L-cysteine is not naturally present in food ingredients; the only natural form is L-cystine.
Cystine is first absorbed by intestine and circulates in blood; Cells import cystine from the blood, reconvert it to cysteine (likely using ascorbate as cofactor).
1- L-cysteine prevents the production of black eumelanin
L-cysteine is in favour of pheomelanin production, a lighter pigment compared to eumelanin. Since it has very high skin-whitening abilities, it is used in various anti-spot medicines and nutrition tablets.
2- L-cysteine stimulates the production of glutathione
Glutathione is an antioxidant that exists in the body and several studies have observed a skin whitening-effect following the administration of glutathione. Therefore, not taking enough L-cysteine can result in various skin troubles.
3- L-cysteine activates skin’s turnover
Skin’s turnover is directly related to wrinkles and skin darkening.
The amount of L-cysteine needed for it to help out with skin troubles is about 200 mg a day or 100 mg of L-cystine. Since it is very hard to get this amount from foods, L-cysteine or L-cystine tablets are recommended.
4- L-cysteine has an antioxidant effect
It stimulates the activity of glutathione peroxidase enzymes, which protect the skin from reactive oxygen species (ROS). It also represses the activity of tyrosinase enzymes, which come from ROS and are involved in the production of melanin.