Selenium is considered a trace mineral – meaning we need only very small amounts. But that doesn’t mean it isn’t important. We want to avoid selenium deficiency, but a little goes a long way and does a lot of good.

Although selenium is essential, too much can be toxic.  

What does selenium do?

Selenium is involved in a number of different processes in the body. Its main functions include:

  • Antioxidant activity: selenium is needed for the function of two important groups of enzymes – glutathione peroxidases and thioredoxin reductases. These are powerful antioxidants in our body and protect our DNA from damage. These effects are protective against cancer and also other oxidative stress diseases such as cardiovascular disease, rheumatoid arthritis, and cataracts.
  • Immune system: selenium is involved in the production of immune T cells. Immune T cells are necessary for destroying cells that have been infected with bacteria or viruses, or that have become cancerous. 
  • Thyroid hormones: selenium is needed to convert the thyroid hormone T4 (thryoxine) into the biologically active form T3 (triiodothyronine). It’s important to have a balanced ratio of these two hormones; if the conversion rate from T4 to T3 is low, the T3 levels will be low, leading to hypothyroid symptoms. Thus selenium is particularly important for those with autoimmune thyroid disorders.
  • Fertility: in men, selenium is necessary for the production of testosterone and the development of sperm. 
  • Toxic metals: selenium might provide some protection against the toxic effects of heavy metals such as mercury.

Why don’t we get enough selenium?

  • Just as we saw with magnesium, if there isn’t much selenium in the soil there won’t be much selenium in the food we eat, whether that be plant-based or animals consumed for meat. Agricultural practices and geographical differences can have a significant impact on the selenium levels in soil. Industrial farming can significantly deplete the soil levels.
  • Processing foods and boiling foods leads to a loss of selenium.

These factors can significantly affect the evaluation of the “average” selenium content quoted for a food. For example, one Brazil nut is said to have 96mcg selenium on average. Yet analysis has shown that the range per Brazil nut is 8mcg to 216mcg! The highest levels of selenium in Brazil nuts seem to be in those grown in Eastern Amazon countries. There’s a big difference in the selenium levels in shelled and unshelled Brazil nuts too – for the higher levels, buy unshelled nuts and shell them yourself.

Who is at particular risk for selenium deficiency?

While selenium deficiency is not thought to be common in healthy adults, the following populations might be at particular risk for selenium deficiency:

  • Pregnant women (low levels seem to be associated with miscarriages)
  • People who:
    • have had bariatric surgery
    • have Crohn’s disease, celiac disease, or severe gastrointestinal conditions
    • are on kidney dialysis
    • have thyroid disease
    • live in countries with very low levels of selenium in the soil (China, for example)
    • take the medications Valproic acid (an anticonvulsant) or cisplatin (a chemotherapy), both of which seem to lower the circulating selenium concentrations in the body, leading to deficiency
  • HIV patients
  • Cancer patients – low selenium levels are frequently seen in patients with different types of cancer, and selenium has been shown to have a protective effect against some cancer. A meta-analysis of 16 observational studies found that higher versus lower selenium status was associated with a 31% lower risk of cancer at any site and a 40% lower risk of cancer-related mortality. 

Recommended daily allowance for selenium

The recommended daily allowance (RDA-USA) for adolescents and adults (both male and female) is 55mcg (micrograms). (This might be written as 55 μg – don’t confuse it with mg (milligrams)!)  In the UK, the recommended nutritional intake (RNI) is 70mcg for men and 60mcg for women.

Higher levels are recommended during pregnancy and lactation. 

The tolerable upper limit for adolescents and adults is 400mcg/day – which means you shouldn’t go above this level. 

Signs of selenium deficiency

Insufficient selenium can reduce the activity of important enzymes and thus reduce antioxidant activity and levels of T3 thyroid hormone. However, even when severe, selenium deficiency doesn’t usually result in obvious clinical symptoms. That said, some of the signs of selenium deficiency include:

  • Muscle weakness or pain.
  • Increased susceptibility to physiological stressors.
  • A specific heart disease (Keshan disease) or a specific type of arthritis (Kashin-Beck) are sometimes seen where severe deficiency exists but they generally occur only in countries where soils are very deficient in selenium (such as China). Both diseases are thought to be caused by the lack of selenium’s antioxidant effects.

How do you test for your selenium level?

Selenium levels can be measured in a blood test. This is often referred to as a Se Serum test. Fasting is not required before this test. Selenium can also be measured along with other micronutrients in the Spectracell micronutrient test

Food sources of selenium

The richest sources of selenium are in organ meats (such as liver) and seafood/fish such as cod, oysters, clams, shrimp, tuna, sardines, and salmon. As mentioned above, there is a wide variation in the selenium content of plants and grains depending on the soil in which they were grown but typically Brazil nuts, garlic, crimini mushrooms, and many Brassica/cruciferous vegetables tend to accumulate selenium, and are good sources. 

Given the variability in selenium levels in food sources, you might wonder about taking supplements instead. However, a study in New Zealand compared selenium levels in people who ate two Brazil nuts a day to the levels in people who took 100mcg selenium supplementation (as selenoemthionine). The researchers found that the Brazil nuts were as effective as the supplement in increasing plasma selenium levels and enyzme activity. It appears that these variations seem to average out to a reasonable level over time.

Looking at the levels needed, the RDA/RNI can easily be fulfilled by one or two Brazil nuts a day. However as the upper limit is 400mcg/day, caution is necessary as a handful of nuts plus selenium in a multi-vitamin could put you over the limit. 

Supplementing with selenium

As we have seen, it is relatively easy to get our daily selenium requirement through food. In general, multivitamins also contain some selenium so it’s worth checking how much you are getting through supplementation and food sources. 

However, if you need to supplement, look for those that contain selenomethionine or yeast-bound selenium. It is not recommended to supplement higher than 200mcg/day – and if you are at that level, do not continue for more than a couple of months. It’s worth monitoring your blood levels to make sure you’re not over-supplementing. 

Selenium “partners”

Some other nutrients are important partners with Selenium:

  • Copper, iron, and zinc partner with selenium to form antioxidant enzymes.  
  • Both selenium and iodine deficiencies are associated with hypothyroidism. 
  • Activated vitamin B6 – which is called P5P – is needed for the absorption of selenium.

Who should avoid supplementation?

You should avoid supplementation if you are taking in adequate levels through food. As with all micronutrients and nutrients, it is really helpful to test your levels periodically to see if you are deficient, sufficient, optimal, or too high. Again, the Spectracell micronutrient test is great for this. 

Take home message about selenium

The key thing about selenium is that we need some every day – but only a small amount. Take a look at whether you are getting any in your multivitamin, which will help you decide whether to focus on food sources every day or not. 

Selenium is really important for immune function and thyroid health, and its antioxidant activity.

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