L-phenylalanine is a hydrophobic essential amino acid, and is the simplest of the aromatic amino acids. Phenylalanine is present in mother’s milk, as well as several different foods, including meat, fish, cheese, lentils, nuts, and seeds (Kapalka, 2010). First, most of the phenylalanine is broken down to yield energy, with a yield of 6.3 kcal/g (Kohlmeier, 2003). Since phenylalanine is an amino acid, it is used as building block for protein in the body. However, it is also part of some other pathways in the body. One of the most important roles of phenylalanine is as precursor for tyrosine. Tyrosine is a non-essential amino acid, formed by adding an -OH group to phenylalanine in the liver (Litwack, 2018), when tyrosine intake via the diet is low. Tyrosine is then metabolized into important substances for the body, such as catecholamines, serotonin, and melanin (Fernstrom & Fernstrom, 2007; Kapalka, 2010; Litwack, 2018). Catecholamines are neurotransmitters such as dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline). The release rates of these neurotransmitters is directly linked to the concentration of amino acid precursors in the brain, which is related to their blood concentration (Fernstrom & Fernstrom, 2007). Supplementation of phenylalanine has therefore been associated with antidepressant effects (Kapalka, 2010).
Not only is there a connection to depression, tyrosine also influences thinking patterns. A study by Colzato et al. (2015) found that tyrosine did have an effect on convergent thinking, called deep thinking. A dose of tyrosine promoted convergent thinking, compared to the placebo group. Tyrosine had no impact on a type of thinking called divergent thinking, for example brainstorming. (Colzato, de Haan, & Hommel, 2015).
Furthermore, phenylalanine is associated with pain sensation. Phenylalanine can be metabolized via tyrosine to serotonin. Among other neurotransmitters, serotonin activates the endogenous analgesia system, which suppresses the neurons that activate pain sensation (Russell & McCarty, 2000). With an increased intake of phenylalanine, the EAS system is activated more. The intake of nutrients such as phenylalanine might amplify the pain-relieving effect of opiate therapy. This is important, since it provides the opportunity to reduce the opiate dosage, giving relief to the many side effects of opiate ingestion (Russell & McCarty, 2000). In short, the amino acid phenylalanine is a precursor for tyrosine, which is metabolized to catecholamines, which play a role in depression, thinking patterns, and pain sensation.