Originally Posted by RLSmi

The sequence of physiological reactions leading to dopamine is;

l-phenylalanine -> l-tyrosine -> l-DOPA -> dopamine.

Humans can't make l-phenylalanine, it has to be provided in the diet, however we can make l-tyrosine, although it is present in dietary protein.

If you eat food with adequate protein in it you get plenty of l-phenylalanine and l-tyrosine in your diet from the breakdown of the protein into its amino acids.

Healthy dopaminergic neurons can perform all three of these reactions provided there are adequate enzymes and co-factors. As I understand it, in PD the problem is the loss of healthy neurons, not a lack of either l-phenylalanine or l-tyrosine for dopamine production.