Phenylalanine, or "Phe", is an amino acid. Amino acids are found in many foods. They are the building blocks of proteins.

Usually, when you eat proteins or amino acids, your body breaks them down. Then it uses the results as building blocks for muscles, organs, hormones, and enzymes. In most people, the body turns Phe into another amino acid called tyrosine. Tyrosine is then used to make other body chemicals.²

In PKU, the enzyme that converts Phe to tyrosine doesn't work right. When the enzyme fails, Phe doesn't turn into tyrosine. Instead, Phe builds up in the body.¹

Too much Phe is toxic to the brain¹

Doctors have known for a long time that too much Phe causes severe mental retardation and other problems in infants with PKU.¹ But, it was thought that higher Phe was not a big problem for teens and adults. That has changed. Many studies show that high Phe levels can cause problems throughout life, including:

• Lower intelligence (IQ)
• Bad moods, being cranky or irritable
• Feeling "foggy"
• Thinking and responding slower
• Being depressed
• Feeling anxious
• Not being able to focus or pay attention^2,3,4,5,6

Even a little bit too much Phe can cause problems. IQ levels may drop as much as 4 points for every 100 μmol/L in lifetime Phe. For some people with PKU, that can be a big loss of IQ.³

To protect your brain and mood, it's important to keep Phe low for life.²

Next: Learn more about high Phe in infants and children and in teens and adults.