NAC Powder, WatermelonCysteine is a semi essential amino acid. Cysteine is found in most high protein foods, such as chicken, turkey, yogurt, cheese, eggs, sunflower seeds and legumes. N acetyl L Cysteine (NAC) is a supplement form of cysteine. NAC is an immediate precursor to a very precious substance that our body makes called glutathione. These amino acids also help with chronic respiratory conditions, fertility and brain health.