nate the possibility of longer-term retention of additional ³²Si. Goldwater (1936) reported daily silicon excretion levels for five subjects averaging 10 mg/day and ranging from 5 to 17 mg/day. Kelsay and coworkers (1979) studied 11 men fed low- and high-fiber diets and found their urinary silicon excretion to be 12 and 16 mg/day, respectively, amounts which were not significantly different.

Silicon appears to be involved in the formation of collagen and bone in animals. A biological role of silicon in humans has not yet been identified. Therefore, neither an Estimated Average Requirement, Recommended Dietary Allowance, nor Adequate Intake was established for silicon.

Concentrations of silicon are higher in plant-based foods than in animal-derived food products. Based on the Food and Drug Administration Total Diet Study, beverages, including beer, coffee, and water, are the major contributors of silicon (55 percent), followed by grains and grain products (14 percent), and vegetables (8 percent) (Pennington, 1991). Refining reduces the silicon content in foods. Silicate additives that have been increasingly used as anti-foaming and anticaking agents can raise the silicon content in foods; however, the bioavailability of these additives is low.

Based on the Total Diet Study, the mean intakes of silicon in adult men and women were 40 and 19 mg/day, respectively (Pennington, 1991). Appendix Table E-8 indicates that the daily median intakes of silicon for adult men and women ranged from approximately 14 to 21 mg/day. Kelsay and coworkers (1979) found intakes of 46 mg/day from a high-fiber diet and 21 mg/day from a low-fiber diet.

The mean concentration of silicon in human milk was reported to be 0.47 mg/L in women up to 5 months postpartum (Anderson, 1992). Based on the mean secretion of 0.78 L of human milk per day (Chapter 2), the mean intake of silicon by infants receiving human milk is approximately 0.37 mg/day.