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What is Phenylalanine?
L-Phenylalanine is an essential amino acid, meaning the body cannot synthesize it, necessitating consumption. In the diet, L-Phenylalanine is predominantly found in protein sources, such as beef, poultry, pork, fish, and milk.  Because other amino acids compete with L-Phenylalanine for absorption, supplementation can ameliorate this conflict. Adequate absorption ensures maximum benefits. 
To say L-Phenylalanine is an essential amino acid is an understatement, as it aids in neurotransmitter production, and is one of the two components necessary to synthesize thyroid hormones.  L-Phenylalanine is commonly used for nootropic purposes due to it augmenting norepinephrine and dopamine production. Both of these neurotransmitters have positive impacts on memory, attention span, and motivation. [3,4,5,6] Many individuals use L-Phenylalanine to aid in body composition and improve health. It can elevate metabolism while assisting thyroid hormone production.
L-Phenylalanine as a Neurotransmitter
Dopamine, often referred to as the reward hormone, plays major roles in cognitive function and is used in almost every part of the brain. L-Phenylalanine can improve working memory and episodic memory (who, what, when, where) by increasing dopamine levels.  Neuronal utilization of L-Phenylalanine is so powerful that cerebral phenylalanine levels decrease significantly following mentally strenuous tasks.  Because dopamine levels influence motivation, L-Phenylalanine is commonly used pre-workout.  Supplementing with L-Phenylalanine can also improve attention span, which is likely secondary to increased motivation. 
Epinephrine, norepinephrine, and adrenaline are notorious for causing the same motivational mindset as dopamine. Norepinephrine in particular plays major roles in executive functioning and intellect. Due to L-Phenylalanine being a precursor to norepinephrine, supplementation can enhance motivation and cognitive functions. 
A wide variety of users prefer L-Phenylalanine over other catecholamine precursors because it supports neurotransmitter production without directly increasing neurotransmitter levels. An increase across the board may eventually lead to tolerance and imbalances, which defeats the purpose. For example, insufficient epinephrine can decrease cognition. However, excess norepinephrine can decrease attention, processing speed, and executive functions. 
Health Benefits of L-Phenylalanine
Ensuring proper neurotransmitter levels can improve physical health and aid in body composition as well. Norepinephrine directly increases free-fatty acid release and oxidation.  Additional norepinephrine secretion occurs while exercising. L-Phenylalanine supplementation can further lipolysis during workouts by ensuring adequate production of norepinephrine. [9,10]
L-Phenylalanine's effects on dopamine levels can improve satiety. Research indicates that L-Phenylalanine can improve satiety following a meal compared to placebo.  Thyroid hormone levels have a major influence on metabolism. Thyroid hormones synthesis requires iodine and tyrosine. L-Phenylalanine indirectly supports proper production of thyroid hormones by being a precursor to tyrosine. 
The recommended dosage of L-Phenylalanine is 500mg (1 rounded 1/8tsp) per serving. This can be taken up to three times per day. Consuming on an empty stomach ensures optimal absorption. Users should NOT exceed 5g per day, as this can have negative effects.
DHEA + L-Phenylalanine
DHEA can increase dopamine as well as testosterone. Gym goers use it to pack on muscle while nootropics enthusiasts use it to improve focus. L-Phenylalanine can augment DHEA's dopaminergic properties to further enhance focus and motivation.
L-Tyrosine + L-Phenylalanine
Anecdotally, L-Tyrosine rapidly boosts dopamine levels. However, it can be short lived. This stack uses L-Phenylalanine to keep catecholamine levels at a normal baseline. L-Tyrosine acts as a dopamine-booster when needed.
- Wood, David R., Fred W. Reimherr, and Paul H. Wender. "Treatment of Attention Deficit Disorder with DL-Phenylalanine." Psychiatry Research16.1 (1985): 21-6. Web.
- Rooyackers, O. E., and K. S. Nair. "Hormonal Regulation of Human Muscle Protein Metabolism." Annual review of nutrition17.1 (1997): 457-85. Web.
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Scientific studies cited are not conclusive and have limitations, due to of their closed environment nature. Referenced studies will not necessarily determine your experience with a supplement or nootropic, since there are many unaccounted variables, which fall outside the scope of the studies.
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