N-acetylcysteine — The Glutathione Precursor

N-acetylcysteine (NAC) acts primarily as a precursor to L-cysteine, the rate-limiting amino acid in the synthesis of glutathione (GSH), the body's master endogenous antioxidant. By replenishing intracellular GSH levels, NAC mitigates reactive oxygen species (ROS) and reduces oxidative stress. It also modulates inflammatory pathways by inhibiting NF-κB activation, thereby lowering the production of pro-inflammatory cytokines. Additionally, NAC contains a free thiol group that can directly scavenge certain free radicals and break disulfide bonds in mucus, explaining its mucolytic properties.

Multiple human trials have investigated NAC, particularly in combination with glycine (GlyNAC). A 2022 randomized, double-blind clinical trial in older adults showed that 16 weeks of GlyNAC supplementation improved glutathione deficiency, oxidative stress, mitochondrial dysfunction, and physical function. Other trials have demonstrated benefits for insulin resistance and endothelial function in aging populations.

600–1,200 mg/day is the standard supplemental range, often taken in divided doses. For longevity and metabolic benefits, it is frequently combined with equal or higher doses of glycine (e.g., 1–3 g/day of each) as 'GlyNAC'. Best taken on an empty stomach or between meals to avoid competition with other amino acids. Intravenous NAC is used clinically for acetaminophen toxicity but is not standard for longevity.

Generally well-tolerated at standard doses. Common adverse effects include gastrointestinal discomfort, nausea, and diarrhea. High doses may cause headaches or rash. NAC can interact with nitroglycerin, potentially causing severe hypotension and headaches. It may also act as a mild anticoagulant, so caution is advised for individuals with bleeding disorders or those taking blood-thinning medications.

Key Papers