Saturated solutions of calcium L-lactate in water or in deuterium oxide continuously dissolve calcium L-lactate by addition of solid sodium D-gluconate and become strongly supersaturated in calcium D-gluconate due to no or slow precipitation. The quantification of total dissolved calcium allied with the calcium complexes equilibrium constants allowed an ion speciation, which shows an initial non-thermal and spontaneous supersaturation of more than a factor of 50 at 25 °C only slowly decreasing after initiation of precipitation of calcium D-gluconate after a lag phase of several hours. A mathematical model is proposed, based on numerical solution of coupled differential equations of dynamics of L-lactate and D-gluconate exchange during the lag phase for precipitation and during precipitation. A slow exchange of L-lactate coordinated to calcium with D-gluconate is indicated with a time constant of 0.20 h⁻¹ in water and of 0.15 h⁻¹ in deuterium oxide and a kinetic deuterium/hydrogen isotope effect of 1.25. Such spontaneous non-thermal supersaturation and slow ligand exchange with a pseudo first order equilibration process with a half-life of 3.5 h in water for calcium hydroxycarboxylates can help to understand the higher calcium bioavailability from calcium hydroxycarboxylates compared to simple salts.