Selenium, that help to make DNA and protect against cell damage and infections

Role of Selenium in the human body
Selenoproteins play critical roles in thyroid hormone metabolism, DNA synthesis, reproduction, and protection from oxidative damage and infection.

Soil and ground water contain inorganic forms of selenium (e.g., selenites and selenates) that plants accumulate and convert to organic forms, mostly selenomethionine and selenocysteine and their methylated derivatives. In foods, selenium is present primarily as selenomethionine along with selenocysteine. Dietary selenium is readily absorbed by the body and absorption is largely not affected by selenium status.

Absorbed selenomethionine and inorganic selenium is rapidly metabolized to a common intermediate that is used for synthesizing selenocysteine, the form of selenium found in the 25 human selenoproteins. Approximately 28% to 46% of the total body’s selenium content is found in skeletal muscle. Selenium homeostasis is maintained primarily by urinary excretion and, in cases of higher selenium intake, through the lungs and feces.

The most commonly used measures of selenium status are plasma and serum selenium concentrations. Plasma or serum selenium concentrations of 8 micrograms (mcg)/dL or higher in healthy people are considered sufficient for selenoprotein synthesis. Concentrations in plasma, serum, and urine reflect recent selenium intake whereas selenium concentrations in whole blood (including erythrocytes) indicate long term status. Analyses of hair and nail selenium content are also used to monitor long term intakes, over months or years.

The two predominant selenoproteins in plasma are glutathione peroxidase 3 and selenoprotein P, and both can be used as functional biomarkers of selenium status. However, some experts question the reliability of these two biomarkers to indicate selenium deficiency because plasma selenoprotein concentrations can be affected by inflammation and oxidative stress. In addition, research suggests that selenium supplementation does not increase selenoprotein P concentration and glutathione peroxidase activity unless individuals are selenium deficient . Urinary methylated selenometabolites are not reliable selenium status biomarkers because many people lack the methylation enzymes due to genetic polymorphisms.

Recommended amounts
1. RDA: The Recommended Dietary AllowanceThe amount of selenium that you need each day depends on your age. Average daily recommended amounts are listed below in micrograms (mcg).

- Birth to 6 months: 15 mcg
- Infants 7–12 months: 20 mcg
- Children 1–3 years: 20 mcg
- Children 4–8 years: 30 mcg
- Children 9–13 years: 40 mcg
- Teens 14–18 years: 55 mcg
- Adults 19–50 years: 55 mcg
- Adults 51–70 years: 55 mcg
- Adults 71 years and older: 55 mcg
- Pregnant teens and women: 60 mcg
- Breastfeeding teens and women: 70 mcg

2. UL: The Tolerable Upper Intake LevelThe daily upper limits for selenium intake for all adults 19 years of age, pregnant and lactating women is 400 micrograms per day. This is the maximum daily intake that is unlikely to cause harmful health effects. It includes intaking from all sources foods, beverages, and supplements. Particularly for young children, the following maximum safe intake as below:

- Birth to 6 months: 45 mcg
- Infants 7–12 months: 60 mcg
- Children 1–3 years: 90 mcg
- Children 4–8 years: 150 mcg
- Children 9–13 years: 280 mcg
- Teens 14–18 years: 400 mcg

How harmful is excess selenium?
Chronically high intakes of selenium can lead to health problems, ranging from muscle tremors, hair loss, stomach upset, and lightheadedness, to more severe outcomes of heart attack, respiratory distress, or kidney failure. Brazil nuts are exceptionally high in selenium even when grown in low-selenium soil, with even one nut containing more than the RDA. Eating too many of these nuts on a daily basis can reach a toxic level, as well as using supplements that contain selenium in excess of the RDA. Early symptoms: Metallic taste, bad breath; Nausea, diarrhea; Hair loss; Nail brittleness or discoloration; Skin rash or lesions; Skin flushing; Fatigue; Irritability; Muscle tenderness.

What will happen if deficiency in selenium
Selenium deficiency alone rarely causes overt illness, but it produces biochemical changes that might predispose people who experience additional stresses to develop certain illnesses. For example, Keshan disease is an endemic cardiomyopathy that was first identified in 1935 in parts of China where the soil is low in selenium. Adults in these areas had average selenium intakes of no more than 10 mcg/day whereas intakes of at least 20 mcg/day protect adults from the disease. Keshan disease mainly affects women of childbearing age and preschool children. Although prevalence is low today, the disease still exists. The etiology of Keshan disease remains unknown, but disease incidence dramatically reduced following several large intervention trials in the 1970s to 1990s that provided selenium selenite supplements. A 2018 systematic review and meta-analysis of 41 studies found that selenium supplements (doses not indicated) reduce the risk of Keshan Disease by 86%.

Selenium deficiency is also associated with Kashin-Beck disease, a type of osteoarthritis that commonly presents in childhood and puberty and occurs in certain low-selenium and low-iodine areas of China, Tibet, Siberia, and North Korea.

Selenium deficiency could exacerbate iodine deficiency, potentially increasing the risk of congenital hypothyroidism in infants. Endemic myxedematous hypothyroidism is a disease that occurs in regions with very low selenium levels, such as Central Africa, and is thought to develop during gestation or early childhood. Individuals with this condition produce insufficient thyroid hormone and present with very low plasma triiodothyronine (T3) and thyroxine (T4) levels and extremely high thyroid stimulating hormone (TSH) levels.

