N Acetyl L Cysteine, commonly abbreviated to NAC, is used to form the powerful antioxidant Glutathione. For those training hard & consequently causing oxidative stress, raising Glutathione levels helps to reduce this damage. NAC is also a cell detoxifier, which helps to support liver function. It can only be obtained via the diet or supplementation & is a more stable form, enabling better absorption, than regular L-Cysteine.