I thought I edited my post to say what did what and where it comes from, but I apparently forgot to submit. To further explain my stance: I work a very active job, so exercise isn't an issue and at my job, where I get most of my anxiety, stimming isn't option. So doing things like fixing my diet and using supplements that can work throughout that day is my only options.
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Theanine is in tea. It's thought to be the major player in the calming effect that tea produces. I read that black tea has more, but it also has more caffeine too. It does have to be actual tea though to get any health benefits. That canned stuff is essentially tea flavored sugar.
Magnesium is, of course, in food; green veggies like spinach and nuts like cashews and almonds. It's an essential mineral and plays many parts in the body, especially cellular function. It's thought that majority of people lack it and it's associated anxiety disorders, among other health problems.
For GABA; many studies have demonstrated a link between Autism and GABA deficiency. I've read that the majority of usable GABA is produced in the gut by good bacteria and since we with Autism have abnormal gut bacteria, it would make sense we would lack it. To address it on a dietary level, you have to consume probiotics, but aim for brands that have a lot of strains. Personally, I drink kefir from Lifeway, as it's the only brand I can find that is both unsweetened and has L. Reutreri.
So, I guess the crux of this post is: fix your diet, even if you take my advice and look into supplements. I personally do both and have seen massive improvements in my life. My anxiety has gone away for the most part, my executive function has improved, and I feel like can actually function.