REVIEW
UPDATE ON THE POSSIBLE NUTRITIONAL IMPORTANCE OF SILICON☆

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Summary

Convincing evidence that silicon is a bioactive beneficial trace element continues to accumulate. The evidence, which has come from human, animal, and in vitro studies performed by several laboratories, indicate that silicon in nutritional and supra nutritional amounts promotes bone and connective tissue health, may have a modulating effect on the immune or inflammatory response, and has been associated with mental health. A plausible mechanism of action for the beneficial effects of silicon is the binding of hydroxyl groups of polyols such that it influences the formation and/or utilization of glycosaminoglycans, mucopolysaccharides, and collagen in connective tissue and bone. In addition, silicon may affect the absorption, retention or action of other mineral elements (e.g., aluminum, copper, magnesium). Based on findings from both animal and human experiments, an intake of silicon of near 25 mg per day would be a reasonable suggestion for an adequate intake that would assure its nutritional benefits. Increased intakes of silicon through consuming unrefined grains, certain vegetables, and beverages and cereals made from grains should be recognized as a reasonable dietary recommendation.

Introduction

Silicon is nutritionally essential for some lower forms of life [1]. Silicon has a structural role in diatoms, radiolarians, and some sponges. Diatoms, which are unicellular microscopic plants, have an absolute requirement for silicon as monomeric silicic acid for normal cell growth. Silicon also may be essential for some higher plants (e.g., rice). Because silicon deprivation has not been shown to interrupt the life cycle in mammals, or to have a defined biochemical function, silicon is not generally accepted as an essential nutrient for higher animals and humans. However, for over 40 years, reports about silicon having beneficial, especially on connective tissue and bone formation, in higher animals and humans have appeared. Initial experiments performed in the 1970s used supra nutritional supplemental amounts of silicon (100 and 500 mg/kg diet) to prevent abnormalities in animal models fed low-silicon diets of questionable nutritional quality based on growth data. The silicon supplementation alleviated abnormal bone structure and strength in chicks and rats; abnormal bone cartilage characterized by decreased hexosamine in chicks; and decreased collagen and prolyhydroxylase activity in skull bone from cultured chick embryos [1], [2]. Concern about dietary quality resulted in the question of whether the supra nutritional supplements were alleviating abnormalities caused by a silicon deprivation or induced by some other sub-optimal dietary factor.

Section snippets

Silicon, bone and connective tissue

Experiments performed since 2000 have indicated that only nutritional amounts of silicon are needed to prevent bone, hexosamine, and collagen metabolism abnormalities, similar to, but of less magnitude than those reported in the 1970s, in animal models fed apparently nutritionally-adequate diets low in silicon. In three studies, silicon supplementation of 10 or 35 mg/kg diet containing < 2.0 mg/kg silicon, was used to show that the silicon deprivation in rats decreased collagen formation [3] and

Silicon and immune and inflammatory response

In 1988, it was suggested that silicon had a regulatory role in the cell cycle of lymphocytes because monomethylsilanetriol at an optimal concentration of 10 mg/L silicon in culture media stimulated peripheral lymphocyte proliferation and decreased lymphoblast proliferation [22]. This possible effect of silicon in nutritional amounts received little attention until 2002 when it was reported that mitogen-induced DNA synthesis of splenic T-lymphocytes from silicon deprived (2.3 mg/kg diet) compared

Silicon and mental health

In 1996, it was reported that an association between cognitive impairment and aluminum in drinking water depended upon the silica concentration in water [26]. High levels of aluminum appeared to have a deleterious effect on cognitive function when the silica concentration was low, and high silica concentrations had a protective effect on the association between high aluminum and impaired cognitive function. In 2000, it was reported that the 8-year follow-up of subjects in the Paquid cohort

Plausible silicon mechanism of action

A plausible biochemical mechanism of action for silicon is some type of structural or binding role that affects the formation of connective tissue where it is strongly bound in significant concentrations [1], [2]. This binding role is supported by the finding that silicon easily forms stable complexes with polyols that have at least four hydroxyl groups [31]. Such polyols include hexosamine and ascorbate used to form glycosaminoglycans, mucopolysaccharides, and collagen involved in connective

Silicon metabolism

Several recent studies of silicon have involved its dietary intake, absorption, transport, retention, and excretion. These studies suggest that silicon may have a significant role in higher animals and humans because silicon is well regulated by the body. Silicon is relatively well absorbed when consumed from various foods and drinks. One study found an average of 41% of silicon in food was excreted in urine, which is an indicator of absorption [35]. A study determining the absorption of

Silicon status indicators

Because reported fasting serum concentrations range from 10 to 31 μg/dL, concentrations in the lower part or below this range might be an indication of a suboptimal silicon status. In addition, a consistent urinary silicon excretion below 14 mg/d may be indicating that an individual has a suboptimal intake of silicon because this excretion compared with 40 mg/d was associated with decreased bone mineral density in humans [15]. This finding also suggests that there is a dose-response to silicon for

Beneficial intakes of silicon

On the basis of extrapolations from animal data, weak balance data from humans, and the usual amount of silicon excreted daily by humans, it has been suggested that an adequate intake to achieve the beneficial effects of silicon might be between 10 to 25 mg/d [39]. Based on the findings from 1251 men and 1596 pre- and postmenopausal women in the Framingham Offspring Cohort, the beneficial intake most likely is near the 25 mg/d intake or slightly higher. The majority of the intakes in this cohort

Conclusion

Recent findings provide additional evidence that silicon in nutritional amounts is beneficial for bone growth and maintenance. In addition, silicon in nutritional amounts might have a beneficial effect in the immune or inflammatory response and in mental health. Supra nutritional amounts of silicon also may promote bone health. Plausible mechanisms of actions have been suggested for the beneficial effects of silicon. Recent epidemiological findings suggest that intakes near 25 mg/d might promote

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