Selenium is an essential mineral needed for optimal health and longevity. Its intake needs to be just right – too much or too little can have the opposite effects. Healthy levels can balance your immune system, set your circadian rhythm, and support reproductive health, while its role in cancer prevention is still controversial. This article covers all the latest findings and what they mean for your health.
What is Selenium?
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 .
The recommended daily allowance (55 µg/day) is based on the amount needed to maximize the activity of the selenium-containing 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 .
Since plants take in selenium from the soil, making selenium intake dependant on its concentration in the surrounding soil. People who live within a wide selenium-poor geographical belt – spanning from northeast to southwest China – are at a greater risk of selenium deficiency. Deficiency can cause serious health problems, including Keshan disease, an extreme weakening of the heart that can result in heart failure.
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). However, this “above-optimal” intake has been associated with better overall health .
- 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
- 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
Health Benefits of Selenium
1) Fights Inflammation
Selenium not only forms selenoproteins, key players in your antioxidant defense but also increases the activity of genes that make them. Selenium offers benefits to people with low antioxidant levels, and any kind of inflammation or autoimmune issues .
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) [3, 4].
2) Helps Reset 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. It helps your body know when it’s time to sleep and rest and when it’s time to be awake and active.
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 [5, 6].
3) Protects the Brain
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.
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 [2+, 8].
4) Supports Thyroid Health
Selenium is most concentrated in the thyroid and even very minor amounts can keep the thyroid working. But severe selenium deficiency will impair its function and is linked to autoimmune thyroid diseases .
Selenium supplementation can be useful for people with Hashimoto’s Thyroiditis, an autoimmune disease that commonly causes the thyroid to become underactive. In a review of clinical studies, selenium improved mood, general well-being, and disease-specific antibodies (thyroid peroxidase antibodies) in people with Hashimoto’s .
Graves disease is another autoimmune disease in which the thyroid becomes enlarged and overactive. In selenium-deficient people with Grave’s disease, supplementation may increase the chances of remission .
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 poor 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 .
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) .
5) May Help Prevent Cancer
The role of selenium in cancer prevention is a hot topic of debate and the findings are conflicting.
Studies Supporting Selenium Supplementation
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 [2+].
Early studies of up to 11k 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 [2+].
In another study, it also increased survival in people with skin cancer by 50% .
Additionally, a combination of Milk thistle (Silymarin) and selenium reduced LDL and total cholesterol in men after prostate removal surgery, which reduced the risk of the prostate cancer of coming back and progressing .
The Selenium Controversy
Based on all these findings, it appears quite clear that selenium deficiency increased the risk of cancer while adequate intake can lessen it. And selenium intake in the US is high, which should decrease the overall cancer likelihood of the population. But the controversy lays elsewhere: almost all of the mentioned studies were purely observational.
Observational studies can make associations, but can’t attribute any causation. It means that if you use a large enough sample, you may find curious links that don’t imply much. People living in the US who own red cars may turn out to have an increased quality of sleep. But this doesn’t mean that buying a red car will solve your sleeping problems.
There have been very few clinical trials using selenium as a therapeutic agent, which are needed to determine its effects. However, cancer prevention is not very easy to study and requires a complex, long-term study design. On top of that, many other factors – including lifestyle, genetics, gender, and age, stage, and type of cancer – will affect cancer outcomes and the preventive action of selenium .
One intervention-based study was done in China, in a region where about 15% of adults are Hepatitis B carriers and 200 times more likely to get liver cancer. In the trial of 226 hepatitis B carriers, none of those who received 200 µg/day of selenium got liver cancer after 4 years, while 7 people in the placebo group did [2+].
In a large US-based trial of over 1k people a history of nonmelanoma skin cancer, 200 µg/day of selenium (as selenium yeast) didn’t reduce the chances of this cancer coming back. But it did lower the risk of dying from any type of cancer by 50% and the chance of getting any type of cancer by 37%. It especially reduced the risk of prostate, colon, and lung cancer [2+].
Studies Cautioning Against Selenium Supplementation
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 available studies .
More clinical research is needed, taking into account genetics, nutritional status, and various forms of selenium supplements.
Animal and Cellular Studies
Lacking proper clinical studies, we can turn to animal and cellular studies to shed some light on the potential mechanisms and health effects of selenium for cancer prevention.
According to cell-based studies, selenium is capable of degrading cancer cells and preventing them from dividing. Selenium reduces DNA damage and oxidative stress, both of which are involved in cancer development. Selenoproteins and their breakdown products may be especially important for selenium’s cancer-preventive action [2+].
The Takeaway Message
Selenium supplementation or increased dietary intake would undoubtedly provide you with benefits if you are deficient. But given that deficiency is quite rare, it would be best to check your selenium blood levels before taking supplements. Whether or not selenium prevents getting cancer or dying from it is still unknown while taking in excessive selenium can only be harmful.
6) Helps Fight Viruses
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) .
Adequate selenium intake protects immune cells and aids the body in producing antibodies.
Lower selenium levels can increase damage (via oxidative stress) in healthy cells, reducing your 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. .
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 [2+].
In one study on children, low selenium levels not only increased the likelihood of dying from HIV but were also linked to a faster spreading of the disease .
7) Boosts the Immune System
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 [2+].
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 [2+].
Selenium is Good for Th2 Dominant People
8) Aids 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 mildly infertile men, selenium supplements greatly increased sperm motility over 3 months. What’s more, 11% of the men who took selenium achieved paternity, compared to none in the placebo group .