Symptoms that may occur when the body is deficient in Selenium: Nausea, vomiting; Headaches; Altered mental state, confusion; Lethargy; Seizures; Coma.

Groups at risk for deficiency
- People living in low-selenium regions who also eat a primarily plant-based diet. This is rarely seen in the U.S., but populations in China, Russia, and Europe are at risk as their soil is generally low in selenium. The risk is further increased in people living in these areas who eat a vegetarian or vegan diet.

- People with HIV. The virus can lead to diarrhea, malabsorption of nutrients, and decreased appetite.

- People with kidney failure undergoing dialysis. This mechanical process of filtering the blood can remove some selenium. The dietary restrictions required with kidney failure can also decrease overall food intake, leading to a risk of selenium deficiency.

Selenium and Health
Selenium is a component of selenoproteins and enzymes. These have antioxidant properties that help to break down peroxides, which can damage tissues and DNA, leading to inflammation and other health problems.

1. CancerA Cochrane review of 13 prospective observational studies showed a 31% reduced risk of cancer and 45% reduced risk of deaths from cancer in the groups with the highest selenium intakes or blood levels compared with the lowest selenium levels, particularly in men. Yet, the authors cautioned about weaknesses in the meta-analysis since there were differences among the study populations and possible inaccuracy when assessing dietary selenium (regional differences in the selenium content of food was not accounted for).

Another Cochrane review of 83 randomized placebo-controlled trials did not find that selenium supplementation reduced the risk of any cancer type. In addition, some of the trials noted a higher incidence of type 2 diabetes with long-term use of selenium supplements (about 7 years) who had normal selenium blood levels at the start the of study. Further research is needed to replicate this finding. Additional research is needed on the association of selenium and cancer before making specific dietary recommendations.

2. Thyroid diseaseThere are high concentrations of selenium in the thyroid gland, where there are several selenium-containing enzymes regulating thyroid function. If the body does not have enough selenium, it can lead to autoimmune thyroid conditions such as Hashimoto’s disease and Graves’ disease. Both of these conditions cause the body to create antibodies that attack the thyroid gland, leading to an overactive thyroid (hyperthyroidism) or underactive thyroid (hypothyroidism).

Randomized controlled trials studying selenium supplementation show mixed results. Supplements have not been shown to significantly affect thyroid function in people with normal thyroid or mild hypothyroidism, despite causing increased blood levels of selenium. Other studies have shown selenium to decrease the amount of these antibodies and promote the activity of selenoproteins, which work to decrease inflammation.

The role of selenium supplementations for people with autoimmune thyroid diseases (ATD) is still unclear. A systematic review and meta-analysis of nine controlled trials did not find that selenium supplements changed thyroid hormone function in people with ATD, nor did a Cochrane review.

3. Cardiovascular diseaseSelenoproteins help to protect cell membranes from damage by free radicals and keep blood platelets from becoming sticky, both of which can lead to heart disease. However, both cohort studies and clinical trials have shown mixed results regarding the relationship of selenium and cardiovascular disease (CVD).

Observational studies have shown that people with both low and high selenium exposure (based on dietary intake and blood levels) have an increased risk of CVD. Others have found no association at all. Clinical trials have not found that selenium supplements reduce the risk of CVD or cardiac death; however these trials were small and included people who were not likely selenium deficient at the start of the study. Additional research is needed on the association of selenium and CVD before making specific dietary recommendations.

Food sources rich in Selenium
Because selenium in foods is protein bound, foods that are high in protein tend to be the best sources of selenium. Brazil nuts, seafood, meat, poultry, and organ meats are the richest food sources of selenium. Other sources include cereals and other grains, and dairy products. The amount of selenium in drinking water is not nutritionally significant in most geographic regions. The major food sources of selenium in the diets of people in the United States are breads, grains, meat, poultry, seafood, and eggs.

The amount of selenium in a given type of plant-based food depends on the amount and form of selenium in the soil and several other factors, such as soil pH and amount of organic matter in the soil. As a result, selenium concentrations in plant-based foods vary widely by geographic location. For example, according to FoodData Central from the U.S. Department of Agriculture (USDA), Brazil nuts have an average of 544 mcg selenium/ounce, but values from other analyses vary widely.

The selenium content of soil affects the amount of selenium in the plants that animals eat. However, this does not substantially affect the amount of selenium in animal products because animals maintain predictable tissue concentrations of selenium through homeostatic mechanisms. Furthermore, formulated livestock feeds generally contain selenium at consistent levels.

Did You Know?
Selenium and iodine have a synergistic relationship that is especially important for a healthy thyroid. Iodine is a component of thyroid hormone, and selenium as a selenoprotein helps to convert the thyroid hormone into its active form. Both minerals are needed by the thyroid in adequate amounts; too much of one can contribute to a deficiency of the other.

Selenium supplements are promoted to offer several benefits, including boosting immune function, improving hair and nail health, and supporting a healthy thyroid. They are sometimes combined with other antioxidant vitamins such as vitamin E or C. These supplements usually contain between 100-400 micrograms of selenium per dose (the Tolerable Upper intake Level is 400 micrograms). However, if a person is not at high risk for a deficiency, there is no evidence that taking a higher amount of selenium promotes the health benefits shown on these supplement labels.

Compiled and written by Crocus Media