Non-Dietary Sources

With the documented prevalence and widespread use of silicon and silica, there are numerous, diverse sources of and exposures to non-dietary silica/silicon. These occur primarily from exposure to dust, pharmaceuticals, cosmetics, medical implants, and medical devices. Often the forms of silicon occur as “silicones,” synthetic organosilicon compounds that, for the most part, are sparse in the human diet and contribute little silicon overall. Moreover, the forms that do result in exposure are not readily absorbed or biologically useful. For example, some pharmaceuticals can increase exposure of silicon to >1 g/day, but the molecular species are largely inert and not absorbed to any significant extent. This seems to be the case with other non-dietary sources such as toiletries, e.g., toothpaste, lipstick, etc., and detergents, tissue implants, etc.8 The safety of nanostructured synthetic amorphous silica (SAS) as a food additive (E551) has also been addressed.58 E551 is commonly used as an anti-caking agent in food products and is included at 2.7-14.5 pg silicon/g food product.59

The forms of SAS used as E551 include fumed silica and hydrated silica (precipitated silica, silica gel, and hydrous silica). Others have concluded, based on the current available database, that there was no indication for toxicity of E551 at the reported uses and use levels.60

Zeolites

One of the most important sources of bioavailable OSA is zeolites. Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents and catalysts.6' Zeolites occur naturally but are also produced industrially on a large scale where they are used extensively in various technological applications, e.g., as molecular sieves, for separating and sorting various molecules, for water and air purification, including removal of radioactive contaminants, for harvesting waste heat and solar heat energy, for adsorption refrigeration, as detergents, etc.62 Current estimates indicate that 245 unique zeolite frameworks exist and have been identified, and over 40 naturally occurring zeolite frameworks are known. In addition to numerous technological applications, zeolites have many applications in biotechnology and medicine. As components of scaffolds for bone tissue engineering, zeolites can deliver oxygen to cells, stimulate cellular differentiation of osteoblasts, and inhibit bone resorption.62,63 In addition, zeolites can function as oxygen reservoirs and improve cellular characteristics associated with vascular and skin tissue engineering and wound healing. For example, silica-based films on silica substrates foster adhesion, growth, viability, and osteogenic differentiation of human osteoblast-like cells.62 Others have shown that zeolites increase cellular proliferation, differentiation, and transforming growth factor beta production in normal adult human osteoblast-like cells in vitro.64

Silica Nanoparticles and Microparticles

Nanomaterials are increasingly becoming prevalent in various consumer products due to technical commercialization.59,65 Recently, concerns have been raised about potential adverse effects of nanoparticles commonly used in food additives, since silica nanoparticles have been detected in food containing E55l.65,66 As a result, current research interests are geared toward the reported knowledge gaps involving risk assessment of nanosilica in food particularly regarding the dissolution and toxicity of different forms.67 Silica nanoparticles are ultrafine, submicroscopic units with dimensions measured in nanometers (10"9m) and occur in the human diet. In one study, detection of silicon dioxide in dietary supplements specifically targeted for women was determined and out of 12 commercial products randomly purchased from retailers with Si02 in the ingredient list but without claims regarding particle size, 11 of the 12 products contained isolated nanoscale materials many with a high degree of aggregation.68 Given the increasing prevalence in the diet, safety concerns have arisen and a critical review of the safety assessment of nanostructured silica additives in food has been conducted.69 In preliminary studies, Liang et al. have demonstrated that neither silica nanoparticles nor silica microparticles induced toxicological effects after subchronic oral exposure in rats.70

 
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