L-Cysteine is conditionally essential. This means that the body generally produces enough on its own; however, during exercise, much of it can be depleted. This may require supplementation.
Cysteine helps to keep protein structures intact and also helps in the formation of collagen indirectly, which helps maintain hair, nail and skin health. It also goes on to promote the production of Glutathione and L-Taurine. Glutathione is an antioxidant that helps the nervous system and protects healthy cells from damage by free radicals. L-Cysteine also heightens the amount of N-Acetyl Cysteine in the body, which in turn may help the body to maintain muscle mass and Glutathione levels more efficiently. It is also an essential part of the metabolic process for a variety of different nutrients and vitamins. Vitamin B6 would be useless without Cysteine.