Last verified: 20 days ago
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.
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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...
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Evidence quality
Evidence base hasn't been formally rated yet. See research below.
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Clinical dose
No established dose
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 EvidenceEffective at: 4–16 µM (in vitro only)
Supporting studies (click to view on PubMed):
Scavenges free radicals and reduces oxidative stress markers in lab and animal models.
Weak EvidenceEffective at: No established dose
Supporting studies (click to view on PubMed):
Reduces inflammatory markers like IL-4, IL-5, and IL-13 in allergic rhinitis animal models.
Weak EvidenceEffective at: No established dose
Supporting studies (click to view on PubMed):
Restored biothiol levels depleted by mercury toxicity in cell and zebrafish lab models.
Weak EvidenceEffective at: No established dose
Supporting studies (click to view on PubMed):
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
Frequently Asked Questions
What does L-Selenocysteine do?
Selenium-containing amino acid with antioxidant and anti-cancer activity in lab studies. No human trials yet.
What is the effective dose of L-Selenocysteine?
No established dose
Is L-Selenocysteine safe?
Zero human clinical trials — all evidence is from cell cultures, animal models, or lab experiments
What doesn't L-Selenocysteine do?
Not proven to treat or prevent cancer in humans.
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