I'm aware of the importance of glycine, particularly for a sub-optimal methylator, have been covering that base with trimethyl glycine (AKA betaine).

Guanidinoacetic acid sounds interesting. As far as 'passing the bbb', well, that seems a fairly complex subject — maybe because I've been digging in. All these years digging into AD (as an amateur, admittedly) without grokking a very key part (or really set of interrelated systems). My bad