Silicon (Si) is present in biologic material as a silanate, an ether (or ester-like) derivative of silicic acid which may play a role in the structure of glycosaminoglycans and their protein complexes. Connective tissues including aorta, trachea, tendon, bone, and skin and its appendages contain much of the silicon that is retained in the body.
Most of the signs of silicon deficiency in chickens and rats indicate aberrant metabolism of connective tissue and bone. Chicks fed a semi-synthetic, silicon-deficient diet exhibited skull structure abnormalities associated with depressed collagen content in bone and long bone abnormalities characterized by small, poorly formed joints and defective endochondral bone growth.
As no appropriate human data are available, extrapolation from limited animal data suggest that the human requirement for silicon is quite small, in the range of 2 to 5 mg/day. However, silicon found in most diets as aluminosilicate and silica is not absorbable or as available as sodium metasilicate. Additionally, factors such as aging and low estrogen status apparently decrease the ability to absorb silicon. Thus, the recommended intake of silicon may be found to be between 5 and 10 mg/day.
Dietary silicon intake of humans varies greatly with the amount and proportions of foods of animal (silicon-low) and plant (silicon-high) origin consumed and the amounts of refined and processed foods in the diet. Normally, refining reduces the silicon content of foods. However, silicate additives have been increasingly used in prepared foods and confections as anti-caking or anti-foaming agents. Although this increases total dietary silicon, most of it is not bioavailable. The silicon content of drinking water, and beverages made thereof, shows geographical variation; silicon is high in hard water and low in soft water areas. The richest sources of silicon are unrefined grains of high fiber content and cereal products. The daily average intakes of silicon apparently ranges from about 20 to 50 mg/day.
Most silicon compounds are essentially nontoxic when taken orally. Magnesium trisilicate, an over-the-counter antacid, has been used by humans for more than 40 years without obvious deleterious effects. Ruminants consuming plants with a high silicon content may develop siliceous renal calculi; renal calculi in humans may also contain silicates.
In rats, bone hydroxyproline is decreased, plasma amino acid and bone mineral composition is altered, and femur alkaline and acid phosphatase are decreased by silicon deprivation. However, growth of rats is not markedly affected by silicon deprivation.