## 1 Introduction
### Sources of Silicon of Mineral Origin Used in Functional Foods and Food Supplements
#### 1.2.1 Silicic Acids and Their Stabilized Forms  
Silicic acids are considered the best sources of silicon due to their high bioavailability (approx. 40%) and natural occurrence in food [43; 44]. Orthosilicic acid (OSA) is the natural form of Si formed by the hydration of silicon dioxide (SiO\({}_{2}\)). OSA plays an important role in providing elemental silicon to human and animal cells. In the industry, OSA is formed by the thermal fusion of SiO\({}_{2}\) with NaOH. The reaction results in soluble silicates and then silicic acids. OSA is the main water-soluble silicon compound. Supersaturation causes OSA to polymerize into less water-soluble forms. The degree of polymerization of orthosilicic acid is positively correlated with its intestinal absorption [20; 23; 27; 45]. OSA is a neutral-charged monomeric silica, which is easily absorbed in the digestive tract, while larger types of colloids and polymers must be broken down to water-soluble monomer in the lumen of the digestive tract before being absorbed [23; 27; 45]. Beverages such as water and beer predominantly contain soluble monomeric species that are readily absorbed [33]. At higher silicon concentrations (above 2-3 mM, in most supplements), less absorbable and larger colloids or polymers are present, with the exception of methylsilanetriol (MMST), in which one hydroxyl group of OSA is replaced by one methyl group, raising the solubility limit and maintaining the silicon in well-absorbed forms [40]. Ch-OSA (choline stabilized orthosilicic acid) contains polymerized silicon in a highly concentrated solution; however, extensive aggregation and polymerization of Si particles are prevented by the presence of choline in the solution [43]. The solubility of silicon depends on the pH of the environment. It dissolves more easily in an almost neutral environment (which is characteristic of the intestinal pH) compared with acidic conditions (i.e., in the stomach) [39]. The availability of silicon compounds depends on their specificity and solubility [46]. The available literature does not provide much comparative data on silicon absorption from different sources. [PERSON] et al. (2009) [43] examined the absorption (based on urinary silicon excretion) from eight sources containing high concentrations of silicon: OSA (control), alcohol-free beer, green beans, bananas, ch-OSA, supplemental colloidal silica (CS), MMST, and magnesium trisilicate (MTBP). The highest absorption was obtained for MMST and beer (64%), followed by green beans (44%), OSA (43%), ch-OSA (17%), bananas and MTBP (4%), and CS (1%). It was noticed that with increasing polymerization, the silicates were less well absorbed [43]. In other studies, assessment of individual substances or in relation to another (usually OSA) was mainly made. The data obtained will be discussed further when discussing the EFSA opinion on the evaluation of these substances. The silicon sources currently approved for use in the production of dietary supplements are listed in Directive 2002/46/EC. These are ch-OSA, SiO\({}_{2}\), OSA gel, and MMST (Table 3) [47]. In addition, the EFSA provided opinions on OSA-VC and MMST as novel food ingredients used in food supplements and also an opinion on the bioavailability and safety of ch-OSA added for nutritional purposes as a source of silicon in food supplements (Table 3). In 2017, the EFSA provided a re-evaluated opinion on the safety of SiO\({}_{2}\) (E551), used as a food additive (Table 3) [48].