Thoughts on gaba?

The blood brain barrier thing is complex. For one, your BBB is not a static thing. Its permeability changes all the time in response to various factors, including exogenous drugs/supplements and endogenous compounds. In fact, it can break down and become much more permeable with head trauma. This is a little talked about deleterious effect of traumatic brain injury. There is a reactive oxidative species called 4-HNE, or 4-hydroxynonenal, that gets released during TBI because of lipid peroxidation. 4-HNE significantly increases BBB permeability, so right after TBI your brain is being bombarded by a lot more systemic compounds that normally don't pass the BBB. People with Multiple Sclerosis also have a more permeable BBB. Exercise can modulate BBB permeability. So we should not think of the BBB as a static thing, but something that varies based on specific scenarios.

Regarding GABA specifically, it is not settled whether or not it can cross the BBB in humans. It likely can in small amounts, or can cross and is then immediately effluxed back out by transporters. However, things don't always have to cross the BBB to have a noticeable effect. Morphine doesn't cross the BBB very well, but nobody would claim morphine doesn't have an effect. This is why heroin (diacetylmorphine) is used more. The two acetyl groups allow heroin to cross the BBB much better than morphine. That doesn't mean morphine is useless, though. You have GABA analogs that cross the BBB much better, like gabapentin, pregabalin, phenibut, baclofen, etc., which is very similar to the morphine/heroin issue. Nobody that has tried them would honestly argue that GABA is as effective as those substances, though. They clearly cross the BBB much more than GABA, and have stronger effects. Then you have the other elephant in the room: the gut-brain axis. This is a way for endogenous compounds in the GI tract to signal the central nervous system without actually crossing the BBB. There are some strains of probiotics that naturally create more GABA, which have been shown to correlate with lower anxiety levels. So there are signaling pathways that can affect the CNS and brain from peripheral mechanisms. It's never black and white. It's always more complex than it seems on the surface.

Regarding GABA supplementation and its effectiveness, that varies from person to person. Some people say they feel effects, while others don't. There is no clear scientific answer. There is conflicting data, and conflicting anecdotes, because the mechanisms are complex. If I was anxious would I personally take GABA? No, not really. I would personally go for things that increase GABA levels in the CNS naturally, like L-theanine or taurine, before I would just start taking GABA. That's not to say that GABA doesn't have a place in some people's regimens. It's just there are better ways to go about it.