L-Cysteine
Commonly used for
Athletic Performance
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What is it? / Overview
Cysteine is the rate-limiting amino acid for glutathione synthesis, the body's main intracellular antioxidant; adequate cysteine supports redox balance and, in one small trial, reduced training-induced DNA oxidation at 0.5 g/day. Very high antioxidant dosing has a theoretical (not well-established) risk of blunting some training adaptations.
How is it used? / Administration
Form: L-cysteine -- N-acetylcysteine (NAC, the existing database entry) is the more bioavailable and better-studied form
Typical dose studied: ~0.5 g/day used in the exercise-oxidative-stress study cited below
Timing: Daily with food
Distinct from the existing NAC (N-Acetylcysteine) entry (see that separate entry) -- NAC is the more bioavailable, better-studied form and is the more reasonable first choice for most people; this entry exists mainly for users who specifically report taking plain L-cysteine.
How well does it work? / Evidence
? Emerging / limited evidence
Who should avoid it?
Already taking it? Reasonable, low-risk to continue -- consider whether NAC (the existing, better-studied database entry) would serve the same purpose, since the evidence base for plain L-cysteine specifically is thinner.
Research & references
Margaritelis NV et al. (2021). Dietary Cysteine Intake Is Associated With Blood Glutathione Levels and Isometric Strength. International Journal of Sports Medicine. Tsakiris S et al. (2006). The Beneficial Effect of L-Cysteine Supplementation on DNA Oxidation Induced by Forced Training. Pharmacological Research.
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