Calcium nutrition. Bioavailability and fortification

Both transcellular, vitamin-D dependent and the quantitatively more important paracellular, nonsaturable calcium absorption depend on the presence of free calcium ions or on the presence of reactive precursors for free calcium ions in the intestines. Complex binding of calcium preventing precipitation of calcium salts under the neutral to slightly alkaline conditions of the intestines accordingly determines calcium absorbability. The presence of suitable ligands for calcium complexation is important already when the partly digested food is transferred from the acidic stomach to the neutral intestines. The competitive complex binding and precipitation of calcium decisive for subsequent absorption in the intestines and further for bioavailability is accordingly influenced by the calcium speciation in the stomach resulting from food digestion. Calcium salt solubilities relative to calcium binding strength in complexes are the thermodynamic factors controlling the level of free calcium ions matching the free calcium level allowing facile absorption. However, kinetic factors like rates of precipitation and rates of complex formation and of complex dissociation seem of equal importance, especially when the same anion precipitates calcium and forms complexes with calcium as for hydroxycarboxylates like lactate. Spontaneous supersaturation resulting from isothermal dissolution overshooting during digestion or from gradually increasing pH is suggested to increase calcium mobility and bioavailability and needs to be incorporated in current theories for calcium bioavailability. Understanding the dynamic aspects of calcium speciation throughout digestion, and not only in the intestines, will also allow for better calcium fortification strategies to ensure optimized calcium bioavailability.