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Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc
RESEARCH RECOMMENDATIONS FOR NICKEL
Identification and clear characterization of a biochemical function for nickel in humans; identification of a reliable indicator of nickel status for use in future studies of nickel deficiency.
Further exploration of the possible role of nickel in vitamin B12 and folate metabolism, including whether nickel nutrition should be a concern for pregnant women or people at risk for cardiovascular disease.
A functional role for silicon in humans has not yet been identified. In view of the distribution of silicon in the body, as well as the biochemical changes that occur in bone with a silicon deficiency, silicon appears to be involved with the formation of bone in chickens and rats (Carlisle, 1980a, 1980b, 1981; Schwarz and Milne, 1972). Silicon contributes to prolylhydrolase activity, which is important for collagen formation (Carlisle, 1984). Chicks fed a silicon-deficient diet exhibited structural abnormalities of the skull and long-bone (Carlisle, 1984). Rats deprived of silicon showed decreased bone hydroxyproline and alkaline and acid phosphatases (Seaborn and Nielsen, 1993, 1994). Silicon has been suggested to have a preventive role in atherogenesis (Mancinella, 1991).
Physiology of Absorption, Metabolism, and Excretion
Findings that as much as 50 percent of ingested silicon is excreted in the urine (Kelsay et al., 1979) suggest that some dietary forms of silicon are well absorbed. Silicon in blood exists almost entirely as silicic acid and is not bound to proteins. Various connective tissues including the aorta, trachea, bone, tendons, and skin contain most of the silicon present in the body (Carlisle, 1984). Significantly higher serum silicon concentrations were seen in patients with chronic renal failure (46 μmol/L) compared to controls (21 μmol/ L) (Dobbie and Smith, 1986).
In a study by Popplewell et al. (1998), 48 hours after ingestion of 32Si, 36 percent of the dose was excreted in the urine and elimination appeared to be complete. This study, however, did not elimi-