Key points
- Absorption saturates around 500 mg — split doses above that.
- Calcium citrate absorbs independently of stomach acid; carbonate needs acid.
- Separate from iron, zinc and levothyroxine by at least 2 hours.
The absorption ceiling
Calcium crosses the intestinal wall by two routes. A saturable active transport path dominates at low intakes, and a passive paracellular path takes over at high intakes but with low efficiency. The practical consequence: fractional absorption falls from roughly 35% at 300 mg to about 12% at 1000 mg in a single dose.
This is why the standard advice is 500 mg per dose, two or three times daily, rather than one large dose. A single 1200 mg pill is not three times as effective as a 400 mg pill.
Citrate versus carbonate
Calcium carbonate requires gastric acid to dissolve and should be taken with food. It is 40% elemental calcium by weight, so a 1250 mg tablet provides 500 mg elemental — a labelling trap worth noting.
Calcium citrate is 21% elemental calcium but absorbs independently of stomach acid, so it can be taken with or without food. For anyone on a proton pump inhibitor or with low stomach acid, citrate is the better choice.
Interactions
Calcium competes with iron and zinc for DMT1, and inhibits iron absorption dose-dependently. Keep two hours between calcium and either mineral.
Calcium binds levothyroxine in the gut and can reduce thyroid hormone absorption by up to a third. Separate them by at least four hours, taking levothyroxine first on an empty stomach.
Calcium chelates tetracycline antibiotics. Separate by two hours before or four to six hours after the antibiotic.
Oxalate-rich foods such as spinach bind calcium and make that plant calcium largely unavailable. Do not count spinach as a calcium source.