Phenylalanine

What Is Phenylalanine?

Phenylalanine is an essential amino acid with a ring-shaped side chain, the same benzene ring found in many aromatic compounds. The body cannot make it, so it has to come from protein in food or from supplements. Some of it is built straight into proteins; much of the rest is changed into tyrosine.

From Phenylalanine to Adrenaline

An enzyme in the liver, phenylalanine hydroxylase, adds one oxygen atom to phenylalanine and makes tyrosine. It needs a helper molecule called tetrahydrobiopterin (BH4) to do it. Tyrosine is then the raw material for several of the body's signals: dopamine, noradrenaline and adrenaline, the thyroid hormones, and melanin, the pigment in skin and hair.

Because the body makes tyrosine this way, the WHO sets one requirement for the pair: 25 mg per kilogram a day of phenylalanine and tyrosine together. The more tyrosine your food brings, the less phenylalanine has to be spent making it.

Phenylalanine and PKU

In phenylketonuria (PKU), an inherited condition, phenylalanine hydroxylase does not work. Phenylalanine then builds up in the blood and, without treatment, damages the developing brain. Babies in the US, Europe and many other countries are screened for it with a heel-prick blood test in their first days.

Treatment is a diet very low in phenylalanine, kept up for life, with a special protein formula that supplies the other amino acids. Some people with milder forms respond to a medicine based on BH4, the helper molecule the enzyme needs. For people with PKU, ordinary protein foods are the main thing to limit, not only sweeteners.

Phenylalanine in Aspartame

Aspartame is made of two amino acids, aspartic acid and phenylalanine, joined together. Phenylalanine makes up a little over half of its weight, about 56%. That is why products sweetened with it carry the line Contains a source of phenylalanine in the EU, or Phenylketonurics: contains phenylalanine in the US.

For anyone without PKU, the amount is small next to food. One tablespoon (16 g) of peanut butter in the Caltide food library has about 190 mg of phenylalanine, as much as roughly 340 mg of aspartame. A 50 g boiled egg has about 330 mg.

Foods High in Phenylalanine

Peanuts stand out among plant foods: in the Caltide food library they have more phenylalanine than isoleucine or valine, while eggs, bacon and mozzarella have less. Soy, seeds, nuts, cheese and meat are among the richest sources, and fish, eggs, beans and oats carry useful amounts too. Every food with protein has some, which is what makes a PKU diet so strict. The full list is in the food library.