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It rarely occurs in its elemental state in nature, or as pure ore compounds.
Selenium is found impurely in metal sulfide ores, where it partially replaces the sulfur.
Selenium salts are toxic in large amounts, but trace amounts are necessary for cellular function in many organisms, including all animals. Selenium is a component of the antioxidant enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants). It is also found in three deiodinase enzymes, which convert one thyroid hormone to another.
Functions:
Selenium is required for the proper activity of a group of enzymes called glutathione peroxidases.
These enzymes play a key role in the body's detoxification system and they also provide protection against oxidative stress. (Oxidative stress is physiological circumstance in which there is excessive risk of oxygen-related damage to the body.) Of the eight known glutathione peroxidase enzymes, five of them require selenium.
In addition to the activity of glutathione peroxidase, selenium-containing enzymes are involved in recycling of vitamin C from its spent form back to its active one, allowing for greater antioxidant protection.
A selenium-containing enzyme is responsible for transforming a less active thyroid hormone called T4 into the more active T3. Selenium and iodine work together to keep thyroid function strong and consistent.
Like the antioxidant protection issue, this is not just an esoteric concern. Researchers have been able to induce problems with the thyroid gland in just two months of a low-selenium diet.
What food Selenium is found in:
The selenium content of plant foods is often closely related to selenium content of soil in which the plants have been grown.
It is found in meat, seafood, grain cereals, egg yolk, milk, brazil nuts, mushrooms and garlic.
One ounce ( 28 grams) of Brazil nuts may contain as much as 10 times the Dietary Reference Intake (DRI) recommendation for selenium intake. Other exceptionally selenium-rich foods include oysters, clams, liver, and kidney. Each of these foods is likely to contain double to triple the DRI in a serving.
World's Healthiest Foods ranked as quality sources of selenium
Food Serving Size Cals Amount (mcg)
Tuna 116 grams/4 oz 147.4 122.70
Shrimp 4 oz 134.9 56.13
Sardines 90 gr /3.20 oz 188.7 47.81
Salmon 4 oz 157.6 43.09
Cod 4 oz 96.4 31.75
Mushrooms, Crimini 1 cup 15.8 18.72
Mushrooms, Shiitake 0.50 cup 40.6 17.98
Asparagus 1 cup 39.6 10.98
Mustard Seeds 2 tsp 20.3 8.32
Turkey 4 oz 166.7 34.25
Chicken 4 oz 187.1 31.30
Lamb 4 oz 350.4 27.90
Scallops 4 oz 125.9 24.61
Beef 4 oz 132.7 23.93
Barley 0.33 cup 217.1 23.12
Tofu 4 oz 164.4 19.73
Eggs 1 each 77.5 15.40
Brown Rice 1 cup 216.4 19.11
Sunflower Seeds 0.25 cup 204.4 18.55
Sesame Seeds 0.25 cup 206.3 12.38
Cow's milk 4 oz 74.4 4.51
Flax Seeds 2 TBS 74.8 3.56
Cabbage 1 cup 43.5 3.45
Spinach 1 cup 41.4 2.70
Garlic 6 cloves 26.8 2.56
Broccoli 1 cup 54.6 2.50
Swiss Chard 1 cup 35.0 1.57
What symptoms and disorders selenium deficiency may cause:
A decline in blood selenium levels in the UK and other European countries has raised concern about possible public health implications, particularly in relation to cancer and cardiovascular disease.
Keshan disease and Kashin-Bek disease (also often referred to as Keshan-Beck disease). These are both associated with selenium deficiency;
Counteracting the development of virulence and inhibiting HIV progression to AIDS;
A possible role in preventing pre-eclampsia;
A possible relationship between mood disturbance and selenium deficiency;
A possible association between elevated selenium intake and reduced cancer risk;
In asthma, some epidemiological studies suggest that deficiencies of selenium, zinc and other nutrients (vitamins A, C, D, and E) may be associated with the development of allergic disorders and asthma;
Cardiovascular disease. Deficiency is associated with cardiomyopathy;
Deficiency in trauma and burns patients. One study reports improved recovery with selenium supplementation in trauma and burns patients;
The thyroid gland is rich in selenium. Selenium deficiency may help to precipitate hypothyroidism and autoimmune thyroid disease and supplementation may contribute to prevention of these diseases.
Problems related to excessive doses:
Chronically high intakes of the organic and inorganic forms of selenium have similar effects. Early indicators of excess intake are a garlic odor in the breath and a metallic taste in the mouth.
The most common clinical signs of chronically high selenium intakes, or selenosis, are hair and nail loss or brittleness. Other symptoms include lesions of the skin and nervous system, nausea, diarrhea, skin rashes, mottled teeth, fatigue, irritability, and nervous system abnormalities.
Respiratory Symptoms
Respiratory symptoms of selenium poisoning include mucous membrane irritation, coughing, garlicky breath odor, bronchitis, bronchial pneumonia, reduced respiratory rate, and ultimately, pulmonary edema and death.Gastrointestinal Symptoms
Gastrointestinal symptoms of selenium poisoning include stomach upset, nausea, vomiting, diarrhea, a bitter metallic taste with burping and intestinal cramps, according to "The New Encyclopedia of Vitamins, Minerals, Supplements and Herbs."Hair, Nail, Teeth and Skin Changes
Common early symptoms of selenium toxicity and poisoning involve the hair and nail beds of the hands and feet. Excessive selenium causes brittle hair, hair loss, deformed nails, brittle nails, sloughing off of nails, tooth discoloration, tooth decay and mottled or discolored skin.
Neurological Signs
Neurological signs of selenium poisoning infer damage to nerves and brain tissue. They include fatigue, irritability, listlessness, reduced mental alertness, emotional instability, hand tremors, tingling or loss of sensation in the arms or legs, reduced blood pressure and ultimately, unconsciousness and death.As discussed earlier, Brazil nuts contain very high amounts of selenium (68–91 mcg per nut) and could cause selenium toxicity if consumed regularly.
Toxic effects probably occur at around 40 x the RDA.
Recommended doses for people in the UK:
The average daily intake of selenium in in the UK has been falling over a 30-year period.
The recommended daily allowance (RDA) of 75 micrograms/day and 60 micrograms/day for men and women respectively in the UK.