Silicon is a chemical element with the symbol Si. In the periodic table, it has atomic number 14 and is in the 3rd period and 4th main group and carbon group, respectively (“tetrels”). Since silicon has the properties of both metals and classical non-conductors, it is one of the typical semimetals or semiconductors (elemental semiconductors). The term silicon is derived from the Latin word “silex” (hard stone, pebble, flint). As one of the most important rock-forming minerals, silicon is the second most abundant element in the earth’s crust after oxygen (element symbol: O) with 27.6%. There, due to its high affinity to oxygen, it occurs predominantly in the form of silicate (SiO4, salts and esters of ortho-silicic acid (Si(OH)4) and its condensates) and silica, which essentially consists of silicic acid anhydride or silicon dioxide (SiO2) and originates from radiolarians (radiolarians, unicellular organisms with an endoskeleton of opal (SiO2)) and diatoms (diatoms with a cell shell of SiO2) deposited in layers. In all compounds occurring in nature, silicon exclusively forms single bonds – Si-O single bonds – in which it primarily appears as a tetravalent electropositive partner – fourfold coordinated, positively charged silicon atom. This enables the tetrahedrally built silication (SiO44-) to form larger compounds (three-dimensional networks), preferably of the composition SiO2. In addition, compounds exist in which silicon has a five- or sixfold coordination. Synthetically produced compounds of divalent silicon (silylenes) are mostly unstable, with only silicon monoxide (SiO) being important, especially in the optical industry. While animal models speak for an essentiality of silicon, this has not yet been proven for the human organism. For this reason, silicon is one of the ultratrace elements (elements whose essentiality has been confirmed in animal experiments and for which deficiency symptoms have been found under extreme conditions without their specific functions being known). Silicon is available to humans both through its natural content in food – in free form as monosilicic acid (orthosilicic acid, Si(OH)4) or silicate (SiO4) and bound as an ether or ester derivative – and through its use as a food additive – silicates (SiO4) as anticaking and antifoaming substances. Plant foods, especially fiber-containing cereals such as barley and oats, and root vegetables, are generally richer in silicon than animal foods, but are presumably less bioavailable due to the predominantly polymeric bonding form of silicates (macromolecules composed of multiple SiO4 units). Beverages, such as beer, also contain high levels of silicon, which is also in a readily utilizable form.
Absorption
Silicon can enter the body both through food by absorption (uptake) in the gastrointestinal (GI) tract and through respiratory air by resorption (uptake) in the pulmonary alveoli (alveoli where gas exchange between blood and alveolar air occurs during respiration). Organically bound silicon or polymeric silicate (a macromolecule composed of several SiO4 units) supplied via the diet must first be cleaved in the digestive tract by hydrolytic enzymes of the pancreas and/or the brush membrane of the enterocytes (cells of the small intestinal epithelium) in order to be absorbed in the small intestine as monomeric silicate (SiO44-). Intestinal absorption of monosilicic acid or monomeric silicate supplied by the diet takes place directly without preceding enzymatic hydrolysis (cleavage by reaction with water). The mechanism by which silicon is absorbed into the enterocytes (cells of the small intestinal epithelium) and subsequently into the bloodstream is unclear. Diatoms, whose cell envelope consists largely of silicon dioxide (SiO2), are permeable to the human intestinal tract and pass through the intact intestinal mucosa and lymphatic circulation. Likewise, they can enter the body through absorption in the pulmonary alveoli.In pregnant women, diatom particles can cross the placental barrier and accumulate in the tissues of newborn and premature infants, respectively. The absorption rate of silicon depends on its binding type, the dietary fiber content, biological age, gender, and the functional state of exocrine glands such as the pancreas (pancreas → production of digestive enzymes that are secreted into the small intestine). Since silicon ingested in food is primarily of plant origin and thus occurs in the form of polymers (macromolecules composed of several identical units – in this case SiO4) or bound to organic molecules that require hydrolytic cleavage before absorption, the absorption rate of silicon from food is very low and is only about 4%. The high dietary fiber content of foods rich in silicon contributes to the low bioavailability, since celluloses and hemicelluloses from cereals, for example, bind silicon and thus remove it from absorption. The majority of the silicon supplied by the diet is thus not absorbed by the body, but leaves it unabsorbed via the feces (stool). Compared to polymeric silica from plant products, orally administered monomeric silica (Si(OH)4) is absorbed directly and rapidly due to the fact that enzymatic hydrolysis is not necessary and that there is no interaction (interaction) with food constituents, and consequently it has a higher bioavailability. Exocrine pancreatic insufficiency (disease of the pancreas), which is associated with insufficient production of digestive enzymes, may lead to a decrease in silicon absorption because of decreased enzymatic cleavage of polymeric and food-bound silicon in the intestinal lumen.
Transport and distribution in the body
Absorbed monosilicic acid and monomeric silicates, respectively, are distributed to the appropriate tissues via the bloodstream. The human organism contains about 1-1.5 g of silicon (~20 mg/kg body weight), which accumulates (accumulates) especially in connective tissues and thus can be found in blood vessels, such as aorta (main artery), trachea (windpipe), tendons, bones, and skin. The highest silicon content is found in bones (up to 100 mg/kg) due to their high weight. In addition, silicon can also accumulate in the lungs and lymph nodes (450 mg/kg). The high silicon concentration of connective tissue-like structures underlies the occurrence of the trace element as an integral component of glycosaminoglycans (acidic polysaccharides built linearly from repeating disaccharide units) and proteoglycans (strongly glycosylated glycoproteins consisting of a protein and one or more covalently bound glycosaminoglycans), respectively. In blood serum, silicon is found primarily in the form of undissociated monomeric silica (Si(OH)4) at concentrations of 190-470 µg/l. Silicon serum concentration is not affected by biological age or gender. Several studies indicate that with increasing age the silicon content in tissues, especially in skin, aorta and bones, decreases.The age-related silicon depletion in bones cannot be attributed to a silicon deficit, but to the reduction of the ash content (mineral content, inorganic fraction of bone) – calcium, phosphorus, magnesium, zinc, manganese. Diseases, such as osteoporosis (bone loss, decrease in bone density due to the excessive rapid degradation of bone substance and structure with increased susceptibility to fracture) and atherosclerosis (arteriosclerosis, hardening of the arteries due to deposits of blood fats, connective tissue, etc. in the walls of the vessels), accelerate the reduction of tissue concentration of silicon.
Excretion
Excretion of absorbed silicon occurs largely through the kidney in the form of magnesium orthosilicate. Adults excrete an average of approximately 9 mg of silicon/day in the urine. In lactating women, an additional silicon loss of 350-700 µg/l can be expected via breast milk. Silicon homeostasis (maintenance of a balance) is mainly regulated by renal (kidney-related) excretion, the level of which depends on the amount absorbed intestinally.When intestinal silicon absorption is low, for example, by increased dietary fiber intake, there is a decrease in renal excretion (excretion), whereas when intestinal silicon absorption is increased, for example, by administration of monomeric silica, elimination via the urine is increased.
