Calcium bioaccessibility as affected by strontium. Temperature effect on citrate binding to strontium and calcium alone and in combination

Strontium as a bone health promoter was found to increase the bioaccessibility of essential calcium from solid calcium citrate tetrahydrate in aqueous suspensions. This is unexpected since calcium citrate binds calcium stronger than strontium with Δ H 0 ass, c  = 5.07 kJ⋅mol −1 , Δ S 0 ass, c  = 78.0 J⋅mol −1 ⋅K −1 for 1:1 Ca-Citrate complex formation and Δ H 0 ass, c  = 1.67 kJ⋅mol −1 , Δ S 0 ass, c  = 58.3 J⋅mol −1 ⋅K −1 for 1:1 Sr-Citrate complex formation as determined both electrochemically and by isothermal titration calorimetry. Calcium citrate tetrahydrate has reverse solubility around physiological temperature in contrast to strontium citrate pentahydrate. This difference results in opposite effect of strontium upon calcium ion liberation from calcium citrate suspensions at both low and high temperature compared to physiological temperature, for which strontium was found to stabilize solid calcium citrate tetrahydrate. This effect, confirmed by quantum mechanical calculation and by comparison of spontaneous aqueous supersaturation of calcium citrate and strontium citrate, is suggested to be involved in the positive effect of strontium on bone and teeth health. The conclusions provide a theoretical foundation for the research of molecular mechanisms among calcium, strontium, and citrate ions in relation to bone and teeth health.