs how much calcium is absorbed at all
24-hour urinary calcium	100–250 mg/24 h	The single best predictor of stone risk on supplementation
Estimated glomerular filtration rate (eGFR)	Above 90 mL/min/1.73 m²	Determines whether supplemental calcium is safe at all
Serum phosphorus	3.0–4.0 mg/dL	Completes the mineral picture alongside calcium and parathyroid hormone
Red blood cell magnesium	5.0–6.5 mg/dL	Magnesium is required for parathyroid hormone secretion and action
Ferritin	50–150 ng/mL	Detects the iron depletion that calcium can aggravate
Bone mineral density by DXA	T-score above −1.0	The outcome calcium is most often taken for
CTX	Lower half of the premenopausal reference range	Bone breakdown rate; falls when intake becomes adequate

Cadence: Serum calcium and 24-hour urinary calcium at three months, then at 12 months and annually thereafter. Parathyroid hormone and 25-hydroxyvitamin D follow the same annual cadence. Ferritin annually for menstruating adults and anyone with a marginal iron status. Repeat DXA only at 24-month intervals.

Qualitative Assessment

Bowel regularity, bloating and abdominal cramping, which flag an excessive single dose
Flank pain or visible blood in urine, which signal a stone and warrant stopping immediately
Muscle cramps, twitching or tingling around the mouth, which point toward inadequate rather than excessive intake
Unexplained fatigue, thirst, frequent urination, confusion or constipation together, the classic picture of hypercalcaemia
Dietary consistency — whether calcium-rich meals are actually being eaten, since the whole target is total intake
Absence of fragility fractures and stable height over years, the only outcomes that ultimately matter for the skeleton

Calcium for Health & Longevity

Protocol

Benefits

Risk & Side Effects

Monitoring

Qualitative Assessment