Basically there are 2 amino acids that come before dopamine one is phenyalanine and one is tyrosine like Junk DNA said, phenylalanine becomes tyrosine after the addition of a hydroxyl group. Theoretically having more of the tyrosine or even phenylalanine could increase dopamine levels but in this study they used tyrosine since its closer to dopamine. So if you eat a lot of high protein foods you will get a lot of tyrosine.