Citrates increase calcium bioavailability from food and beverages such as milk and are important for transport of calcium during calcium phosphate biomineralisation in mammal skeletons. Long-lasting spontaneous supersaturation of calcium citrate in presence of excess citrate or isocitrate counteracts the low aqueous solubility otherwise limiting calcium transport with isocitrate as a growth inhibitor for calcium citrate crystallisation and citrate as a growth inhibitor for calcium isocitrate crystallisation. The aim of this present review is to understand why the metastable calcium citrate tetrahydrate of higher solubility becomes important for bioavailability and for mineralisation under physiological conditions rather than the stable hexahydrate of lower solubility. Transformation from the tetrahydrate is kinetically hindered by trapping into a polymorph tetrahydrate with four water molecules bound in the first coordination sphere of calcium.