Differences in 5-HT function have been the subject of extensive research in psychiatric studies. Many studies have manipulated L-tryptophan (Trp) levels to temporarily decrease (depletion) or increase (loading) 5-HT synthesis. While most researchers have used a 100g formulation, there has been ongoing interest in using smaller-sized formulations.
This study examined the time-course of multiple plasma indicators of brain 5-HT synthesis following 50g depletion and loading as a comparison to the typical 100g formulation.
Plasma was collected from 112 healthy adults at 7 hourly intervals following consumption of either a 50g or 100g depletion or loading. Self-ratings of mood and somatic symptoms were completed before and after Trp manipulations.
The primary findings were that: (1) the 50g and 100g formulations produced the expected changes in plasma indicators following both depletion (−89% and −96%, respectively) and loading (+570% and +372%, respectively); (2) the 100g depletion showed more robust effects at the 4, 5, and 6 h measurements than the 50g depletion; (3) there was significant attrition following both the 100g depletion and loading, but not after either 50g formulation; and (4) both 50g and 100g depletions produced increases in negative self-ratings of mood and somatic symptoms, while loading significantly increased negative ratings following the 100g only.
There are important considerations when choosing among formulation sizes for use in Trp manipulation studies, and the complete 7-hr time-course data set of the typical plasma Trp measures presented here may help researchers decide which methodology best suits their needs.