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November 1995

Seasonal Variation in Plasma L-Tryptophan Availability in Healthy Volunteers: Relationships to Violent Suicide Occurrence

Author Affiliations

Affiliations of the authors are given in the acknowledgment section at the end of the article.

Arch Gen Psychiatry. 1995;52(11):937-946. doi:10.1001/archpsyc.1995.03950230051008

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.

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