Silicon is also available as a food supplement in tablet and solution forms. These show varying bioavailability (<1 to >50%) and most show negligible-low bioavailability. Biosil® or choline-stabilised orthosilicic acid (BioMineral NV, Destelbergen, Belgium), is a concentrated solution of orthosilicic acid (2% solution) in a choline (47%) and glycerol (33%) matrix. This is promoted as 'biologically active silicon' and studies in man have suggested that it is a readily bioavailable source of Si ( 53 ) and biologically active ( 54 - 56 ). Silica+® (Pharmafood, Belgium) is made from the dry extract of horsetail and contains 12 mg Si per tablet, of which 85% is suggested to be bioavailable. However, studies conducted in man have shown it to be significantly less bioavailable than Biosil®

Thus natural levels of soluble (available) silica are low. Chemical and biological (plants, algae and lichens) weathering, however, releases silicon from these stable minerals, increasing its bioavailability. Dissolution of Si, from soil minerals in water results in the formation, by hydrolysis, of soluble silica species. Below pH 9, and at a total Si concentration below 2 mM, silicon is present predominately as Si(OH) 4 the most stable specie at low Si concentration. This monomeric form of silica, 'monomeric silica', is water soluble and a weak acid (pKa of 9.6), thus also referred to as 'monosilicic acid' or 'orthosilicic acid' ( 22 ). At neutral pH, this tetrahedral, uncharged (ie neutral) species is relatively inert, but does undergo condensation reactions (polymerisation) to form larger silica (polysilicic acid) species, especially at Si concentrations > 2-3 mM. Indeed, only in very dilute solutions, it is suggested, that the monomer will be found in its pure form, as often the dimer [(HO) 3 Si-O-Si(OH) 3 ] is also present (but never > 2%), even in solutions greatly below 2 mM Si ( 22 , 22 ). Above 2 mM Si, Si(OH) 4 undergoes polymerization to form small oligomers (linear and cyclic trimers and tetramers or cyclic decamers) and, at concentration much above 2 mM, small colloidal species will also be present, which upon aggregation will eventually results in the formation of an amorphous precipitate, which at neutral pH (pH 6-7) is a gel ( 20 , 22 - 24 ). Thus polymerisation of Si(OH) 4 reduces its solubility and hence bioavailability.

Silica+® (Pharmafood, Belgium) is made from the dry extract of horsetail