Selenium is part of an important antioxidant, glutathione peroxidase. Glutathione peroxidase protects against oxidation inside cells, in cell membranes with vitamin E, in blood plasma, in sperm, and in the intestines. Glutathione peroxidase has the ability to transform harmful antioxidants such as hydrogen peroxide into water.
Glutathione is recharged with the assistance of the B vitamins riboflavin and niacin, as seen in Figure 13-3.
Selenium is also found in another antioxidant, selenoprotein P. Selenoprotein P is capable of neutralizing nitrogen free radicals in the lining of blood vessels. Other minerals that are necessary components of antioxidant enzymes include copper, zinc, and iron.
SELENIUM AND THYROID HORMONES
Thyroid hormones exist as a storage form, thyroxine, with four iodide ions (T4), and as the active form with three iodide ions (T3). An enzyme that requires selenium removes the extra iodine ion to transform T4 into T3. Without selenium, the active form of thyroid hormone cannot be made. Thus, selenium and iodine are both necessary for proper thyroid function.
Summary for Selenium
Main functions: needed for the important antioxidant glutathione peroxidase. Needed to convert thyroxine to the more active thyroid hormone.
RDA: adults, 55 mcg.
Toxicity: toxic in large amounts with skin and digestive symptoms.
Tolerable upper intake level is 400 mcg for adults.
Deficiency can reduce antioxidant activity and thyroid function.
Sources: Brazil nuts, seafood, whole grains.
Forms in the body: the major form is part of the enzyme glutathione peroxidase.
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