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10 Selenium Benefits + Sources, Dosage & Toxicity

Written by Ana Aleksic, MSc (Pharmacy) | Last updated:
Jonathan Ritter
Puya Yazdi
Medically reviewed by
Jonathan Ritter, PharmD, PhD (Pharmacology), Puya Yazdi, MD | Written by Ana Aleksic, MSc (Pharmacy) | Last updated:

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Selenium is an essential mineral needed for immunity, thyroid health, antioxidant protection, and more. Its intake needs to be just right – too much or too little can both have adverse effects. This article covers all the benefits of selenium supplementation, optimal dosage, and toxicity.

What is Selenium?

Intro & Roles

Selenium (Se) is a mineral found in Brazil nuts, poultry, fish, cereal, and eggs. It was considered a toxin until 1957 when its health benefits started to be discovered. Balanced selenium intake is crucial. Both selenium deficiency and excess are harmful while taking in just the right amount can have wide-ranging health benefits [1].

The recommended daily allowance – 55 µg/day – is based on the amount needed to maximize the activity of the selenium-dependant master antioxidant enzyme glutathione.

Selenium also binds to the sulfur-containing amino acid cysteine and forms antioxidant selenoproteins in the body. Selenoproteins carry selenium to the tissues, reduce inflammation, support a healthy immune system and thyroid gland [1].

Snapshot

PROs

  • Fights inflammation and improves the immune response
  • Vital for a healthy thyroid
  • May help prevent cancer
  • Can reset the circadian rhythm
  • Supports a balanced mood

CONs

  • Narrow optimal range, you need to be careful not to take in too much
  • May contribute to type 2 diabetes and lower antioxidants in excess
  • May lower thyroid hormones in men

Intake & Deficiency

Plants take in selenium from the soil, making selenium intake dependant on its concentration in the surrounding soil. People who live within a selenium-poor geographical belt – spanning from northeast to southwest China – are at a greater risk of selenium deficiency [2, 3].

Deficiency can cause serious health problems, including Keshan disease, an extreme weakening of the heart that can result in heart failure [3].

On the other hand, very high blood selenium is associated with diabetes, high short-term (fasting blood glucose) and long-term diabetes markers (HbA1c) [4].

Intake of selenium is also low in Eastern Europe, while it can exceed the recommended daily amounts in Venezuela, parts of the US, Canada, and Japan. The average person living in the US takes in 107 – 151 µg/day of selenium from food and supplements, which is 2-3x the official recommended daily allowance (RDA) [4].

Anti-inflammatory and Antioxidant Effects

Selenium forms selenoproteins, key players in antioxidant defense, and increases the activity of genes that make them [5].

Selenium blocks the activation of NF-kB, the main controller of inflammation in the body. As a result, fewer inflammatory substances are released into the bloodstream (including IL-6, IL-8, and TNF-alpha) [5, 6].

Adequate selenium intake may also lower the inflammatory marker CRP, which is high in many inflammatory and autoimmune diseases [5].

Effects on the Circadian Rhythm

Selenium is a powerful zeitgeber – a cue that synchronizes your internal clock to the natural 24-hour light/dark cycles of the Earth (zeit=time, geber=giver). Selenium helps restore an imbalanced circadian rhythm, which often affects people with insomnia.

The selenium- and sulfur-containing amino acid methyl selenocysteine could reset circadian rhythm genes in cell-based studies. It caused the activity of these genes to peak at nighttime, as they normally should. This amino acid is found in garlic, astragalus, onions, and broccoli [7, 8].

Health Benefits of Selenium

Possibly Effective:

1) Thyroid Disorders

Selenium is most concentrated in the thyroid and helps regulate its function. Severe selenium deficiency may impair its function and is linked to different thyroid disorders [9].

Hashimoto’s Disease

Selenium supplementation can be useful for people with Hashimoto’s thyroiditis or autoimmune hypothyroidism. In a review of four clinical studies, selenium improved mood, general well-being, and thyroid peroxidase antibodies in people with Hashimoto’s. The direct benefits on thyroid structure and function were less clear, though [10].

