Study	Reason for exclusion
Helms 1995	The intervention was either methionine supplementation, or methionine and cysteine supplementation.
Laine 1991	The study did not include two simultaneous groups but had a before/after design.
Pointdexter 2000	The intervention was N‐acetyltyrosine and cysteine hydrochloride supplementation
Shew 2000	None of the predetermined outcome variables for this systematic review were analyzed.
Van Goudoever 1994	The intervention was not limited to cysteine supplementation; it included different amounts of cysteine and tyrosine, or N‐acetyl‐L‐tyrosine and N‐acetyl‐L‐cysteine.