Serotonin deficiency is actually super easy to research and replicate and has been for a long time with tryptophan depletion so I don't think that's the new part from this study.

Here's one but there's a zillion:

"Acute tryptophan depletion (ATD) is a well-established dietary method in translational brain research used to briefly lower central nervous serotonin (5-hydroxytryptamine (5-HT)) synthesis. A simplified two amino acid ATD formula (ATDPHE/LEU) was developed while reducing the overall amount of amino acids (AAs), with the objective of administration especially in children and adolescents in future studies."