I didn't find anything first pass, which is unsurprising given the paper. Once I had the abstract however, it was trivial to find the whole paper.
None of your conclusions are supported by the paper.
What is true is that if one follows their protocol, one can temporarily reduce the free tryptophan available to the brain via competition with preferred amino acids, with somewhat deleterious effects. There is no mention of down or up regulation of the receptors. It is a lack of tryptophan for synthesis, not changes to the receptors.
As I said multiple times, you idiot, cutting contact between neurotransmitters and their receptors causes their receptors to upregulate across the brain and body.
Collagen's reduction in tryptophan therefore must reduce serotonin and upregulate serotonin receptors by reducing contact between serotonin and serotonin receptors, just as shilajit upregulates serotonin by reducing serotonin. Anyone who has noticed a cognitive boost from shilajit will notice the same cognitive benefits from taking collagen on an empty stomach 2 hours before a meal (collagen will not have this effect if you take collagen after a meal).
This is the exact same principle that causes beta blockers like propranolol to upregulate beta receptors: Beta blockers prevent the contact of adrenaline with beta receptors, which in turn causes the beta receptors to upregulate.
Edited by Kelvin, 16 March 2025 - 08:55 PM.