Phenylalanine: definition, benefits & diet

What is phenylalanine?

Definition of phenylalanine

Phenylalanine is a so-called ‘essential’ amino acid: it is essential for the body to function properly, but must be provided in the diet as the body cannot manufacture it itself.

More specifically, it plays a key role in the brain, affecting essential functions such as motivation, attention and mental well-being.

A key player in the production of neurotransmitters

As well as being a component of proteins, phenylalanine is essential for the production of tyrosine, another amino acid that helps to produce all kinds of neurotransmitters and hormones:

• adrenaline (or epinephrine), a hormone secreted when under stress or in preparation for physical activity
• noradrenaline, which plays a role in arousal, selective attention, vigilance, emotions, the sleep/wake transition, learning, etc.
• dopamine, a neurotransmitter that plays a decisive role in behaviour, motivation and risk-taking
• melanin, the natural pigment responsible for our skin colour
• thyroid hormones, which act on the body to increase basic metabolism, affect protein production and make the body more receptive to certain hormones

The role of phenylalanine hydroxylase

In the body, phenylalanine is converted into tyrosine by an enzyme found in the liver: phenylalanine hydroxylase (1).

This enzyme sometimes fails, leading to a rare genetic disease known as phenylketonuria (PKU), which may cause serious neurological complications. It affects one in 16,000 newborns and can lead to severe mental disability if left untreated.

What are the sources of phenylalanine?

The 15 foods richest in this amino acid

Since the body is not capable of manufacturing phenylalanine, and since it plays a decisive role in cognitive function, we need to regularly consume foods containing it.

These are mainly protein- and peptide-rich foods of animal and plant origin (particularly legumes).

Here is a list of 15 potential sources, ranked from the richest to the least rich in phenylalanine:

1. beef, 2400 mg per portion (150 g)
2. tofu, 2,334 mg per portion (100 g)
3. poultry, 2330 mg per portion (150 g)
4. mung beans, 1,473 mg per portion (100 g)
5. lentils, 1,383 mg per portion (100 g)
6. salmon, 1,100 mg per portion (100 g)
7. eggs, 967 mg per portion (3 eggs)
8. milk, 873 mg per portion (50 cl)
9. Parmesan cheese, 680 g per portion (30 g)
10. leeks, 559 mg per portion (100 g)
11. peppers, 550 mg per portion (100 g)
12. walnuts, 437 mg per portion (30 g)
13. Brie cheese, 350 mg per portion (30 g)
14. feta cheese, 270 mg per portion (40 g)
15. spirulina, 83 mg per portion (3 g)

However, beware of excessive protein consumption, which, if prolonged, can contribute to premature ageing through mechanisms such as oxidative stress, renal overload and metabolic dysfunctions.

Daily phenylalanine requirements

By way of information, the phenylalanine intake required by a healthy adult varies from 1,000 to 3,500 mg per day (39 mg/kg according to the WHO), depending on the research carried out and the individual's weight.

Increasing tyrosine intake

There is a possible solution: you can slightly reduce your consumption of these foods if you increase your tyrosine intake at the same time.

Tyrosine is found in meat, almonds, avocado, bananas, cheeses (especially hard cheeses), soya and seeds (pumpkin, sesame, sunflower, etc.).

This approach is particularly useful for people suffering from PKU.

Can I take phenylalanine supplements?

Phenylalanine-rich food supplements for the brain

In general, sources of phenylalanine are sufficiently numerous and varied to avoid deficiencies and provide the daily intake needed to manufacture tyrosine.

There are, however, brain nutrition supplements containing phenylalanine, which are reputed to act progressively and to be ideal over the long term (the synergistic NeuroNutrition Formula, for example, combines phenylalanine with other compounds studied for their role in cognitive health and mental well-being: L-pyroglutamic acid, sodium R-lipoate, etc.).

Of course, people with PKU should avoid phenylalanine supplements, as their bodies cannot metabolise it properly.

Tyrosine food supplements

Given its direct role in endogenous hormone production, brain performance and protein synthesis, tyrosine itself is used in the composition of many dietary supplements, in its active levorotatory form: L-tyrosine.

Tyrosine supplements are therefore particularly popular with sports enthusiasts and people concerned about their cognitive abilities (L-Tyrosine, for example).

The action of tyrosine supplements is reputed to be faster than that of phenylalanine supplements.

Increased blood levels of tyrosine and effects on cognition and mood

As early as the 1980s, studies showed that phenylalanine or tyrosine supplementation resulted in an increase in tyrosine levels in the blood, one hour after ingestion and up to 8 hours afterwards (2).

Other studies have subsequently evaluated the level of neurotransmitters synthesised from this tyrosine and their potential benefits in conditions marked by low levels of neurotransmitters (3-5), including fatigue linked to intense physical activity, emotional or mental exhaustion, cognitive stress and mood problems.