SEASONAL-VARIATION IN PLASMA L-TRYPTOPHAN AVAILABILITY IN HEALTHY-VOLUNTEERS - RELATIONSHIPS TO VIOLENT SUICIDE OCCURRENCE

Objective: To investigate the seasonal variation in levels of plasma L -tryptophan and competing amino acids (CAAs) in healthy humans in rela tion to climatic variables, total serum protein levels, and violent su icide occurrence. Methods: Twenty-six healthy volunteers (13 men and 1 3 women; mean [+/-SD] age, 38.7+/-13.4 years) had monthly blood sampli ngs 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, phe nylalanine, valine, and leucine, and semiannual rhythms in L-tryptopha n values and in L-tryptophan/CAA ratios. Plasma L-tryptophan and the L -tryptophan/CAA ratio were significantly lower in the spring than in t he 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-trypt ophan values and L-tryptophan/CAA ratios, respectively The amplitude o f the yearly variation in all CAAs was low, ie, less than 7%. An impor tant part of the variance in L-tryptophan availability (ie, 12% to 14% ) could be explained by the composite effects of present and past clim atic 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 wer e noted between total serum protein level and all amino acid concentra tions, 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 sea sonal pattern in the availability of plasma L-tryptophan that matches seasonal patterns in the prevalence of violent suicide in the local po pulation and depression in other studies.