Physiological silicon incorporation into bone mineral requires orthosilicic acid metabolism to SiO4/4

Under physiological conditions, the predominant form of bioavailable silicon (Si) is orthosilicic acid (OSA). In this study, given Si's recognized positive effect on bone growth and integrity, we examined the chemical form and position of this natural Si source in the inorganic bone mineral hydroxyapatite (HA). X-ray diffraction (XRD) of rat tibia bone mineral showed that the mineral phase was similar to that of phase-pure HA. However, theoretical XRD patterns revealed that at the levels found in bone, the 'Si effect' would be virtually undetectable. Thus we used first principles density functional theory calculations to explore the energetic and geometric consequences of substituting OSA into a large HA model. Formation energy analysis revealed that OSA is not favourable as a neutral interstitial substitution but can be incorporated as a silicate ion substituting for a phosphate ion, suggesting that incorporation will only occur under specific conditions at the bone-remodelling interface and that dietary forms of Si will be metabolized to simpler chemical forms, specifically SiO44-. Furthermore, we show that this substitution, at the low silicate concentrations found in the biological environment, is likely to be a driver of calcium phosphate crystallization from an amorphous to a fully mineralized state.