L-cysteine is a Sulfur-containing semi-essential amino acid which can be synthesized from methionine and serine AAs but if these are low, you need to acquire cysteine directly from diet. Abundant in protein-rich foods and together with the amino acids glutamine and glycine creates the useful antioxidant glutathione.
Making antioxidant Glutathione is the biggest role of L-cysteine since it does a bunch of stuff inside our body like supporting immune function, making DNA, regenerating vitamins C and E, aiding people who are suffering from heart disease and infertility or other ailments caused by oxidative stress, transporting dangerous chemical mercury out of the brain, helping reduce the effects of aging on the skin, helping the liver and gallbladder deal with lipids and so on.
Cysteine may play a role in the normal growth speed of hair. It may help healing after surgery or burns and protect the skin from radiation injury.
In an experiment involving 10 male basketball players, the authors of a 2007 study published in Clinical Chemistry and Laboratory Medicine determined that one week of supplementation with L-cysteine helped boost antioxidant capacity and reduce free radical production.
Although it is believed that cysteine may help with controlling diabetes, Cysteine may interfere with how insulin works so if you are suffering from diabetes or any other type of blood glucose related ailments, you must talk to a doctor first before supplementing with cysteine.
A 2012 literature review published in the Canadian Journal of Physiology and Pharmacology found that cysteine-rich whey protein improves glucose metabolism in people with type 2 diabetes. However, the study authors note that more research is needed. It also appeared to inhibit blood vessel inflammation, a key contributor to heart disease among those with diabetes.
N-acetyl cysteine which is a form of amino acid cysteine has been shown to be beneficial for lung and trachea related ailments(especially Chronic bronchitis) but this has not been fully researched and proven yet.
flu and some inflammatory diseases have also been helped with cysteine to some degree which maybe because of the antioxidant property of it but scientist still cannot say for sure.
research on Cysteine is still ongoing and it’s potential has not been fully discovered yet.