Selenium - a comprehensive guide


Selenium is a mineral we need in tiny amounts – only 55 micrograms a day, which is less than one 10,000th of a gram! Despite this, it’s still vital for our health, including for our thyroid and our metabolism, for supporting our cells against free radical damage, and for making healthy sperm. The best food source of selenium is Brazil nuts, with other good sources including organ meats, fish and shellfish.

Primary functions of Selenium

  • Selenium contributes to normal thyroid function. The hormones produced by our thyroid gland govern our metabolism but also have a role in growth, heart rate, body temperature, energy levels, mood and much more. Selenium is needed by the enzyme that converts the inactive thyroid hormone T4 (thyroxine) into the more active hormone called T3 (triiodothyronine).
  • Selenium is important for our immunity. Among its various potential roles, it is involved in the function of our T helper cells – a type of immune cell that coordinates the activity of other cells. Selenium may also play a role in making immune chemicals such as interleukin-2 which is released as part of the body’s response to infection.[1,2]
  • Selenium is needed for healthy sperm production and motility (how well they swim!), and therefore for fertility and reproduction.
  • Selenium helps protect our cells against oxidative stress (free radical damage). Its main role here is as part of an antioxidant enzyme called glutathione peroxidase [3], which is found in all our cells, protecting them against damage.
  • Selenium supports hair and nail health.
  • Selenium may also help protect against heart disease. This could be via its role in antioxidant enzymes such as glutathione peroxidase, protecting LDL cholesterol against oxidative damage.[4]

How much selenium do we need?

The nutrient reference value for selenium in the UK is just 55 µg (micrograms) for adults. Nutrient reference values refer to the amount needed to ensure that the needs of nearly all the population (97.5%) are being met.

Food sources

The best common food sources of selenium are:

  • Brazil nuts can be by far the highest source. Just one Brazil nut can provide up to 95 µg of selenium. However, if they are grown in selenium-poor soils, this can fall to as little as 10 µg per kernel [6] – and it’s not usually possible to tell what you’re getting!
  • Oysters – 100g of oysters can provide around 80 µg of selenium. They are also extremely rich in zinc.
  • Lamb or chicken liver, which can contain up to around 100 µg selenium per 100g.
  • Oily fish: tuna, anchovies, herring, salmon, sardines and mackerel may provide between 50 and 80 µg per 100g.
  • Sunflower seeds provide up to 55 µg per 100g – although this would be equivalent to just about 8 µg per tablespoon.

Deficiency signs and symptoms*

Symptoms of selenium deficiency may include [5]:

  • Reduced thyroid function (symptoms include difficulty losing weight, feeling cold, tiredness and fatigue, and low mood)
  • Increased susceptibility to infections / poor immunity
  • Abnormal sperm motility and/or reduced fertility
  • Increased cholesterol levels
  • Low mood
  • Premature ageing

*If you experience any of these symptoms, please consult your doctor or health practitioner.

Forms and bioavailability / What to look for when buying a supplement

There are a few different forms of supplemental selenium on the market. These include:

  • Sodium selenite. This form is ‘inorganic’ and is not widely found in food. It is said to be about 50% absorbed, which is less than L-selenomethionine, below [7]. However, it is still considered a good form for supplementation.
  • L-selenomethionine is selenium bound to the amino acid methionine. It is one of the primary forms found in food, and is considered one of the more bioavailable forms of selenium, with up to 90% being absorbed.
  • Yeast bound selenium, or selenium-enriched yeast. This is a popular form to take and is often used in clinical trials. It is thought to be an excellent form for absorption and retention in the body: a specific formulation of selenium-enriched yeast has been found to be 89% absorbed and 74% retained in the body.[8] It contains naturally occurring L-selenomethionine, but also a different organic form called methylselenocysteine, and small amounts of various other forms. Yeast-bound selenium is sometimes seen as ‘food state’ or ‘food form’ selenium. Although a good choice for most people, it’s not suitable for anyone who needs to avoid yeast.

As well as these differences in absorption, these various forms of selenium may have different benefits for the body. For example, it’s said that organic forms of selenium (such as selenomethionine and yeast-bound selenium) may be better for increasing blood levels of selenium, yet the inorganic forms (such as sodium selenite) may actually be better for increasing levels of the antioxidant enzyme glutathione peroxidase.[7]

In summary, it seems that there may be advantages to all of these different types of selenium, and there is not one that particularly stands out as better than the others. 

Dosages: Lifestyle Labs’ recommendations

Adults: Selenium supplements for adults typically provide between 100 and 200 µg of selenium per daily dose. In general, we don’t advise taking more than 200 µg in supplement form on a long-term basis (also see ‘Safety’ below) unless otherwise advised by your healthcare practitioner.

Children: Selenium is not generally given as an individual supplement to children, although those over 8 years can take half an adult dose – up to 100 µg a day. Multivitamin and mineral supplements for children under this age may contain up to around 50 µg a day. More than this may be recommended by a healthcare practitioner.


If you are taking any medications or have any medical condition, please consult your healthcare practitioner before taking selenium.  

For most people, selenium is a very safe supplement to take at the advised levels. However, high doses of this mineral can be toxic. It’s not advisable to regularly consume more than 400 µg of selenium a day in total from food and supplements, and so we recommend no more than 200 µg in supplement form if taken on a long-term basis (unless otherwise advised by your healthcare practitioner). If you’re already taking a multivitamin and mineral supplement, check how much selenium it contains

If you are regularly eating several Brazil nuts a day, it can be best to avoid supplements containing selenium.



  1. Baum MK et al. Selenium and interleukins in persons infected with human immunodeficiency virus type 1. J Infect Dis. 2000 Sep;182 Suppl 1:S69-73.
  2. Huang Z, Rose AH, Hoffmann PR. The role of selenium in inflammation and immunity: from molecular mechanisms to therapeutic opportunities. Antioxid Redox Signal. 2012 Apr 1;16(7):705-43.
  3. Haas, E. and Levin, B. (2006). Staying healthy with nutrition. Berkeley: Celestial Arts. P.203
  4. Nève J. Selenium as a risk factor for cardiovascular diseases. J Cardiovasc Risk. 1996 Feb;3(1):42-7.
  5. Osiecki, H. (n.d.). The Nutrient Bible. 9th ed. Bio Concepts Publishing, pp.198–199.
  6. Chang JC, Gutenmann WH, Reid CM, Lisk DJ. Selenium content of Brazil nuts from two geographic locations in Brazil. Chemosphere. 1995;30(4):801-802.
  7. [This link leads to a website provided by the Linus Pauling Institute at Oregon State University. Lifestyle Labs is not affiliated or endorsed by the Linus Pauling Institute or Oregon State University.]
  8. Bügel S et al. Absorption, excretion, and retention of selenium from a high selenium yeast in men with a high intake of selenium. Food Nutr Res. 2008;52.