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L-Selenocysteine

Also known as: Sec, selenocysteine, L-CySe, 21st amino acid

Evidence under review. — Not yet rated

Selenium-containing amino acid with antioxidant and anti-cancer activity in lab studies. No human trials yet.

  • What it does

    L-Selenocysteine is a naturally occurring amino acid that contains selenium — an essential trace mineral. In laboratory and animal studies, it has shown antioxidant, anti-inflammatory, and...

  • Evidence quality

    Evidence base hasn't been formally rated yet. See research below.

  • Clinical dose

    No established dose

See full breakdown ↓ Methodology →

What the Science Says

L-Selenocysteine is a naturally occurring amino acid that contains selenium — an essential trace mineral. In laboratory and animal studies, it has shown antioxidant, anti-inflammatory, and antibacterial properties, and has been shown to trigger cancer cell death in liver cancer cell lines via a reactive oxygen species pathway. It has also been explored as a building block for selenium-delivery materials and as a potential detoxifying agent against heavy metal toxicity like mercury. No human clinical trials have been conducted, so its real-world benefits in people are unknown.

What It Doesn't Do

Not proven to treat or prevent cancer in humans. No evidence it boosts immunity or detoxifies heavy metals in real people. Not a proven antioxidant supplement in humans — all antioxidant data comes from test tubes or mice. Don't confuse it with selenomethionine, which has more human research.

Evidence-Based Benefits

Triggers programmed death in liver cancer cells in laboratory studies via ROS signaling.

Weak Evidence

Effective at: 4–16 µM (in vitro only)

Scavenges free radicals and reduces oxidative stress markers in lab and animal models.

Weak Evidence

Effective at: No established dose

Reduces inflammatory markers like IL-4, IL-5, and IL-13 in allergic rhinitis animal models.

Weak Evidence

Effective at: No established dose

Restored biothiol levels depleted by mercury toxicity in cell and zebrafish lab models.

Weak Evidence

Effective at: No established dose

Absorption & Bioavailability

Unknown — no human pharmacokinetic studies in the provided data. Animal data suggests organic selenium forms bioaccumulate more than inorganic forms, but L-selenocysteine specifically has not been studied for oral bioavailability in humans.

Red Flags to Watch For

  • Zero human clinical trials — all evidence is from cell cultures, animal models, or lab experiments
  • Selenium has a narrow safety window; excess selenium causes selenosis (hair loss, nerve damage, GI issues)
  • Organic selenium forms like selenocysteine may bioaccumulate more than inorganic forms, raising toxicity risk at higher doses
  • 177 registered supplement products contain this ingredient despite no clinical trial evidence supporting human use
  • Often confused with better-studied selenium compounds like selenomethionine or Se-methylselenocysteine — these are different molecules

Research Sources

  • PubMed
  • NIH DSLD

This information is for educational purposes only and is not medical advice. Always consult a healthcare professional before starting any supplement regimen. Last updated: 2026-05-25