Phenylalanine is an essential amino acid which cannot be endogenously synthesised and must be acquired from the diet. It is then metabolised into other, non-essential amino acids, which increase the levels of beneficial neurotransmitters in the blood plasma.
These include dopamine, epinephrine and norepinephrine, which play an important role during conditions of stress, such as prolonged periods of work, fatigue, sleep deprivation, and cold. In animal trials, researchers have found phenylalanine reduces stress hormone levels and weight loss induced by stress, while in humans they found it improved physical and cognitive performance, especially 'working memory' when multi-tasking.
Phenylalanine occurs naturally in breast milk, but good dietary sources can be found in most proteins, notably soya beans, beef, liver, chicken, eggs and milk, as well as some in leafy greens, spinach and tofu. It is a common additive in dietary supplements and in processed food and beverages, especially in the production of the artificial sweetener aspartame.
Concern over use of aspartame is founded on the incidence of phenylketonuria (PKU), a rare metabolic disorder which occurs when one of the enzymes required to process phenylalanine is lacking. Excess build-up of phenylalanine can cause severe intellectual impairment and autistic symptoms.