Hi Alexandra,
as far as I know at least one theory behind it is that leucine competes with tryptophan for uptake into the cerebrospinal fluid. Tryptophan is a precursor for serotonin, so it shifts the ratio of other neurotransmitters to serotonin in the direction of said other transmitters (e.g. dopamine). Dopamine is effectively a cerebral vasodilator, while serotonin is a vasoconstrictor – so you might get more nutrient support. But that can't be the whole story because most pwME seem to have problems with the regulation of liquor-flow more so than strictly just opening or closing vessels.
Another theory that was discussed somewhere on these boards is that overproduction of serotonin plays a role at least for a subset of pwME. Who knows.
BCAA in general also have some role in using fatty acids for energy and Leucine is involved in activating mTor-pathways. Both might play a role for us, someone mentioned recently they responded well to Beta Hydroxybutyrate and there is a recent thread on how an mTor-inhibitor helped some patients – so maybe our brains run better if they can use more ketone bodies, and maybe the regulation of mTor-pathways doesn't work very well (since the supposedly helpful medicine is inhibitory there, if BCAA do indeed help here in some way it cannot be as simple as them activating the same pathway, unless you e.g. need to do a bit of both at different times).
As far as supplements go, BCAA are pretty much as safe as they come. They are basically literally broken down food mostly without allergens. That doesn't mean someone in a sick state can't have a bad response to them by any means, but you generally won't find many things with less risk. Do keep in mind though that many commercial BCAA tabs contain a bit of B6 (pyridoxine) which tends to be a bit less safe for quite a few pwME, especially in higher quantities, so maybe recheck the label if you haven't.
I do not think it has been super-well established how well this stuff works for us, but I am really just posting because this is literally the first post in several years I felt i could at least provide some kind of answer to until
@Hip shows up and drops the knowledge bombs that kept me from signing up here for a decade
Btw, glutamine may do something with the gut lining because it is used somehow for the tight junctions there, theoretically decreasing intestinal permeability which in turn might help if you have dysbiosis, yeast etc etc. I don't know if this works in a practical setting though or how relevant that is. It is also used as a building block for glutathione, but normally it is not the rate-limiting step there (cysteine is). How good or bad this may be for us seems to be pretty individual. For what it's worth, I seem to do pretty bad on it unless I take quite a bit of vitamin c alongside it. A guess to why that is would be that the glutathione oxidizes quicker than it gets recycled and the oxidized version builds up – but again, I don't really know what I am talking about, so take it for what it's worth.
As a side note, when I was relatively new to this whole supplement game I thought I could judge pretty quickly how well stuff was working for me – until I realized that I have the mild problems with fluid retention many of us do and what was (probably) actually doing the trick was the glass of water I was drinking to take the pills. It is super hard to work around that (and obviously placebo and the normal ups-and-downs) when trying to evaluate how good supplements work for you because there is no practical way (...that i know of) to accurately gauge how much said glass of water alone would have affected you at each point in time, so I'd say don't fret too much over it. But that's my personal take after the years.