Selenium: forms available and which to choose

On supplement shelves selenium comes in several chemical forms, and manufacturers are eager to explain why theirs is «the best». In reality each form has its own features: one is better absorbed, another accumulates less, a third is cheaper and more stable. The editors compare the main forms of selenium and explain which to choose depending on your goals.

Organic and inorganic forms: what the difference is

All forms of selenium in supplements can be divided into two groups. Organic ones are compounds in which selenium is bound to carbon, primarily the amino acids selenomethionine and selenocysteine, as well as their derivatives. Inorganic ones are salts in which selenium is bound to oxygen and sodium: sodium selenite and sodium selenate.

The difference between the groups lies not only in chemistry but also in how the body processes them. Inorganic selenium enters the general «pool» of selenium used to synthesize selenoproteins relatively quickly, with the excess excreted in urine. Selenomethionine, in addition, can be incorporated into tissue proteins in place of ordinary methionine, forming a store that takes no direct part in enzyme function.

The review by Rayman, Infante and Sargent (2008), devoted to selenium «speciation» — that is, its chemical forms in the food chain — stresses that different forms have different bioavailability, metabolism and, possibly, different biological effects. So the notion of a «selenium dose» without specifying the form is incomplete.

For the consumer it is important to understand: better absorption does not always mean a better supplement. For a trace element with a narrow safety window, accumulation in tissues has both advantages and risks.

Selenomethionine and selenium yeast

L-selenomethionine is the main form of selenium in grains, legumes and nuts, and so it is often positioned as the «natural» form. It is well absorbed in the gut via the same transporters as methionine. The review by Schrauzer (2000) describes the high bioavailability of selenomethionine and its ability to maintain the body's selenium stores.

The flip side of this advantage is accumulation. Because selenomethionine is incorporated into the proteins of muscle, liver and other tissues, prolonged intake of high doses gradually raises the body's total selenium content. For a person with a deficiency this is useful; for a person with already adequate status it is potentially superfluous.

Selenium yeast is baker's yeast grown on a medium high in selenium. It contains predominantly selenomethionine, but also other selenium compounds whose composition may depend on the production technology. It was selenium yeast that was used in the NPC study, whereas the SELECT trial used pure L-selenomethionine.

When choosing between pure selenomethionine and yeast, consider standardization: a quality manufacturer of the yeast form states the selenium content and, preferably, the proportion of selenomethionine. For people with yeast intolerance or dietary restrictions, pure selenomethionine is simpler.

Sodium selenite and selenate

Sodium selenite is one of the oldest and cheapest forms of selenium. It is well studied and has been used in many clinical trials, including the randomized study by Marcocci and colleagues (2011) in mild endocrine ophthalmopathy. Inorganic selenium is readily available for selenoprotein synthesis.

The drawbacks of selenite are sensitivity to redox reactions. In combination with high doses of vitamin C in a single dose, selenite can be reduced to elemental selenium, which is poorly absorbed. That is why in complexes with large doses of ascorbic acid manufacturers more often choose other forms.

Sodium selenate is absorbed very well, but a significant part of it is rapidly excreted in urine. This limits accumulation, which can be an advantage for safety, but it means that maintaining status requires regular intake.

Inorganic forms do not build up a store in tissue proteins, so for a person who wants to cover the current need without long-term accumulation they may be a rational choice. However, as with any form, the dose is key.

Other forms and dietary sources

Methylselenocysteine— an organic form that occurs naturally in garlic, onion and broccoli grown on soils high in selenium. It is not incorporated nonspecifically into proteins the way selenomethionine is. This form has been studied mainly in laboratory and animal research, and human clinical data are far scarcer than for selenomethionine and selenite.

Selenium «chelates»such as the glycinate or aspartate are often positioned as improved forms. The chemical nature of such products is not always clearly characterized, and independent comparative studies with the classic forms are scant. The editors recommend treating such marketing promises with restraint.

Brazil nuts— the richest dietary source of selenium. However, their selenium content depends on the soil the tree grew on and can vary several-fold between nuts. So they are not suitable as a means of precise dosing, though in moderate amounts they are a useful food.

Multivitamin complexesusually contain selenium as selenomethionine or sodium selenite in a dose of 50–70 mcg. For most people who need «insurance», this is the simplest and safest option.

How to choose a selenium supplement

The choice of form is secondary to the choice of dose and quality. Even the best form becomes a problem if the capsule contains an excessive amount of selenium or if the manufacturer does not control the content. The story of mass poisoning by a liquid supplement in the United States (MacFarquhar et al., 2010) shows how important quality control is.

• To correct low intake:L-selenomethionine or selenium yeast at a dose of 50–100 mcg.
• For intake without long-term accumulation:sodium selenite at a moderate dose, separate from large doses of vitamin C.
• As general insurance:a multivitamin with 50–70 mcg of selenium.
• For medical indications:the form and dose are determined by a doctor.

On the label look for the amount of selenium itself in micrograms, not the mass of the compound. Pay attention to the presence of independent testing or quality certificates. Avoid liquid forms dosed «by eye» and products where selenium is present in several sources at once without the total amount being stated.

If you already take several supplements, first calculate how much selenium you are getting. It often turns out that the need is already met and a new jar is not needed at all.

Editorial conclusions

Selenomethionine and selenium yeast are well absorbed and build a store in tissues; selenite and selenate accumulate less and are well studied in clinical trials. No form is universally «the best».

The most important criteria for choosing are a moderate dose, a transparent composition and quality control. Brazil nuts and non-standardized «chelates» are not suitable for precise dosing.

We also recommend reading our materials «How to take selenium: dosage, timing, duration», «Side effects of selenium» and «Iodine: forms available and which to choose».

References

1. Rayman MP, Infante HG, Sargent M. Food-chain selenium and human health: spotlight on speciation. Br J Nutr. 2008;100(2):238–253.
2. Schrauzer GN. Selenomethionine: a review of its nutritional significance, metabolism and toxicity. J Nutr. 2000;130(7):1653–1656.
3. Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256–1268.
4. Marcocci C, Kahaly GJ, Krassas GE, et al. Selenium and the course of mild Graves' orbitopathy. N Engl J Med. 2011;364(20):1920–1931.
5. MacFarquhar JK, Broussard DL, Melstrom P, et al. Acute selenium toxicity associated with a dietary supplement. Arch Intern Med. 2010;170(3):256–261.
6. Lippman SM, Klein EA, Goodman PJ, et al. Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2009;301(1):39–51.
7. EFSA Panel on Nutrition, Novel Foods and Food Allergens. Scientific opinion on the tolerable upper intake level for selenium. EFSA J. 2023;21(1):7704.