In women, adequate selenium levels are especially important during early pregnancy. In one study, women who suffered from first-trimester or 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 [2+, 27].
9) May Improve Mood and Anxiety
Selenium supports the cells in your nervous system. Even a mild selenium deficiency may lead to low mood or anxiety by subtly reducing your production of neurotransmitters.
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 .
Selenium supplementation over 8 weeks noticeably improved depression and mildly reduced anxiety in a study on seniors living in nursing homes .
10) May Support a Healthy Heart
Selenium may protect against heart disease, probably by boosting the antioxidant glutathione, reducing the oxidative stress and preventing plaque-forming platelet clumping [28+].
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 causes platelets to clump together and narrow blood vessels, worsening heart health [28+].
The results are still mixed, though. In one study, low selenium was linked to a two- to three-fold increase in suffering from or dying from heart disease, while in another study men with low selenium were at an increased risk of heart attacks or stroke. But half a dozen other small studies didn’t find any clear link between heart disease risk and selenium levels [28+].
Given the importance of selenium for general health, a balanced diet rich in fish and poultry will help you avoid deficiencies. But there is no proof that additional selenium intake through supplements in people who are not deficient will improve heart health.
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 :
- Elemental selenium (normal range: 70 – 160 ng/mL)
- Glutathione peroxidase
- Selenoprotein P, the main supplier of selenium to tissues
The normal selenium blood levels are dynamic and depend on a combination of factors. For example, scientists discovered the optimal levels needed for the ideal activity of Selenoprotein P are on the higher end of the normal range and amount to about 130 ng/mL, which would bring up the recommended selenium intake to 100 µg/day.
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 .
Aside from blood tests, long-term selenium status can alternatively be assessed by measuring selenium concentrations in toenails and hair.
Causes of low selenium include:
- Lack of selenium in the diet [2, 30]
- Inflammatory bowel disease (ulcerative colitis and Crohn’s disease) [31, 32, 33]
- Parenteral nutrition, in ill people who receive nutrients through the veins 
- Kidney disease 
- Alzheimer’s disease [36, 37]
- Grave’s disease (hyperthyroidism) or an underactive thyroid (hypothyroidism) 
- Specialized diets for people with health conditions (such as phenylketonuria, a rare birth defect that causes phenylalanine amino acid buildup in the body) [39, 40]
The following drugs may also decrease selenium levels:
- Corticosteroids, drugs used to reduce inflammation 
- Birth Control pills [42, 43]
- Clozapine (Clozaril, FazaClo, Versacloz), antipsychotic medication used to treat schizophrenia 
Having low selenium levels are associated with:
- Muscle weakness [45, 34]
- Worsening of iodine deficiency 
- Anemia 
- Rheumatoid arthritis 
- Heart disease, including Keshan disease, which affects the heart muscle [49, 50, 51]
- Progression of HIV to AIDS 
- Various cancers, including lung, prostate, breast, esophageal, and stomach cancer [53, 54, 55]
- Miscarriage 
Symptoms of low selenium levels include :
The Dangers of High Selenium
While optimal selenium intake has many health benefits, excess selenium can be dangerous.
Causes of high selenium levels include:
- Excess selenium intake, either from a diet high in selenium or from selenium supplements [57, 58, 59, 60]
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 [57, 58, 59].
Increased Risk of Type 2 Diabetes
Higher selenium levels were linked to type 2 diabetes and high blood sugar, even after other factors are accounted for .
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 .
All in all, only excess selenium supplementation in non-deficient people may increase the risk of type 2 diabetes.
Lower Active Thyroid Hormone (T3) in Men
Increased Risk of Dying from Prostate Cancer
Selenium supplementation of 140 μg/day or more in people with prostate cancer (non-metastatic) was linked to an increased risk of dying from prostate cancer in one observational study. Blood selenium levels were not measured, so it’s impossible to know how many of the included men were actually selenium-deficient or what other supplements they took .
Increased Blood Fats
Fetuses exposed to high selenium from mothers during pregnancy were more likely to get ADHD in one study, but this remains an isolated finding that hasn’t been further studied. Selenium was measured in the umbilical cord, so it’s unknown what blood levels or selenium intake it would translate to [68+].
Although some studies suggest that selenium supplements may be harmful even in non-toxic doses, this is still a matter of debate .
- Stomach pain
- Skin rash (dermatitis)
- Low blood pressure
- Fast heartbeat (above 100 bpm)
Selenium toxicity can cause :
- Garlic-like breath
- Metallic taste in the mouth
- Muscle spasms
Symptoms of selenium toxicity can persist for a long time, even up to 3 months after stopping a supplement. In more rare and serious cases, selenium overdose can even cause fever, memory loss, or damage to the heart and kidneys .
Chronic selenium poisoning also causes :
- Skin changes, such as darker patches, moles, lumps, or skin ulcers
- Brittle nails
- Hair loss
Sources & Dosage
Selenium exists naturally in a variety of foods, but it can also be purchased as a supplement.
- Brazil nuts
- Young barley seedlings
- Brewer’s yeast
- Wheat germ
- Shiitake and button mushrooms
- Whole-wheat bread
- Brown rice
- Red swiss chard
Fish is an especially a good source of selenium because it’s also high in omega 3s and iodine, which all work in synergy.
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 .
I take 100 mcg per day long-term in supplemental form and usually recommend at least 40 mcg/day for avoiding deficiency .
Selenium on SelfDecode
- SNPs that impact selenium measurements
- More information about Selenium on SelfDecode
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