Clayton's Health Facts: L-Cysteine.

Clayton South, SPN (ISSA), is a recognized expert in the bodybuilding / fitness industry with over 150 bodybuilding, fitness and nutrition publications to his credit.

What is it and where does it come from?

L-cysteine is a conditionally sensitive, sulfur bearing, amino acid present in protein.

L-cysteine is a conditionally essential because it can be made endogenously in sufficient quantities under normal circumstances, but it may have to be exogenously supplemented if physical demands on the body (from physical exercise, stress or sickness) become too great.

What does it do and what scientific studies give evidence to support this?

L-cysteine performs a number of functions in the body.

It is used to manufacture L-taurine and L-glutathione. Taurine is important because it plays a key role in central nervous system function. Taurine also helps to regulate blood pressure, helps maintain good vision and eye function, and aides in thermogenesis (fat burning) and muscle building.

Glutathione is critical for immune system function because it is a powerful antioxidant that ensures healthy cellular function and nerve signal efficiency. Ageing and physical decline are linked to glutathione deficiency. While direct glutathione supplementation has proven ineffective at increasing cellular glutathione levels, L-cysteine levels has proven effective for this purpose.

L-cysteine supplementation elevates NAC - N-acetyl cysteine (C3H7NO2S). NAC is known to increase blood glutathione levels and preserve lean muscle mass. Therefore, the antioxidative and glutathione elevating effects of L-cysteine are attributed to its ability to raise NAC levels.

L-cysteine is essential for T-cell production and immune system activation. L-cysteine is also a component of human hair (research has shown it can increase hair health and hair diameter) and is a component of the hormone insulin. L-cysteine can also be converted into glucose (for energy production) when blood glucose levels are low and this conversion may enhance athletic endurance and prevent muscle catabolism.

The gastrointestinal system may also benefit from L-cysteine. Aspirin and other drugs may stress the digestive system and stomach, and L-cysteine may protect against these side-effects by strengthening stomach lining.1

L-cysteine supplementation may be most effective when used in conjunction with vitamin E, vitamin C, vitamin B6 and calcium and selenium.

Who needs it and what are some symptoms of deficiency?

Everyone may benefit from L-cysteine supplementation.

Physical training is known to increase oxidative stress, and this may lead to immune suppression. L-cysteine can help speed recovery, and support health. Its role as a taurine precursor can also assist in lean muscle tissue gain and thermogenesis.

Persons with compromised immune function (the young, the elderly and people exposed to pollutants) can benefit from L-cysteine supplementation due to its immuno-protective and antioxidative effects. L-cysteine can enhance immune system function and maintain optimal health.

Those with HIV - Human Immunodeficiency Virus - or AIDS - Acquired Immunodeficiency Syndrome - frequently suffer from L-cysteine deficiency and may thus benefit from supplementation.2

How much should be taken? Are there any side effects?

Strictly adhere to label recommendations.

Optimal doses of L-cysteine supplementation may produce beneficial effect, but high dosages of L-cysteine are linked to oxidative stress and possible DNA damage.3

Be sure to drink enough water when supplementing with L-cysteine.

Consult with a physician prior to use. Persons with cystinuria should avoid using l-cysteine as they can form cysteine gallstones from its use.


  1. Salim AS. Sulfhydryl-containing agents in the treatment of gastric bleeding induced by nonsteroidal anti-inflammatory drugs. Can J Surg 1993;36:53-8.
  2. Eck HP, Gander H, Hartmann M, et al. Low concentrations of acid-soluble thiol (cysteine) in the blood plasma of HIV-1 infected patients. Biol Chem Hoppe Seyler 1989;370:101-8.
  3. Kleinveld HA, Demacker PNM, Stalenhoef AFH. Failure of N-acetylcysteine to reduce low-density lipoprotein oxidizability in healthy subjects. Eur J Clin Pharmacol 1992;639-42.