## 1 Introduction
### Diatomaceous Earth as a Supplement  
A good form of organic silicon supplementation is diatomaceous earth or amorphous diatomaceous earth, e.g., Fossil Shell Flour(r) (FSF), obtained from diatoms. Dating back to the Jurassic, Cretaceous, and even Devonian periods, diatoms are a group of single-celled algae that form diatomic sediments. These deposits occur in the form of slate or loose diatomaceous earth and, in its pure, unprocessed form, comprises, on average, of 85-90% of silicon dioxide in amorphous form. The basic element of the silica structure is the silicon-oxygen tetrahedron (SiO\({}_{4}\))\({}^{4-}\), with the molecular formula SiO\({}_{2}\). The amorphous form, as opposed to the crystalline one, results from the lack of an ordered spatial arrangement of these tetraeders. Figure 1a,b shows the microstructure of diatoms from amorphous diatomaceous earth containing SiO\({}_{2}\) in amorphous form and saltwater diatomaceous earth, with a high content of SiO\({}_{2}\) in crystalline form. In amorphous diatomaceous earth (a), single forms of diatoms are clearly visible, and in saltwater sources (b), they are joined into large clusters with a poorly preserved diatomaceous shape. Its unique porosity (between 10 and 200 \(\upmu\)m), chemical inertness, small particle size, large surface area, good permeability, and low heat conduction properties make it very popular among naturally occurring materials [73]. Diatomaceous earth of saltwater origin, due to its high content of crystalline silica (up to 60%) and arsenic admixtures, serves only as a technical product and should not be approved for use in the food industry as it may pose a real health risk [74]. Amorphous diatomaceous earth originating from freshwater consists mainly of amorphous silica, and the content of the crystalline form is negligible (0.5-2%). As a result, this product is considered completely safe and suitable for ingestion. There are several types of food-grade diatomaceous earth on the market, differing in chemical composition and origin, of which Fossil Shell Flour(r) is the most studied. The product comes from four Melosira Preicelancia freshwater reservoirs and is certified by the FDA as the purest food quality (Food Grade Codex). During manufacturing, this product is subjected to a purification process that uses hot air and water vapor and has a high amorphous silica content (89-95%), with the proportion of crystalline silica not exceeding 1% (average 0.3%). The content of other minerals is negligible. Amorphous silica, the main constituent of amorphous diatomaceous earth, including Fossil Shell Flour(r), is an approved food additive of the E551 silicate group (silicon dioxide, amorphous). Amorphous diatomaceous earth is insoluble in water. However, it is presumed that when silica is hydrated in the body, it is converted to OSA, which is, in turn, soluble in water, increasing its bioavailability and facilitating absorption in the gastrointestinal tract [30]. In the available literature, there is no data on the bioavailability of  
Figure 1: Images of the microstructure of diatoms from diatomaceous earth containing silica (**a**) in amorphous form and (**b**) from diatomaceous earth, with a high content of crystalline silicon. (Own research, images were taken with a scanning electron microscope Quanta 200 XL, FEI, Oregon, USA). diatomic organic silicon in amorphous and crystalline forms. There is not enough evidence that the structure of silica affects bioavailability. It is assumed that both forms are similar to SAS E551. However, the crystalline form of silica is not strictly approved by the EFSA as a food additive, which may be due to its safety. It should be assumed that its safety is similar to or higher than silicon from synthetic silica (SAS). Diatomic earth is of great interest not only as a supplement of organic silicon but also as an additive with a significant, multidirectional health-promoting effect, confirmed by studies on its use in animal nutrition.