Grave’s Disease

Graves’ disease (GD) is an autoimmune disease in which the thyroid becomes enlarged and overactive. In 38 selenium-deficient patients with Grave’s disease, Se supplementation significantly reduced thyroid hormones [11].

However, it showed no short-term benefits in another trial of 30 GD patients with normal selenium levels [12].

The benefits of Se supplementation for Graves’ disease may be limited to Se-deficient patients.

Other

Some pregnant women experience mildly reduced thyroid function, also known as subclinical hypothyroidism (increased TSH, normal T4).

In an analysis of over 300 pregnant women, lower selenium blood levels were linked to thyroid enlargement and damage. Selenium supplements helped restore thyroid health, especially after birth. Selenium was used in the form of Selenomethionine, a naturally-occurring selenium-bound amino acid [13].

In mice with autoimmune thyroiditis, selenium increased T-reg immune cells, which are crucial for overcoming the destructive autoimmune response. It also lowered blood levels of disease-related antibodies (thyroglobulin TgAb) [14].

2) Viral Infections

Lack of selenium can set off the immune system and cause even harmless virus infections to progress and become dangerous. In the most severe cases, this can result in complete heart failure (Keshan disease) [15].

Adequate selenium intake protects immune cells and aids in producing antibodies. Lower selenium levels can increase damage (via oxidative stress) in healthy cells, reducing immune defense, activating viruses, and triggering their faster division.

HIV infections can trigger selenium deficiency, while supplementation may help fight HIV and improve outcomes. In one trial of 18 HIV-positive people, selenium in combination with conventional drugs improved the quality of life. Selenium also blocked the replication of the virus in cellular and animal studies [16].

Selenium levels may predict the disease outcome in HIV-positive people. In one study, selenium-deficient HIV patients were almost 20 times more likely to die from HIV than those with normal selenium levels [17].

In one study on children, low selenium levels not only increased the likelihood of dying from HIV but were also linked to faster spreading of the disease [18].

3) Kashin-Beck Disease Prevention

Kashin-Beck disease (KBD) is an endemic disease of the joints and bones, mainly occurring in certain China regions, particularly Tibet. Selenium deficiency plays an important role, along with different environmental and genetic factors [19].

According to a meta-analysis of 11 Chinese clinical trials with 2652 patients, adding Se-enriched salt to a diet reduces the odds of Kashin-Beck disease by 84% in healthy children, compared with iodized salt [20].

However, Se supplementation didn’t improve joint pain or mobility in 324 children with Kashin-Beck disease. The authors emphasized the role of iodine deficiency in KBD [21].

4) Preeclampsia

Preeclampsia is characterized by high blood pressure, swelling, and the presence of protein in the urine during pregnancy [22].

In a meta-analysis of 3 trials with 439 pregnant women, Se supplements (60-100 mcg daily for up to 6 months) reduced the risk of preeclampsia by 72% compared with placebo. According to 13 observational studies (1,515 participants), pregnant women with low Se levels have a higher risk of preeclampsia [23].

Insufficient Evidence:

No valid clinical evidence supports the use of selenium for any of the conditions in this section. Below is a summary of up-to-date animal studies, cell-based research, or low-quality clinical trials which should spark further investigation. However, you shouldn’t interpret them as supportive of any health benefit.

5) Brain Protection

The benefits of fish consumption are often attributed to polyunsaturated fatty acids, omega-3s, but high amounts of selenium found in fish may be equally important. Selenium works in synergy with the fatty acids, preventing their breakdown and enhancing their beneficial effects on cognition.

In one study on 200 older people, fish intake increased selenium blood levels more than omega-3 levels. According to the authors, better cognitive function in the elderly may depend directly on selenium [24].

Selenium is very important for your brain health. Low selenium levels are linked to poor cognition, memory problems, and low neurotransmitter levels in the brain. Alzheimer’s patients had only 60% of the brain selenium levels of healthy people in one study [4, 25].

