L-Tyrosine & L-Phenylalanine - Dopamine & Thyroid Precursors
The most significant and studied aspects of L-Tyrosine and L-Phenylalanine are that both amino acids are precursors to Dopamine, and Thyroid hormone.
The building blocks of thyroid hormone production:
Iodine + Tyrosine --> monoiodotyrosine (T1) and diiodotyrosine (T2)
T1 + T2 --> T4 (thyroxine) & T3 (triiodothyronine, needs selenium too)
The biological pathway to dopamine (mediated by enzymes):
L-Phenylalanine --> L-Tyrosine --> L-Dopa --> Dopamine.
Dopamine is one of those buzzwords that gets thrown around very often in day-to-day scenarios. It's not always completely out of context, but it often misses the larger picture.
While dopamine is involved in getting rewarded and excited by things, it also plays an important role in not getting over-rewarded or feeling rewarded or stimulated by things that are not necessarily serving us for the better.
Interestingly, clinical trials study this by creating a Tyrosine and Phenylalanine deficiency in patients or animal models and then applying the stimulus or variable that they wish to test. Some fascinating results of studies like this have been quicker reaction time and less inclination towards smoking.
Another fascinating aspect of L-tyrosine is its crucial component in melanin production. "The critical step in melanin biogenesis is the oxidation of tyrosine by the enzyme tyrosinase." Riley, P. (1997).
This means that Tyrosine is a light-absorbing amino acid that helps us produce pigment for our hair and skin. It's important to note that our exposure to natural light also helps promote the conversion of tyrosine into dopamine.
It may seem that Tyrosine is the main goal - but here's why consuming Phenylalanine is still beneficial and important:
While Tyrosine can be synthesized in the human body, it is created by converting Phenylalanine, which cannot be synthesized and must come from the diet.
Therefore, we can deduce the importance of consuming Phenylalanine but also understand the importance of Tyrosine itself. Consuming both amino acids allows the body to have some supply of Tyrosine and a reserve of its building blocks in case it needs to convert more.
Moreover, a study by Wang, H. L. (1962) called "Effect of dietary phenylalanine and tryptophan on brain serotonin" showed that not dietary Tyrosine but Phenylalanine consumption was able to lower body and brain serotonin. If you have studied or are a fan of the work of Dr. Ray Peat and his extensive explanations on serotonin and its harmful effects in excess, then you will appreciate the value in that.
As Fernstrom, J. (2007) says, "physiologic factors that influence brain pools of these amino acids, notably diet, influence their rates of conversion to neurotransmitter products, with functional consequences."
