I've been reading Julia Ross (The Diet Cure, The Mood Cure).
She talks in the Mood Cure about several different amino acid deficiencies which can lead to mood disturbance. She talks about low norepinephrin causing lack of motivation and concentration. Her cure is Tyrosine. I've tried it and it seems to make no difference. It also comes with health warnings because it is a precursor to thyroid hormone - although since I have no discernable thyroid function that probably isn't a problem for me.
And when tyrosine doesn't cut it, she suggests D and L phenylanine which is the precursor to lots of other hormones, including tyrosine.
Has anyone tried these two? What were your results?