6) Cancer Prevention

The role of selenium in cancer prevention is a hot topic of debate and the findings are conflicting. Selenium supplements aren’t approved for cancer prevention or treatment.

Supportive Research

Ever since the 70s, studies have been reporting that increased selenium intake reduces cancer deaths and vice versa. In one study, the dietary intake of selenium in 27 countries was linked to fewer deaths from cancer. In the US, deaths from cancer were much higher in low selenium counties, in which people are more likely to be deficient [26].

Early studies of up to 11,000 people reported that low selenium status is linked with an increased cancer risk of cancer and death. The estimated risk ranged from two to six-fold and was at times only applicable to men [27, 28].

Some recent studies support early findings. Slightly increased selenium intake – still well below toxic levels – was associated with a lower risk of colorectal and prostate cancer [4, 29].

In 37 men, a combination of Milk thistle (Silymarin) and selenium reduced LDL, inflammation, and total cholesterol after prostate removal surgery, potentially preventing prostate cancer relapse and progression [30].

Conflicting Research

A significant limitation is that most studies of selenium intake and cancer risk are observational. Observational studies can make associations, but can’t attribute any causation.

There have been very few clinical trials using selenium as a therapeutic agent, which are needed to determine its effects. Many other factors – including lifestyle, genetics, gender, and age, stage, and type of cancer – will affect cancer outcomes and the preventive action of selenium [1].

In one clinical trial of over 1,000 people with a history of nonmelanoma skin cancer, 200 µg/day of selenium (as selenium yeast) didn’t reduce the chances of relapse. But it did lower all-type cancer mortality by 50% and cancer rates by 37%. It especially reduced the rates of prostate, colon, and lung cancer [31].

According to a meta-analysis of 15 clinical trials, Se supplementation may lower the risk of lung cancer in deficient patients but increase the risk in patients with normal Se levels [32].

Once again, the benefits of Se supplementation actually stemmed from fixing a deficiency.

Discouraging Research

Despite a couple of promising clinical trials, the largest review up-to-date concluded that there is no evidence to support increased selenium intake through diet or supplements in preventing cancer in humans. This review included 83 studies and emphasized the above-mentioned drawbacks of the available studies [33].

Selenium supplementation or increased dietary intake would undoubtedly provide benefits to Se-deficient individuals. But given that deficiency is quite rare, it would be best to check selenium blood levels before taking supplements. Whether or not selenium prevents cancer is still unknown while taking in excessive selenium can only be harmful.

More clinical research is needed, taking into account genetics, nutritional status, and various forms of selenium supplements.

7) Immune Response

Selenium deficiency impairs immune function. Accordingly, selenium supplementation stimulates the immune system, even in people who are not deficient. Lymphocytes of people who supplemented with selenium (200 µg/day) were more active and efficient at destroying foreign invaders and tumor cells [34].

Selenium may be more important for proper immunity than was previously realized. Activated immune cells have an increased need for selenium and its amino-acid bound form (selenocysteine), according to recent studies [4].

Good for Th2-Dominant People

Selenium supplementation may help divert the immune response away from the Th2-type and promote the Th1-type, which protects against asthma, allergic reactions, viral infections and cancer [35].

More studies are needed to evaluate the immune-stimulating properties of selenium.

8) Fertility and Reproductive Health

Balanced selenium levels are important for reproductive health in both men and women. In men, selenium is important for producing sperm and testosterone.

In a study of 69 men with fertility issues, selenium supplements mildly increased sperm motility but didn’t improve sperm density. 11% of the men who took selenium achieved paternity, compared to none in the placebo group [36].

However, Se supplementation had no benefits on sperm count, motility, or quality in a similar trial of 33 patients [37].

In studies on male rats, selenium increased testosterone levels but only in rats who were selenium-deficient [38, 39, 40].

In women, adequate selenium levels are especially important during early pregnancy. In one study, women who suffered from recurrent miscarriages were more likely to have lower selenium blood levels. Selenium deficiency was also linked to an increased likelihood of miscarriages in animal studies [4, 41].

