Silica (silicon dioxide), a nonmetallic crystalline mineral composed of silicon and oxygen, is the most abundant mineral in the earth's crust, where it is most commonly found in the form of quartz or sand. Only recently recognized as having a role in human nutrition, silica's functions in the body are not well understood. It appears to play a role in the mineralization of bones and teeth, and in the formation of collagen, the primary protein component of bones, connective tissues, hair, skin, and nails. Supplemental forms of silica include colloidal silicic acid, sodium metasilicate, and the silica-rich herb horsetail grass. Silica is also available in a homeopathic form known as silicea.
Good sources of silica include whole grains, green leafy vegetables, root vegetables, asparagus, and sunflower seeds.
There is no established RDI for silica. The average diet is estimated to provide 5 to 20 mg of silica per day, and this appears to be sufficient for most people. No toxicity has been associated with high silica intake. Inhalation of large amounts of silica (as occurs in people with high levels of industrial exposure) can cause a respiratory disease known as silicosis, and is associated with an increased risk for lung cancer.