Hint of a biological role for silicon.

Hints of a biological role for silicon

With every sip of tapwater, humans swallow trace amounts of dissolved silicon. Though no one has identified a specific cellular or biochemical function for silicon, chemist J. Derek Birchall of Imperial Chemical Industries in Runcorn, England, says he has found a candidate role that might even have implications for Alzheimer's disease. The toxicologic and chemical evidence he has assembled suggests that silicon -- present in body tissues as silicic acid -- binds to aluminum, reducing that metal's toxic potential.

In the 1970s, researchers observed that young rats and chicks fed silicon-deficient diets gained too little weight and developed abnormalities in bone, cartilage and other tissues. Replenishing their diets with silicon reversed these trends.

Birchall and his colleagues reported in NATURE last year that young salmon suffered gill damage and died within 48 hours of being placed in water that contained toxic levels of aluminum and scant silicic acid. But in aluminum-laden waters with high levels of silicic acid, all fish survived without obvious gill damage. The researchers proposed that silicic acid prevents systemic absorption of aluminum by preventing it from binding to gills.

Even when present in low levels, silicon- and aluminum-based oxides and hydroxides eagerly combine in a solution to form aluminosilicates, which are also commonly found in soil minerals, Birchall found. He suspects that this coupling effectively imprisons -- and thus detoxifies -- the aluminum.

Birchall now suggests a possible connection between Alzheimer's disease and silicon levels in the body. Scientists have detected abnormally high concentrations of aluminum in the autopsied brains of Alzheimer's patients, but no one knows whether the metal is a cause or effect of the disease, he notes. To help resolve this ambiguity, Birchall proposes comparing the amounts of silicon and aluminum ingested by Alzheimer's patients.