9) Heart Health

Selenium may protect against heart disease, probably by boosting the antioxidant glutathione, reducing oxidative stress and preventing platelet clumping [42].

Selenium deficiency eventually causes harmful, reactive substances to build up, making blood vessels less flexible and platelets more “sticky”. For example, in men with heart disease, low selenium may cause platelets to clump together and narrow blood vessels, potentially worsening heart health [42].

The clinical results are still mixed, though. In one study, low selenium was linked to a two- to three-fold increase in heart disease, while in another study men with low selenium were at an increased risk of heart attacks or stroke. But other small studies didn’t find any clear link between heart disease risk and selenium levels [43, 44].

In 668 elderly Swedish people, supplementation with selenium (200 μg) and coenzyme Q10 (200 mg) for four years significantly reduced the risk of heart failure. It dropped by 50% in previously Se-deficient patients and by 62% in patients with moderate Se levels [45].

Patients with high Se levels didn’t benefit from supplementation, suggesting once again that the benefits might be limited to Se-deficient individuals [45].

Further research should determine whether selenium supplementation protects against heart disease and under which occasions.

10) Mood and Anxiety

Low selenium levels have been linked to depression, anxiety, tiredness, and mental confusion. In one study of 50 people, selenium supplementation (100 mcg/day) over 5 weeks improved mood, energy levels, and lessened anxiety [43].

Selenium supplementation over 8 weeks noticeably improved depression and mildly reduced anxiety in a study on seniors living in nursing homes [46].

Selenium Blood Test & Normal Ranges

The margin between selenium deficiency, optimal and above-optimal levels and toxicity is very narrow. Knowing your selenium status can be helpful in identifying potential health problems.

The most common way to measure selenium status is using blood biomarkers that reflect selenium intake, which can include the following [1]:

  • Elemental selenium (normal range: 70 160 ng/mL)
  • Glutathione peroxidase
  • Selenoprotein P, the main supplier of selenium to tissues

To increase longevity, fight inflammation, and aid in cancer prevention, even higher blood levels have been researched. Slightly above-optimal intake may reduce the risk of lung, prostate and colorectal cancers, but the evidence is inconclusive [1].

Aside from blood tests, long-term selenium status can alternatively be assessed by measuring selenium concentrations in toenails and hair.

Low Selenium

Selenium levels are a marker of antioxidant protection and overall health. Low or high levels don’t necessarily indicate a problem if there are no symptoms or if your doctor tells you not to worry about it.

Causes

Causes of low selenium include:

  • Lack of selenium in the diet [4, 47]
  • Inflammatory bowel disease (ulcerative colitis and Crohn’s disease) [48, 49, 50]
  • Parenteral nutrition, in ill people who receive nutrients through the veins [51]
  • Kidney disease [52]
  • Alzheimer’s disease [53, 54]
  • Grave’s disease (hyperthyroidism) or an underactive thyroid (hypothyroidism) [55]
  • Specialized diets for people with health conditions (such as phenylketonuria, a rare birth defect that causes phenylalanine amino acid buildup in the body) [56, 57]

The following drugs may also decrease selenium levels:

  • Corticosteroids, drugs used to reduce inflammation [58]
  • Birth Control pills [59, 60]
  • Clozapine (Clozaril, FazaClo, Versacloz), antipsychotic medication used to treat schizophrenia [61]

Associated Conditions

Low selenium levels may be associated with:

  • Muscle weakness [62, 51]
  • Worsening of iodine deficiency [63]
  • Anemia [64]
  • Rheumatoid arthritis [65]
  • Heart disease, including Keshan disease, which affects the heart muscle [66, 67, 68]
  • Progression of HIV to AIDS [69]
  • Various cancers, including lung, prostate, breast, esophageal, and stomach cancer [70, 71, 72]
  • Miscarriage [41]

High Selenium

Selenium levels are a marker of antioxidant protection and overall health. Low or high levels don’t necessarily indicate a problem if there are no symptoms or if your doctor tells you not to worry about it.

