title: Mood effects of the amino acids tryptophan and tyrosine: 'Food for Thought' III.
abstract: OBJECTIVE
  Reflecting increased scientific interest in any nutritional contribution to the onset and treatment of mood disorders, we overview research into two neurotransmitter precursors - the amino acids tryptophan and tyrosine - particularly examining whether any deficiency increases risk to depression and whether those amino acids have any antidepressant properties.
  
  
  METHODS
  The theoretical relevance of the two amino acids was overviewed by considering published risk and intervention studies, technical papers and reviews.
  
  
  RESULTS
  There is some limited evidence, suggesting that depressed patients, especially those with a melancholic depression, have decreased tryptophan levels. Whether such findings reflect a causal contribution or are a consequence of a depressed state remains an open question. There is a small database supporting tryptophan preparations as benefitting depressed mood states. There is no clear evidence as to whether tyrosine deficiency contributes to depression, while the only randomized double-blind study examining tyrosine supplementation did not show antidepressant benefit.
  
  
  CONCLUSIONS
  Acute tryptophan depletion continues to provide a research tool for investigating the relevance of serotonin to depression onset. There is limited evidence that tryptophan loading is effective as a treatment for depression through its action of increasing serotonin production. Most clinical studies are dated, involve small sample sizes and/or were not placebo controlled. The development of the new serotonin reuptake inhibitor drugs seemingly signalled an end to pursuing such means of promoting increased serotonin as a treatment for depression. The evidence for tyrosine loading promoting catecholamine production as a possible treatment for depression appears even less promising, and depletion studies less informative.
authors.affiliation: School of Psychiatry, University of New South Wales, Black Dog Institute, Randwick, Sydney, NSW, Australia. g.parker@unsw.edu.au
authors.forename: G
authors.initials: G
authors.lastname: Parker
authors.forename: H
authors.initials: H
authors.lastname: Brotchie
mesh_terms: D000928:Antidepressive Agents / Q000378:metabolism / Q000493:pharmacokinetics; D001682:Biological Availability; D002395:Catecholamines / Q000378:metabolism; D002986:Clinical Trials as Topic; D003863:Depression / Q000378:metabolism*; D006801:Humans; D019964:Mood Disorders / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000378:metabolism; D009751:Nutritional Requirements; D012701:Serotonin / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects*; D014364:Tryptophan / Q000378:metabolism / Q000493:pharmacokinetics / Q000627:therapeutic use; D014443:Tyrosine / Q000378:metabolism / Q000493:pharmacokinetics / Q000627:therapeutic use
chemical_list: D000928:Antidepressive Agents; D002395:Catecholamines; D018377:Neurotransmitter Agents; D012701:Serotonin; D014443:Tyrosine; D014364:Tryptophan