Calcium Carbonate
Calcium Carbonate is a tier 2 form of Calcium, used in 2,482 on-market products and providing 40.0% elemental calcium by weight.
2,482
on-market products
tier 2
quality tier
40.0%
elemental fraction
Basis: CaCO3, anhydrous (no commercial hydrate exists)
Why this tier: Absorption from calcium citrate was 20-27% higher than from carbonate in a meta-analysis (15 studies, 184 subjects; Sakhaee 1999), i.e. carbonate absorbed roughly 17-21% less; carbonate absorption is better with meals/gastric acid (general pharmacology, not from the abstract).
Source: Sakhaee K et al. 1999, Am J Ther 6(6):313-321, PMID 11329115