Effects of Tyrosine on Parkinson's Disease: A Randomized, Double‐Blind, Placebo‐Controlled Trial

Individuals with Parkinson's disease ( PD ) can suffer from orthostatic hypotension ( OH ) resulting from reduced levels of norepinephrine ( NE ), which inhibits the sympathetic nervous system. Levodopa reduces NE levels even further, leading to a greater decrease in blood pressure ( BP ) and increased OH . Tyrosine is a nonessential amino acid that is the major precursor to NE . Reduced levels of tyrosine have been shown after administration of l ‐dopa. This study was a single‐center, randomized, double‐blind, placebo‐controlled trial to test the effects of supplementing l ‐tyrosine on BP , plasma tyrosine, NE levels, and autonomic responses to exercise in PD . Thirty‐six subjects with PD receiving l ‐dopa medication that suffer from OH participated. Random assignment was to a placebo group or l ‐tyrosine 1,000 mg (500 mg of 2× daily) group for 7 days. OH testing and exercise testing was performed pre‐ and postsupplementation. There was no effect of tyrosine on BP after OH testing postsupplementation (tyrosine, n = 17; placebo, n = 19). There was an increase in plasma tyrosine in the tyrosine group ( P  > 0.05). There were no significant changes in any of the secondary outcome measures. l ‐tyrosine at 1,000 mg (500 mg/2× day) for 7 days is safe and well tolerated in PD . Our results were inconclusive as to whether an increase in plasma tyrosine has an effect on OH in subjects with PD . An increase in plasma tyrosine had no effect on BP or autonomic responses in subjects with PD during acute exercise stress. (Trial registration: ClinicalTrials.gov .; identifier: NCT 01676103)