(Source: SaluGenecists, Inc.)

A sulfur-containing amino acid that occurs naturally in food, cysteine can also be manufactured in the body from the amino acid methionine. This endogenous synthesis of cysteine begins with methionine being converted to S-adenosyl methionine (SAM), which is then converted to homocysteine, which then reacts with serine to form cysteine. Cysteine can also be found in the body as the amino acid cystine, which is composed of two cysteine molecules joined together.


Physiological functions of cysteine

  • Assists in the detoxification of chemicals and heavy metals
  • Protects cells from free radical damage
  • Aids in the breakdown of extra mucous in the lungs

Physiological events that may signal a need for greater cysteine intake

  • Frequent colds


Functions of cysteine

Promotes antioxidant activity

Along with glutamic acid and glycine, cysteine is a key constituent of glutathione, a potent antioxidant that is found in all human tissues with the highest concentrations being found in the liver and eyes. Glutathiones ability to protect fatty tissues from the damaging effects of free radicals is attributed specifically the to presence of cysteine in this compound.

Detoxification and immune support

Glutathione plays an important role in the livers detoxification of harmful substances. It has been found to be able to chelate (attach to) heavy metals such as lead, mercury and cadmium, thus preparing them for excretion from the body. Glutatione may also support the immune system through its suggested role in carrying nutrients to lymphocytes and phagocytes.

Helps eliminate mucous

Cysteine may be a useful therapy for bronchitis and other problems of the respiratory tract since it has been shown to be able to breakdown proteins found in mucous that settles in the lungs.


Deficiency Factors

Causes and symptoms of cysteine deficiency

Dietary deficiency of vitamin B6, vitamin B12, folic acid, methionine and s-adenosyl methionine (SAMe) can lead to a deficiency of cysteine since these nutrients are involved in its production in the body.

While there is no known medical condition directly caused by a deficiency of cysteine, inadequate levels of this amino acid may impair immune system function and diminish ones ability to prevent free radical damage. Cysteine deficiency is not very common, although vegetarians with a low intake of plant foods rich in cysteine and methionine may be at risk for deficiency.

Toxicity Factors

Causes and symptoms of cysteine toxicity

Toxicity symptoms are not likely to occur with consumption of foods containing cysteine, or its precursor methionine, except in select individuals who are unable to metabolize the amino acid correctly. Since cysteine is a brain excitoxin, susceptible individuals can experience brain cell damage that can put them at risk for certain neurodegenerative diseases including multiple sclerosis, Alzheimers disease and amylotrophic lateral sclerosis (Lou Gehrigs disease).

High doses of supplemental oral N-acetyl cysteine, given to patients with acetaminophen (TylenolTM) toxicity, may cause vomiting, nausea and diarrhea. A small percentage of individuals may have allergic reactions to intravenous administration of N-acetyl cysteine with symptoms manifesting as skin flushing, irregular heartbeat, respiratory distress and a drop in blood pressure. Accidental overdose of intravenous N-acetyl cysteine can be fatal.

Cooking, Storage and Processing

Effects of cooking, storage and processing on cysteine

Currently, no research is available in the scientific literature regarding how cooking, storage or processing affects cysteine levels in foods.

Drug & Nutrient Interactions

Interactions between medications and cysteine

Cysteine may beneficially affect treatment with the following medications:

  • Nitroglycerin Intravenous
    • N-acetyl cysteine may prevent the development of tolerance to nitroglycerin, which is used in the treatment of chest pain, although the combination of these two compounds can cause severe headaches.
  • Acetaminophen (Tylenol)
    • Since n-acetyl cysteine helps to quickly metabolize acetaminophen, protecting against the subsequent development of liver damage, oral and intravenous N-acetyl cysteine is used in the treatment of acetaminophen (Tylenol) poisoning.
  • Chemotherapy
    • N-acetyl cysteine may reduce associated nausea and vomiting.
    • Researchers are investigating the potential of n-acetyl cysteine to prevent heart damage caused by certain chemotherapy drugs.
  • Corticosteroids
    • N-acetyl cysteine may increase the effectiveness of this class of anti-inflammatory drugs.
  • Interferon
    • Researchers are investigating the potential of N-acetyl cysteine to enhance this drugs effectiveness in treating hepatitis C.


Nutrient Interactions

Interactions that occur between cysteine and other nutrients

Currently, no research is available in the scientific literature regarding how other nutrients interact with cysteine.

Health Conditions

Health conditions that require special emphasis on cysteine

Individuals who have the following health conditions should pay special attention to their cysteine status:

  • Acute respiratory distress syndrome
  • Asthma
  • Cancer
  • Cataracts
  • Hair loss
  • Heart disease
  • Heavy metal toxicity or exposure
  • Liver disease
  • Parkinsons disease
  • Psoriasis
  • Rheumatoid arthritis
  • Viral infections

Forms in Dietary Supplements

Forms in which cysteine is found in dietary supplements.

As a dietary supplement, cysteine is most commonly available as n-acetyl cysteine (NAC), although some supplements feature it in the form of L-cysteine hydrochloride. NAC is believed to be have greater bioavailability than L-cysteine hydrochloride, potentially due to its greater water solubility. Supplemental cysteine is used by many people to increase their glutathione levels.

Food Sources

Foods that are concentrated sources of cysteine

Cysteine can be found in an array of foods including broccoli, Brussels sprouts, egg yolks, garlic, oats, onions, poultry, red bell peppers, wheat germ and yeast.