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===Health effects resulting from the deficiency and excessiveness of selenium===

===Health effects resulting from the deficiency and excessiveness of selenium===

According to Fordyce (2010), selenium is found in phosphatic rocks, coals, organic-rich shales and sulphide mineralization. It occurs in the soil in the form of selenites (Se4+), selenates (Se6+), selenides (Se2-) and elemental selenium (Se0).

According to Fordyce,<ref name=Fordyce>Fordyce, F., 2010, Selenium Deficiency and Toxicity in the Environment. In: Selinus, O., Alloway, B., Centeno, J.A., Finkelman, R.B., Fuge, R., Lindh, U. and Smedley, P., (eds.) 2010. Essentials of Medical Geology. Elsevier, Amsterdam, 375-413.</ref> selenium is found in phosphatic rocks, coals, organic-rich shales and sulphide mineralization. It occurs in the soil in the form of selenites (Se4+), selenates (Se6+), selenides (Se2-) and elemental selenium (Se0).

It can accumulate in the kidneys, liver, marrow, myocardium, pancreas, lungs, skin and hair (Fordyce, 2010).

It can accumulate in the kidneys, liver, marrow, myocardium, pancreas, lungs, skin and hair. <ref name=Fordyce />

Deficiency of selenium manifests as a disease known as Keshan disease– a disease that occurs due to the deterioration of the heart muscle leading to chronic cardiomyopathy. Generally, WHO (1987, 1996) recommended a daily Se intake of 30 μg of selenium for females and 40 μg of selenium for males. Excess concentration of Se in the body results in garlic scent from the mouth and skin (due to the formation of dimethyl selenide), hair loss, nail fragility, edema of the lungs, malfunctioning of the liver, redness of the skin, rhinitis and bronchoalveolitis (Fordyce, 2010).

Deficiency of selenium manifests as a disease known as Keshan disease– a disease that occurs due to the deterioration of the heart muscle leading to chronic cardiomyopathy. Generally, WHO (1987, 1996) recommended a daily Se intake of 30 μg of selenium for females and 40 μg of selenium for males. Excess concentration of Se in the body results in garlic scent from the mouth and skin (due to the formation of dimethyl selenide), hair loss, nail fragility, edema of the lungs, malfunctioning of the liver, redness of the skin, rhinitis and bronchoalveolitis.<ref name=Fordyce />

In living systems, selenium occurs as selenocysteine and is incorporated in two enzymes, namely: glutathione peroxidase and 1 5'-iodothyronine deiodinase. These enzymes are important in the thyroid gland where they help in thyroid hormone production. Glutathione peroxidase protects cells against oxidative damage by acting as a catalyst in the breakdown of hydrogen peroxide. According to Davies<ref name=Dvies /> thyroxine is broken down to triiodothyronine–an important thyroid hormone– through deiodination catalysis by iodothyronine deiodinase. These hormones –triiodothyronine and thyroxine– contain iodine. So selenium and iodine are involved in the formation of thyroid hormone. It follows, therefore, that combined iodine and selenium deficiencies affect the metabolism of thyroid hormone leading to increased rate of IDD. This explains the reason for the severity of IDD in Central Africa and Democratic Republic of Congo as both countries have low concentrations of selenium and iodine in their soils.

In living systems, selenium occurs as selenocysteine and is incorporated in two enzymes, namely: glutathione peroxidase and 1 5'-iodothyronine deiodinase. These enzymes are important in the thyroid gland where they help in thyroid hormone production. Glutathione peroxidase protects cells against oxidative damage by acting as a catalyst in the breakdown of hydrogen peroxide. According to Davies<ref name=Dvies /> thyroxine is broken down to triiodothyronine–an important thyroid hormone– through deiodination catalysis by iodothyronine deiodinase. These hormones –triiodothyronine and thyroxine– contain iodine. So selenium and iodine are involved in the formation of thyroid hormone. It follows, therefore, that combined iodine and selenium deficiencies affect the metabolism of thyroid hormone leading to increased rate of IDD. This explains the reason for the severity of IDD in Central Africa and Democratic Republic of Congo as both countries have low concentrations of selenium and iodine in their soils.

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* Fordyce, F., 2010. Selenium Deficiency and Toxicity in the Environment. In: Selinus, O., Alloway, B., Centeno, J.A., Finkelman, R.B., Fuge, R., Lindh, U. and Smedley, P., (eds.) 2010. Essentials of Medical Geology. Elsevier, Amsterdam, 375-413.

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* Fuge, R., 2010. Soils and Iodine Deficiency. In: Selinus, O., Alloway, B., Centeno, J.A., Finkelman, R.B., Fuge, R., Lindh, U. and Smedley, P., (eds.) 2010. Essentials of Medical Geology. Elsevier, Amsterdam: 417-433.

* Fuge, R., 2010. Soils and Iodine Deficiency. In: Selinus, O., Alloway, B., Centeno, J.A., Finkelman, R.B., Fuge, R., Lindh, U. and Smedley, P., (eds.) 2010. Essentials of Medical Geology. Elsevier, Amsterdam: 417-433.

* Hisashi, N., Kunio, F. and Takashi, K., 2010. Medical Geology in China. In: Selinus, O., Finkelman, R.B. and Centeno, J.A., (eds) 2010. Medical Geology: A Regional Synthesis, 329–338.

* Hisashi, N., Kunio, F. and Takashi, K., 2010. Medical Geology in China. In: Selinus, O., Finkelman, R.B. and Centeno, J.A., (eds) 2010. Medical Geology: A Regional Synthesis, 329–338.