What can high-cysteine foods do for you?

  • Help your body detoxify chemicals and heavy metals
  • Protect cells from free radical damage
  • Help breakdown extra mucous in your lungs

What events can indicate a need for more high-cysteine foods?

  • Frequent colds

Food sources of cysteine include poultry, yogurt, egg yolks, red peppers, garlic, onions, broccoli, Brussel sprouts, oats, and wheat germ.



What is cysteine?

Cysteine is a sulfur-containing amino acid that occurs naturally in foods and can also be manufactured by the body from the amino acid methionine. In the production of cysteine, methionine is converted to S-adenosyl methionine (SAM), which is then converted to homocysteine. Homocysteine then reacts with serine to form cysteine. Cysteine is also found in the body and in food as cystine, an amino acid that contains two cysteines joined together.

How it Functions

What is the function of cysteine?

Promoting Antioxidant Activity

As a key constituent of glutathione, cysteine has many important physiological functions. Glutathione, formed from cysteine, glutamic acid, and glycine, is found in all human tissues, with the highest concentrations found in the liver and eyes. Glutathione is a potent antioxidant, protecting fatty tissues from the damaging effects of free radicals. The antioxidant activity of glutathione is attributed specifically to the presence of cysteine in the compound.


Glutathione also plays a vital role in the detoxification of harmful substances by the liver and can chelate (attach to) heavy metals such as lead, mercury, and cadmium. It is also believed that glutathione carries nutrients to lymphocytes and phagocytes, important immune system cells.

Help Eliminate Mucous

Cysteine also has the ability to breakdown proteins found in mucous that settles in the lungs. As a result, this amino acid may be useful in the treatment of bronchitis and other respiratory problems.

Deficiency Symptoms

What are deficiency symptoms for cysteine?

Cysteine deficiency is relatively uncommon, but may be seen in vegetarians with low intake of the plant foods containing methionine and cysteine. There is no known medical condition directly caused by cysteine deficiency, but low cysteine levels may reduce one’s ability to prevent free radical damage and may result in impaired function of the immune system.

Toxicity Symptoms

What are toxicity symptoms for cysteine?

Consumption of foods containing cysteine, or its precursor methionine, is not likely to cause toxicity symptoms. However, cysteine is a brain excitoxin that can cause damage to brain cells in susceptible individuals. Such individuals do not metabolize the amino acid correctly, and, as a result, may be at risk for certain neurodegenerative diseases, including multiple sclerosis, amylotrophic lateral sclerosis (Lou Gehrig’s disease) and Alzheimer’s disease.

High doses of oral N-acetyl-cysteine, such as those given to patients with acetaminophen (Tylenol) toxicity, can cause nausea, vomiting, and diarrhea. Intravenous administration of N-acetyl cysteine can cause allergic reactions in a small percentage of people, characterized by skin flushing, a drop in blood pressure, irregular heart beat, and respiratory distress. Accidental overdose of intravenous N-acetyl-cysteine has proven to be fatal.

Impact of Cooking, Storage and Processing

How do cooking, storage, or processing affect cysteine?

There is no research available about the impact of cooking, storage or processing on cysteine.

Factors that Affect Function

What factors might contribute to a deficiency of cysteine?

The production of cysteine involves several nutrients. As a result, dietary deficiency of methionine, vitamin B6, vitamin B12, s-adenosyl methionine (SAMe) and folic acid may decrease the production of cysteine.

Drug-Nutrient Interactions

What medications affect cysteine?

  • Supplemental cysteine, given intravenously as N-acetyl cysteine, may prevent the development of tolerance to nitroglycerin, a drug used in the treatment of chest pain. Unfortunately, the combination of nitroglycerin and N-acetyl-cysteine can cause severe headaches.
  • Oral and intravenous N-acetyl cysteine is used in the treatment of acetaminophen (Tylenol) poisoning. N-acetyl cysteine helps to metabolize acetaminophen quickly, so that liver damage is prevented.
  • Supplemental cysteine, given as N-acetyl cysteine, may reduce the nausea and vomiting caused by chemotherapy.
  • N-acetyl cysteine may increase the effectiveness of corticosteroids, a class of drugs with anti-inflammatory activity.
  • Researchers are investigating the role of N-acetyl cysteine in preventing heart damage caused by certain chemotherapy drugs and in enhancing the effectiveness of interferon in treating hepatitis C.

Nutrient Interactions

How do other nutrients interact with cysteine?

There is no research available about how other nutrients interact with cysteine.

Health Conditions

What health conditions require special emphasis on cysteine?

Cysteine may play a role in the prevention and/or treatment of the following medical conditions:
  • Acute respiratory distress syndrome
  • Asthma
  • Cancer
  • Cataracts
  • Hair loss
  • Heart disease
  • Heavy metal toxicity or exposure
  • Liver disease
  • Parkinson’s disease
  • Psoriasis
  • Rheumatoid arthritis
  • Viral infections

Form in Dietary Supplements

What forms of cysteine are found in dietary supplements?

Many people take supplemental cysteine as a way to increase the level of glutathione in the body. As a dietary supplement, cysteine is available as L-cysteine hydrochloride, and more commonly as n-acetyl-cysteine (NAC). NAC is a more water-soluble form of cysteine, and as such is believed to be more bioavailable than oral L-cysteine hydrochloride.

Food Sources

Introduction to Nutrient Rating System Chart

The following chart shows the foods which are either excellent, very good or good sources of this nutrient. Next to each food name you will find the following information: the serving size of the food; the number of calories in one serving; DV% (percent daily value) of the nutrient contained in one serving (similar to other information presented in the website, this DV is calculated for 25-50 year old healthy woman); the nutrient density rating; and the food's World's Healthiest Foods Rating. Underneath the chart is a table that summarizes how the ratings were devised. For more detailed information on our Nutrient Rating System, please click here.


Food Source Analysis not Available for this Nutrient

Public Health Recommendations

What are current public health recommendations for cysteine?

A Recommended Dietary Allowance has not been established for cysteine.


  • Breuille D, Obled C. Cysteine and glutathione in catabolic states. Nestle Nutr Workshop Ser Clin Perform Programme 2000;3:173-91; discussion 191-7.
  • Droge W. Cysteine and glutathione in catabolic conditions and immunological dysfunction. Curr Opin Clin Nutr Metab Care 1999 May;2(3):227-33.
  • Droge W, Hack V, Breitkreutz R, et al. Role of cysteine and glutathione in signal transduction, immunopathology and cachexia. Biofactors 1998;8(1-2):97-102.
  • Groff JL, Gropper SS, Hunt SM. Advanced Nutrition and Human Metabolism. West Publishing Company, New York, 1995.
  • Lininger SW, et al. A-Z guide to drug-herb-vitamin interactions. Prima Health, Rocklin, CA, 2000.
  • Mahan K, Escott-Stump S. Krause's Food, Nutrition, and Diet Therapy. WB Saunders Company; Philadelphia, 1996.
  • Puerto M, Guayerbas N, Victor V. Effects of N-acetylcysteine on macrophage and lymphocyte functions in a mouse model of premature ageing. Pharmacol Biochem Behav 2002 Nov;73(4):797-804.
  • Quig D. Cysteine metabolism and metal toxicity. Altern Med Rev 1998 Aug;3(4):262-70.

This page was updated on: 2003-12-13 00:27:29
© 2002 The George Mateljan Foundation