"Glutamic acid. A free-form amino-acid, and glucose are the only two sources of energy the brain can use. Glutamic acid does not cross the ""brain-blood-barrier"" easily, so it is not very bioavailable, but l-glutamine, also a free-form amino acid, can cross the barrier easily. It is then converted to glutamic acid for energy production. L-glutamic acid is also a precursor for gaba (gamma amino butyric acid), an important relaxing or inhibitory neurotransmitter. "