Barnes, 2001; Nelson et al., 1999), the effect of low-selenium bioavailability on the risk of prostate cancer or benign prostatic hyperplasia (BPH) has not been addressed. With the availability of data from community-based studies on the natural history of BPH and placebo-controlled clinical trials, interest is shifting beyond short-term effects on symptoms to reducing the risk of long-term negative outcomes and BPH progression (Roehrborn, 2000).

Zinc as a “Protective Factor”

Zinc is a homeostatically regulated essential mineral present in red meat, poultry, grains, dairy, legumes, and vegetables. It is a critical soil nutrient, and deficiency of zinc in soil can impact crop yield and the nutritive quality of the resulting food crop (Adriano, 2001). Human zinc deficiency has also been associated with geophagia, where the ingestion of soils rich in zinc actually decreased zinc absorption (Hooda et al., 2004). Zinc is a component of numerous metalloenzymes and is important for cell growth and replication, osteogenesis, and immunity. Zinc may also act as an antioxidant by stabilizing membranes in some cell types.

The normal human prostate accumulates the highest zinc levels of any soft tissue in the body—10 times higher than for other soft tissue (Costello and Franklin, 1998). Zinc levels in prostate cancer cells are markedly decreased compared with nonprostate tissues, and there is evidence that zinc inhibits human prostate cancer cell growth (Liang et al., 1999). Cancer cells from prostate tumors have been found to lose their ability to amass zinc (Costello and Franklin, 1998).

Reduced red meat consumption and increased cereals in the diet may reduce the intake and bioavailability of zinc (Gibson et al., 2001). Both dietary and biochemical data suggest that thecurrent Western diets of the elderly may result in a risk of zinc deficiency.

Arsenic

The distribution of naturally occurring arsenic and the health effects of arsenic exposure have been reviewed in several recent review articles (Oremland and Stolz, 2003; Smedley and Kinniburgh, 2005; Centeno et al., 2005) and in Chapter 3 above. Here the microbial role in determining the speciation and bioavailable concentrations of arsenic in soils and the resultant effects from arsenic ingestion through food are described (see Box 5.4).

Soil microorganisms can transform and metabolize arsenic species found in soil, both as a pathway to conserve energy and to provide a defense mechanism against the toxic effects of arsenic. Some soil microbes



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