While optimal selenium intake has many health benefits, excess selenium can be dangerous.

Causes

Causes of high selenium levels include:

  • Excess selenium intake, either from a diet high in selenium or from selenium supplements [73, 74, 75, 76]

If you have high selenium levels, stop taking selenium supplements. You should also make sure that your total selenium intake from any other supplements does not exceed the recommended daily intake [73, 74, 75].

Curcumin supplements may reduce the damage from high selenium in the body [77].

Associated Conditions

Type 2 Diabetes

Higher selenium levels were linked to type 2 diabetes and high blood sugar, independent of other factors [78].

However, the relationship between diabetes and selenium is not that straightforward. The association was much stronger in men who supplement with selenium despite getting sufficient amounts from food. On the other hand, women who develop diabetes during pregnancy are often selenium-deficient [79].

Excess selenium supplementation in non-deficient people may increase the risk of type 2 diabetes.

Lower Active Thyroid Hormone (T3) in Men

Excess selenium supplementation (300 mcg/day) lowered T3 in men in one study. However, a larger study couldn’t replicate these results [80, 81].

Prostate Cancer Mortality

Selenium supplementation of 140 μg/day or more in people with prostate cancer (non-metastatic) was linked to increased mortality in one observational study. Blood selenium levels were not measured, so it’s hard to draw meaningful conclusions [82].

Increased Blood Fats

High blood selenium levels may be associated with increased blood levels of total, LDL, and HDL cholesterol, apo-B, and apo-A1 [83].

Symptoms

General symptoms selenium toxicity include [84, 76]:

  • Nausea
  • Vomiting
  • Diarrhea
  • Stomach pain
  • Skin rash (dermatitis)
  • Low blood pressure
  • Fast heartbeat

Selenium toxicity may also cause [84]:

  • Garlic-like breath
  • Metallic taste in the mouth
  • Tremors
  • Muscle spasms
  • Confusion
  • Delirium
  • Coma

Symptoms of selenium toxicity can persist for a long time, even up to 3 months after stopping a supplement [74].

That said, short-term supplementation with an appropriate dosage of Se is safe and well-tolerated, even in children and pregnant women [85, 86, 87].

Selenium Sources & Dosage

Sources

Selenium exists naturally in a variety of foods, but it can also be purchased as a supplement.

Some food sources include [2, 47, 88]:

  • Brazil nuts
  • Fish
  • Shellfish
  • Beef
  • Chicken
  • Young barley seedlings
  • Brewer’s yeast
  • Wheat germ
  • Shiitake and button mushrooms
  • Oats
  • Whole-wheat bread
  • Bran
  • Brown rice
  • Red swiss chard

Fish is an especially good source of selenium because it’s also high in omega 3s and iodine, which all work in synergy.

Dosage

The below doses may not apply to you personally. If your doctor suggests using a selenium supplement, work with them to find the optimal dosage according to your health condition and other factors.

Although the RDA is 55 mcg, most clinical studies used selenium doses of 100-200 mcg/day, taking into account all sources.

For example, one brazil nut already contains 50 mcg of selenium on average. If you eat 2 brazil nuts per day and plenty of fish, you probably don’t need to supplement with selenium as you are already taking in 100-150 mcg/day through diet.

However, the selenium content in Brazil nuts can vary depending on the soil they were grown on, which is why supplements may be a more reliable option for some people [89].

The Tolerable Upper Intake Level for selenium in adults is 400 mcg/day. This is the maximum daily intake at which no adverse health effects are expected in most people [74, 90].

About the Author

Ana Aleksic

Ana Aleksic

MSc (Pharmacy)
Ana received her MS in Pharmacy from the University of Belgrade.
Ana has many years of experience in clinical research and health advising. She loves communicating science and empowering people to achieve their optimal health. Ana spent years working with patients who suffer from various mental health issues and chronic health problems. She is a strong advocate of integrating scientific knowledge and holistic medicine.

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