L-cysteine is a protein amino acid naturally present in the proteins of life forms. L-cysteine is a sulfur amino acid and contains a sulfhydryl group. Although most cysteine is found in proteins, small amounts of free cysteine are found in body fluids and in plants. The normal diet contributes approximately 1 gram of L-cysteine daily.

L-cysteine is considered a nonessential amino acid, meaning that, under normal physiologic conditions, sufficient amounts of this amino acid are formed from the dietary essential amino acid L-methionine and the nonessential amino acid L-serine via a transsulfuration reaction. L-cysteine is a conditionally essential amino acid under certain circumstances, for example, for preterm infants.

L-Cystein is used in Bagels, Croissants, Hard Rolls, Cake Donuts (from human hair in Dunkin Donut's cake donuts only, Yeast raised donuts do not contain L-Cysteine), Pita Bread, some Crackers and Melba Toast. It is also used as a nutrient in baby milk formula and dietary supplements.

Some reports suggest that European Union countries are thinking to ban the use of L-Cystiene from human hair in food products. There is no pressure from consumers n USA and Canada to ban the L-Cysteine from human hair. It is a haram product for Muslims because it is a part of human body. Islam does not allow to consume any part of human body. Some Rabbis accept it as Kosher ingredient saying it is dead part of the body but some do not.