Calcium

Supplements · Minerals · Supplement, Typed Supps

More than 99% of it sits in your bones and teeth.

Stage by stage

Intake (Tablet): Absorbed actively, dependent on vitamin D, up to a saturation point per dose.
Small intestine (Vitamin-D dependent): Split doses under 500mg absorb more efficiently than one large dose.
Bloodstream (Tightly regulated): Blood calcium is tightly regulated by hormones, kept within a narrow range regardless of intake.
Bones & teeth (Structural reserve): Provides structure and a mineral reserve the body draws from as needed.

Reducing fractures in older adults, combined with vitamin D — moderate evidence. Benefit is clearest in institutionalised older people and those with low intake. In healthy community-dwelling adults the effect is small at best.
Increasing bone mineral density — moderate evidence. Produces a small increase — around 1–2% — which largely plateaus rather than accumulating year on year.
Preventing fractures in healthy adults with adequate dietary intake — insufficient evidence. Adding supplements on top of a sufficient diet has not shown a meaningful fracture reduction.
Reducing pre-eclampsia risk in women with low calcium intake — moderate evidence. Supported enough that the WHO recommends it in populations where dietary calcium is low.

500–600 mg per dose; more than that in one go absorbs poorly

Upper limit: 2,500 mg daily for adults; 2,000 mg from 51

Take with vitamin D, split into smaller doses.

Levothyroxine: Calcium substantially blocks absorption. Separate by at least four hours. (high)
Bisphosphonates: Absorption is badly reduced — take the bisphosphonate on an empty stomach well away from calcium. (high)
Quinolone and tetracycline antibiotics: Bind with calcium and lose effectiveness. (high)
Thiazide diuretics: Reduce calcium excretion, so combined with supplements blood calcium can rise too far. (moderate)

“More calcium always means stronger bones.” In reality: Beyond a certain intake, extra calcium doesn’t keep building bone.