L-Cysteine

L-Cysteine is a semi-essential (conditionally essential) amino acid that the body can synthesize from methionine under normal conditions. It's the rate-limiting precursor for glutathione (the body's master antioxidant), and also forms taurine, sulfate, and contributes to coenzyme A synthesis. Used for hair/nail health (keratin contains cysteine). More commonly supplemented as N-acetyl cysteine (NAC), which has better bioavailability and is better researched. Safe at normal doses but very high doses may have pro-oxidant effects. NO specific GRADED OUTCOMES for isolated L-cysteine; NAC has more research.

Quick Answer

What it is

Key findings

Grade C: Antioxidant Status
Grade C: Erythrocyte Membrane Integrity
Grade D: Alcohol-Induced Oxidative Damage

Safety

No specific caution or interaction language was detected in the current summary/outcome notes.

ℹ️ Quick Facts: L-Cysteine

Quick Facts: L-Cysteine

Best Evidence:Grade C
Conditions Studied:3
Research Outcomes:5
Key Effect:Antioxidant Status

Outcomes by grade:

3 conditions · 5 outcomes

Detailed Outcomes

Antioxidant Status

Multiple human studies found that L-cysteine supplementation protected against exercise-induced oxidative stress, preventing DNA oxidation, preserving erythrocyte G6PD activity, and maintaining erythrocyte membrane ATPase activities during forced training.

60 participants

moderate↑Improves

Erythrocyte Membrane Integrity

Human studies showed L-cysteine supplementation preserved erythrocyte membrane acetylcholinesterase and Na+,K+-ATPase activities and protected G6PD activity from reduction during exercise-induced oxidative stress.

40 participants

small↑Improves

Alcohol-Induced Oxidative Damage

In rodent models, L-cysteine (often combined with ascorbate and L-methionine) reduced alcohol-induced oxidative stress in the brain, stomach, testes, ovary, kidney, and lung of chronically alcohol-treated rats.

moderate↓Improves

Skin Pigmentation

A single study found that simultaneous administration of vitamin C, L-cysteine, and vitamin E inhibited melanogenesis, suggesting a potential skin-lightening effect, though the contribution of L-cysteine alone is difficult to isolate from the combination.

small↓Worsens

Hair and Nail Health

An animal study in lambs showed that methionine/cysteine infusion influenced follicle protein synthesis rates and mRNA expression in skin, consistent with L-cysteine's known role as a structural component of keratin in hair and nails.

small↑Improves

Evidence by Condition

Best grade per condition (each condition may have multiple outcomes)

CAntioxidant Status

1 outcome · 3 studies

↑Antioxidant StatusImproves

CBlood Health

1 outcome · 2 studies

↑Erythrocyte Membrane IntegrityImproves

DAlcohol Use Disorder

1 outcome · 3 studies

↓Alcohol-Induced Oxidative DamageImproves

Research Citations (8)

Protective effects of cysteine, methionine and vitamin C on the stomach in chronically alcohol treated rats.

Journal of Applied Toxicology (2008)

PMID: 17966116

L-cysteine supplementation prevents exercise-induced alterations in human erythrocyte membrane acetylcholinesterase and Na+,K+-ATPase activities.

Clinical Chemistry and Laboratory Medicine (2007)

PMID: 17243918

The beneficial effect of L-cysteine supplementation on DNA oxidation induced by forced training.

Pharmacological Research (2006)

PMID: 16517176

L-cysteine supplementation protects the erythrocyte glucose-6-phosphate dehydrogenase activity from reduction induced by forced training.

Clinical Biochemistry (2006)

PMID: 16959232

Does ascorbate/L-Cys/L-Met mixture protect different parts of the rat brain against chronic alcohol toxicity?

Advances in Therapy (2006)

PMID: 17142205

Alcohol-induced oxidative stress and reduction in oxidation by ascorbate/L-cys/L-met in the testis, ovary, kidney, and lung of rat.

Advances in Therapy (2005)

PMID: 16510372

Effect of simultaneous administration of vitamin C, L-cysteine and vitamin E on the melanogenesis.

BioFactors (2004)

PMID: 15630239

Transsulfuration, protein synthesis rate and follicle mRNA in the skin of young Merino lambs in response to infusions of methionine and serine.

British Journal of Nutrition (2000)

PMID: 10858698

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