Phenylalanine sensitivity is caused by a rare genetic condition called phenylketonuria, or PKU. People who are born with PKU are missing a liver enzyme required to break down phenylalanine, an amino acid found in almost every type of protein and some artificial sweeteners. Phenylketonuria is often detected during newborn screening, and a special diet can often prevent symptoms from developing. However, these people need to limit their intake of phenylalanine permanently. If those with PKU consume too much phenylalanine, it can cause a variety of symptoms, ranging from mild to severe.
Because people with PKU cannot break down phenylalanine, its consumption causes levels of the amino acid to rise in blood and body tissues. This can cause the skin, breath and urine to develop a musty odor. High levels of phenylalanine in the skin may also cause a rash or eczema. Eating too much phenylalanine can also result in vomiting by those with PKU.
If children with PKU are not placed on a special diet, several developmental effects may occur. These include delayed or slow physical development, which often results in a very small head. Phenylalanine is involved in the synthesis of melanin, the pigment that gives hair and skin their color. As a result, children with PKU often have lighter colored hair, skin or eyes than other members of the family.
Left untreated, PKU can cause severe mental retardation in children. In less severe cases, a child with PKU may experience delayed development of mental abilities and social skills. These children may also display behavioral problems, such as hyperactivity, or psychiatric disorders such as self-mutilation.
Phenylketonuria can cause severe neurological effects, such as seizures or tremors. Children with PKU may also develop uncontrolled jerking movements of the arms and legs, and may tend to rest their hands in unusual positions.