Let us start with the beginning:

Our feeding is composed of 3 principal elements:

• Proteins
• The lipids (greases)
• Glucides or carbohydrates (sugars)

The proteins are in the majority of food, but particularly in great quantity in: meat , the fish , eggs , dairy products and in certain cereals and leguminous plants such as rice, lenses, pea, beans... etc.
A protein consists of a whole of amino acids of different types and in different proportions, which gives an aspect and a different taste to each food.
There are many types of amino acids.
We thus eat food proteins, of origin animal or vegetable, which will be digested , i.e. the connections which join together the amino acids to form proteins will be broken, and the amino acids thus released will pass on the level intestine in blood circulation .
They will be used by the various bodies, for the construction tissue, the growth, or degraded (destroyed) according to processes' various and specific to each amino acid.

L-Phenylalanine is an essential amino acid not only for the man, but also for any alive being including the bacteria.
Like all the other acid-amino ones, it is brought by the feeding, and will be degraded by the liver of the patients.

Phenylalanine is a cyclic amino acid with benzene core .

There is another amino acid near to phenylalanine: Tyrosin or Para-hydroxyph�nylalanine.

These two amino acids are known as ketogenetic .

Phenylalanine is essential, nonreplaceable , because our organization cannot manufacture it, which is not the case of the tyrosin which it is essential, but replaceable. Indeed one can obtain it thanks to phenylalanine by oxidation.