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

Abstract

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: NCT01676103).