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What is selenocysteine?

Selenocysteine is a nonessential amino acid [1]. It can be produced in your body from another amino acid cysteine by replacing the sulfur by selenium [1]. In foods, selenocysteine is incorporated into proteins.

Selenocysteine abbreviation (symbol): Sec

Selenocysteine Functions in the Human Body

Selenocysteine is:

  • A building block of selenium-containing proteins called selenoproteins, which act mainly as enzymes, for example, in the conversion of the thyroid hormone T4 (thyroxine) into its active form T3 (triiodothyronine) [2,3]
  • A glucogenic amino acid — it can be converted into glucose [6]

Foods High in Selenocysteine

  • ANIMAL FOODS: meat, fish, cheese [3,4]
  • PLANT FOODS: cereals (corn, wheat, oats, rice), Brazil nut, walnuts [3,4]

Foods low in selenocysteine: fruits, vegetables, legumes

Selenocysteine Supplements

Available nonprescription (over-the-counter) oral forms of selenocysteine supplements:

  • L-selenocysteine
  • Se-methyl selenocysteine

Selenocysteine Health Benefits

Selenocysteine supplements are one of the sources of selenium, which is an essential mineral and antioxidant.

Heavy metals, such as mercury (Hg), bind to selenocysteine in the gut and inhibit the absorption of both selenocysteine and mercury, but there is INSUFFICIENT EVIDENCE about the effectiveness of selenocysteine supplements in removing heavy metals from the body, boosting immunity or treating cancer [5,7].

Selenocysteine Safety: Side Effects, Toxicity

Side effects. Selenocysteine supplements in high doses may cause nausea, vomiting, diarrhea, garlic breath odor, fatigue, itchiness, tingling and hair and nail brittleness [5].

  1. Turanov AA et al, 2011, Biosynthesis of Selenocysteine, the 21st Amino Acid in the Genetic Code, and a Novel Pathway for Cysteine Biosynthesis  Advances in Nutrition
  2. Selenocysteine: the 21st amino acid  University of Pittsburgh
  3. Selenium  National Institutes of Health, Office of Dietary Supplements
  4. Selenium  Linus Pauling Institute
  5. Gropper SS et al, 2009, Advanced Nutrition and Human Metabolism, Fifth Edition, p. 507-509
  6. Amino acid  ChemPep
  7. Hatfield DL et al, 2014, Selenium and selenocysteine: roles in cancer, health and development  PubMed Central

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