- 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
- L-cysteine PubChem
- Amino acid ChemPep
- List of food high and low in cysteine US Department of Agriculture
- van Vliet D et al, 2014, Single amino acid supplementation in aminoacidopathies: a systematic review PubMed Central
- L-CYSTEINE HYDROCHLORIDE – cysteine hydrochloride injection, solution DailyMed
- L-cysteine hydrochloride injection Drugs.com
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 . In foods, cysteine is incorporated into proteins.
Cysteine abbreviation (symbol): Cys
Cysteine Functions in the Human Body
Cysteine is :
- 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 
- 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 
- PLANT FOODS: nuts (butternuts, peanuts), seeds (pumpkin, sunflower), legumes (beans, soybeans, lentils) 
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.
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 .
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 .