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What is cysteine?

Cysteine is a conditionally essential amino acid, which can be produced in your body from another amino acid methionine, but infants may need to obtain additional amounts from foods to be healthy [1]. In foods, cysteine is incorporated into proteins.

Cysteine abbreviation (symbol): Cys

Cysteine Functions in the Human Body

Cysteine is [2]:

  • A building block of proteins, especially of collagen in the skin and beta-keratin in hair and nails
  • A glucogenic amino acid, which can be converted into glucose in your body [3]
  • A source of sulfur
  • A precursor of the amino acid cystine and the antioxidant glutathione

Cysteine Rich Foods

  • ANIMAL FOODS: chicken, turkey, fish (bluefish, yellowtail, tuna, salmon), pork (ham) beef, veal, lamb, bison, crabs, mollusks, cheese [4]
  • PLANT FOODS: nuts (butternuts, peanuts), seeds (pumpkin, sunflower), legumes (beans, soybeans, lentils) [4]

NOTE: Cysteine contains sulfur, so diet high in cysteine can cause sulfur smelling gas.

Cysteine as a Food Additive

L-cysteine obtained from animal foods, may be added to certain foods, like leavened breads, as a stabilizator. In the European Union it is labeled as the E-number E920.

Cysteine Supplements

Available forms of cysteine supplements:

  • L-cysteine powder without prescription. L-cysteine may be obtained from human hair, duck feathers or other animal or plant proteins, produced using microbes or can be artificially synthesized.
  • L-cysteine/N-acetyl cysteine combination
  • L-cysteine hydrochloride as a prescribed solution for intravenous infusions in infants

Cysteine Health Benefits

There is INSUFFICIENT EVIDENCE about the effectiveness of L-cysteine supplements in the prevention or treatment of homocystinuria [5].

Cysteine Safety: Side Effects, Toxicity

L-cysteine hydrochloride injections can cause local reactions, such as a warm sensation, skin redness, vein inflammation and blood clotting [7].

  1. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients) ( 2005 ) /10 Protein and Amino Acids  National Academic Press
  2. L-cysteine  PubChem
  3. Amino acid  ChemPep
  4. List of food high and low in cysteine  US Department of Agriculture
  5. van Vliet D et al, 2014, Single amino acid supplementation in aminoacidopathies: a systematic review  PubMed Central
  6. L-CYSTEINE HYDROCHLORIDE – cysteine hydrochloride injection, solution  DailyMed
  7. L-cysteine hydrochloride injection  Drugs.com

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