N-Acetylcysteine and Cysteine Supplementation

Introduction to N-Acetylcysteine and Cysteine

N-Acetylcysteine (NAC) and cysteine are supplements with potential antioxidant properties, but their use in various medical conditions requires careful consideration of the evidence, as noted by the American College of Nutrition and the American Academy of Family Physicians 1, 2

In preterm infants, NAC supplementation does not significantly increase plasma cysteine levels, unlike direct cysteine supplementation, which is recommended to maintain adequate plasma levels, according to the American Academy of Pediatrics 3
Cysteine supplementation significantly increases plasma cysteine levels in preterm neonates, whereas N-acetylcysteine (NAC) supplementation does not, as observed in clinical studies 3
Bioavailable cysteine (50-75 mg/kg/day) is recommended for preterm neonates receiving parenteral nutrition, with higher doses showing no additional benefits, and is important for glutathione synthesis, which has antioxidant properties 3
NAC supplementation has not shown significant benefits for clinical outcomes such as death, bronchopulmonary dysplasia, retinopathy of prematurity, necrotizing enterocolitis, periventricular leukomalacia, and intraventricular hemorrhage in preterm neonates 3

For methemoglobinemia treatment, ascorbic acid is preferred over NAC when methylene blue is contraindicated 4
NAC is used in methemoglobinemia treatment, particularly in patients with G6PD deficiency or acetaminophen-induced methemoglobinemia 4

Despite theoretical benefits as an antioxidant precursor, there is insufficient evidence to recommend routine L-cysteine supplementation for general health promotion, as stated by the American Academy of Family Physicians 2
The American College of Nutrition notes that current evidence does not support L-cysteine supplementation for improving cognitive function or preventing cognitive decline in persons with dementia 1
L-cysteine supplementation is not recommended for routine use in improving cognitive function, preventing disease, or enhancing general health outcomes in adults, due to insufficient evidence, as stated by the American College of Nutrition 1

NAC is not recommended for the prevention of contrast-induced acute kidney injury (AKI) in patients undergoing angiographic procedures, based on high-quality evidence, as stated by the American College of Cardiology Foundation/American Heart Association (Level of Evidence: A) 5
No established guidelines exist for the combined use of dihydromyricetin and L-cysteine for medical treatment, and their combined use lacks sufficient evidence for safety and efficacy, highlighting the need for human clinical trials to establish appropriate dosing regimens, safety profiles, and efficacy for specific medical conditions, as well as potential drug interactions 3
The stability of cysteine is low in solution, making it challenging to administer, though it can be added to amino acid solutions just before administration in preterm neonates 3
L-cysteine has better bioavailability (approximately 100%) compared to N-acetylcysteine (NAC), which has only about 50% bioavailability, as noted by the American College of Nutrition 3