Blood-Brain Barrier

The blood-brain barrier (BBB) is a highly selective semipermeable membrane formed by tight junctions between brain capillary endothelial cells, surrounded by astrocyte end-feet and pericytes. It prevents most circulating substances — including many supplements — from entering the brain. This is critically important in supplement research because many compounds with promising in vitro neural effects cannot cross the BBB in meaningful quantities. For example, most oral GABA supplements may not cross the BBB significantly, while magnesium L-threonate is specifically formulated to do so. Understanding BBB permeability helps evaluate whether a supplement can realistically affect brain function.