## 1 Introduction
### Sources of Silicon of Mineral Origin Used in Functional Foods and Food Supplements
#### 1.2.5 Re-Evaluation of Silicon Dioxide (Silica, E551) as a Food Additive  
In 1974, the Food and Agriculture Organization of the United Nations/World Health Organization (FAO/WHO) committee established an \"unlimited acceptable intake\" for silicon dioxide and some silicates. Based on the available research results, the committee concluded that the oral administration of silicon dioxide and silicates appears to be biologically inert and that there is no evidence of toxic accumulation of silicates in the body due to their easy excretion in the urine. Safe upper levels (SUL) for silica has been set at 25 mg/kg/day for lifelong consumption [16]. This amount was determined based on a study of mice and rats using amorphous silica [63]. The EFSA assessed that using up to 1500 mg of silicon dioxide per day for nutritional purposes in dietary supplements is safe. Therefore, silicon dioxide can be used in the production of dietary supplements. In 2004, the EFSA concluded that silicon in the form of silica, silicates, and dimethylpolysiloxane can be added to food and dietary supplements as an anticaking and antifoam agent. The Si availability from the abovementioned sources varies but is quite low. The EFSA stated that there is no adequate dose-response data, and, therefore, an UL cannot be established. In 2017, the EFSA re-assessed the safety of SiO\({}_{2}\) (E551), used as a food additive. The forms of synthetic amorphous silica (SAS) used as E551 include fumed silica and hydrated silica (precipitated and hydrous silica and silica gel). According to Regulation (EU) No. 231/2012 [60; 64], silicon dioxide (E551) is defined as an amorphous substance that is produced synthetically by vapor-phase hydrolysis to give a fumed silica or by a wet process to give precipitated or hydrated silica or silica gel. According to [PERSON] (2012) [65], there are three main types of silicon dioxide: (1) crystalline silica, (2) amorphous silica (naturally occurring or as a byproduct), and (3) synthetic amorphous silica (SAS), including silica gel, precipitated silica, pyrogenic (fumed) silica, and colloidal silica. Among these three types of silicon dioxide, only synthetic SAS is authorized as a food additive (E551) but without \"colloidal silica\". Authorized uses and maximum permitted levels of silicon dioxide (E551) have been defined in Regulation (EC) No. 1333/2008 [66]. SiO\({}_{2}\) and silicates are characterized by a maximum level of additive, oscillating from 2000 to 30,000 mg/kg, and, in some food categories, can also be used at \"quantum satis\". Silicon dioxide (E551) can also be authorized together with silicates (E552, E553a, and E553b). Silicon dioxide (E551) and calcium silicate (E552) can also be used as food additives in nutrients, with the exception of those that are used for infants and young children (mentioned in Regulation (EC) No. 1333/2008) in dry powdered formulas containing all nutrients, which have the maximum level of 50,000 mg/kg [67]. Silicon dioxide (E551) is authorized in potassium chloride preparations (used in salt substitutes) up to the maximum level of 50,000 mg/kg and is also used as a food additive in foods for infants and young children in a dry powdered form up to the maximum level of 10,000 mg/kg. Silicon dioxide (E551) is on the labels of nearly 5000 food products: half of them are dietary supplements, others are, for example, meal replacements, creamers, instant noodles, malt, and drinks. The percentage of foods in each subcategory labeled as containing silicon dioxide (E551) ranged from less than 0.1% to 24.5% in the \"vitamins and dietary supplements\" subcategory [48]. There are few toxicological studies on silicon, including those in which nano silicon dioxide was examined. In the few available animal studies, the content of silicon in the liver, kidneys, and spleen was slightly increased after the oral administration of SAS. Studies using rats showed no accumulation of Si after multiple oral intakes of silica.