Cysteine is a conditional or semi-essential amino acid. You can find it in high-protein foods such as beef, chicken, eggs, and whole grains.
What makes cysteine semi-essential is that your body can produce it from two other amino acids: serine and methionine. If your body is low on these two amino acids, then cysteine supplementation may become essential. 
N-acetyl cysteine (NAC) is a supplement form of cysteine. Your body doesn’t make it and it’s not found in foods, but it still plays an important role. Like cysteine, NAC bonds with glutamine and glycine to form glutathione, a powerful antioxidant. Glutathione performs many functions throughout your body, such as helping to maintain your immune system.
NAC supplements can help to fill the gaps in your natural cysteine levels, ensuring that you have what you need. 
One of NAC’s most important jobs is to help your body replenish its glutathione stores. As one of your body’s most potent antioxidants, glutathione helps to get rid of cell-damaging free radicals.
Reducing oxidative stress can help to lower your risk of developing chronic conditions such as heart disease, diabetes, and infertility. 
NAC does more than just aid in the production of glutathione. Other benefits of the nutrient include:
Detoxification the Liver and Kidneys
NAC plays a role in the detoxification of your kidneys and liver. It can help to prevent potential side effects of drugs and environmental factors — such as exposure to heavy metals — that may impact these organs.
In instances of acetaminophen overdose, doctors often give NAC intravenously to reduce damage to the liver and kidneys. 
Improved Symptoms of Respiratory Issues
NAC supplements can help those with chronic obstructive pulmonary disease (COPD) by improving symptoms and managing lung decline. The supplement can also help to decrease the severity of wheezing and coughing in people with bronchitis
Improved Immune Health
There aren’t many studies on NAC and the immune system, but current research suggests that it and glutathione may help to improve your immune function. Most of the studies focus on NAC and people with human immunodeficiency virus (HIV).
Some research shows that the supplement may help to suppress HIV-1 reproduction. Test-tube studies also show that NAC may help to prevent the flu virus from replicating, possibly reducing its lifespan and symptoms. 
Insulin Resistance in people with PCOS
Research suggests that NAC may help to improve insulin resistance, thereby helping to regulate blood sugar levels. Studies involved human subjects with polycystic ovary syndrome (PCOS), a hormonal condition that can affect women of childbearing age. 
Healthy Brain Function
NAC helps to replenish glutathione levels in the body. It also aids in regulating glutamate. These functions may help to improve brain health and benefit people with conditions such as Alzheimer’s and Parkinson’s diseases. 
Mental Health Management
NAC may help with psychiatric disorders and addictive behaviors. Some research shows that NAC can help improve symptoms of depression and bipolar disorder.
Other studies say that the supplement may help decrease symptoms of withdrawal and lower the risk of relapse in people with addictions
Reduction of Heart Disease Risk
Test-tube studies suggest that combining NAC and green tea may help to reduce damage caused by oxidized LDL-cholesterol, a risk factor for heart disease. NAC also helps to boost nitric oxide production, which can improve your blood flow and lower your heart attack risk. 
Improvement in Fertility 
NAC may help to improve fertility in both men and women. One study found that men supplementing with both NAC and selenium experienced an increase in semen quality. NAC may also help to improve fertility in women with PCOS. 
While NAC does provide important health benefits, there are a few potential risks to keep in mind.
Slow Blood Clotting
NAC may impair blood clotting. If you have a bleeding disorder such as hemophilia or you take blood-thinning medications, you should avoid taking NAC supplements. 
Medication Interactions
NAC has few known medication interactions. It may, however, interfere with medications for angina and diabetes. NAC may intensify the effects of nitroglycerin, which could contribute to dizziness and fainting. For those with diabetes, it may cause hypoglycemia
Kidney Stones
In rare cases, NAC may cause kidney stones to form. 
As your body can produce cysteine, there are no dietary recommendations. Many protein-rich foods contain the nutrients you need for your body to produce it. 
NAC supplements help to fill in any gaps and help to increase cysteine levels. These supplements have low bioavailability, however — meaning they are not well-absorbed by the body. The typical recommendation for NAC is between 600 and 1,800 milligrams. 
Acta Obstetricia et Gynecologica Scandinavica: “N-Acetyl Cysteine and Clomiphene Citrate for Induction of Ovulation in Polycystic Ovary Syndrome: A Cross-Over Trial.”
Antioxidants: “Protective Effect of Glutathione Against Oxidative Stress-Induced Cytotoxicity in RAW 264.8 Macrophages Through Activating the Nuclear Factor Erythroid 2-Related Factor-2/Heme Oxygenase-1 Pathway.”
Alternative Medicine Review: “Cysteine Metabolism and Metal Toxicity.”
Biochemical Pharmacology: “N-Acetyl-L-Cysteine (NAC) Inhibits Virus Replication and Expression of Pro-Inflammatory Molecules in A549 Cells Infected with Highly Pathogenic H5N1 Influenza A Virus.”
Blood Coagulation and Fibrinolysis: “The Effect of N-Acetylcysteine on Blood Coagulation and Platelet Function in Patients Undergoing Open Repair of Abdominal Aortic Aneurysm.”
Cell Journal: “A Review on Various Uses of N-Acetyl Cysteine.”
Clinical Interventions in Aging: “Oxidative Stress, Aging, and Diseases.”
European Journal of Clinical Investigation: “N-Acetyl-L-Cysteine Exerts Direct Anti-Aggregating Effect on Human Platelets.”
European Journal of Pharmacology: “Green Tea Polyphenols Inhibit Human Vascular Smooth Muscle Cell Proliferation Stimulated By Native Low-Density Lipoprotein.”
European Respiratory Journal: “The Effect of Oral N-Acetylcysteine in Chronic Bronchitis: A Quantitative Systematic Review.”
Fertility and Sterility: “N-acetyl-cysteine treatment improves insulin sensitivity in women with polycystic ovary syndrome.
Innovations in Clinical Neuroscience: “Getting a Knack for NAC.”
International Journal of Chronic Obstructive Pulmonary Disease: “The Role for N-Acetylcysteine in the Management of COPD.”
Journal of Biological Chemistry: “Resurgence of Serine: An Often Neglected but Indispensable Amino Acid*.”
Neuropsychopharmacology: “N-Acetylcysteine Normalizes Glutamate Levels in Cocaine-Dependent Patients: A Randomized Crossover Magnetic Resonance Spectroscopy Study.”
Proceedings of the Nutrition Society: “Glutathione and Immune Function.”
Stroke: “Dietary Cysteine and Other Amino Acids and Stroke Incidence in Women.”
The Journal of Clinical Psychiatry: “N-Acetylcysteine in Depressive Symptoms and Functionality: A Systematic Review and Meta-Analysis.”
The Journal of Urology: “Efficacy of Selenium and/or N-Acetyl-Cysteine for Improving Semen Parameters in Infertile Men: A Double-Blind, Placebo-Controlled, Randomized Study.”
Virus Research: “Influence of Glutathione Availability on Cell damage Induced By Human Immunodeficiency Virus Type 1 Viral Protein R.”

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