Seasonal Variation in Plasma L-tryptophan Availability in Healthy Volunteers. Relationships to Violent Suicide Occurrence

Arch Gen Psychiatry. 1995 Nov;52(11):937-46. doi: 10.1001/archpsyc.1995.03950230051008.

Abstract

Objective: To investigate the seasonal variation in levels of plasma L-tryptophan and competing amino acids (CAAs) in healthy humans in relation to climatic variables, total serum protein levels, and violent suicide occurrence.

Methods: Twenty-six healthy volunteers (13 men and 13 women; mean [+/- SD] age, 38.7 +/- 13.4 years) had monthly blood samplings for assays of L-tryptophan, valine, leucine, isoleucine, tyrosine, and phenylalanine during 1 calendar year.

Results: Significant annual rhythms were detected in L-tryptophan, the L-tryptophan/CAA ratio, phenylalanine, valine, and leucine, and semiannual rhythms in L-tryptophan values and in L-tryptophan/CAA ratios. Plasma L-tryptophan and the L-tryptophan/CAA ratio were significantly lower in the spring than in the other seasons. The peak-trough differences in the yearly variation expressed as a percentage of the mean were 17.1% and 16.1% for L-tryptophan values and L-tryptophan/CAA ratios, respectively. The amplitude of the yearly variation in all CAAs was low, ie, less than 7%. An important part of the variance in L-tryptophan availability (ie, 12% to 14%) could be explained by the composite effects of present and past climatic factors; higher ambient temperature and relative humidity in the face of lower air pressure are the most important predictors of low L-tryptophan availability. Important and positive time relationships were noted between total serum protein level and all amino acid concentrations, and a significant time relationship was also noted between the seasonal variation in L-tryptophan availability and the occurrence of violent suicide in Belgium.

Conclusion: Our results show a bimodal seasonal pattern in the availability of plasma L-tryptophan that matches seasonal patterns in the prevalence of violent suicide in the local population and depression in other studies.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Amino Acids / blood
  • Blood Proteins / analysis
  • Brain / metabolism
  • Circadian Rhythm
  • Depressive Disorder / epidemiology
  • Female
  • Humans
  • Male
  • Prevalence
  • Regression Analysis
  • Seasons*
  • Suicide / statistics & numerical data*
  • Tryptophan / blood*
  • Tryptophan / metabolism
  • Violence*
  • Weather

Substances

  • Amino Acids
  • Blood Proteins
  • Tryptophan