Hartnup disorder is an inherited metabolic disorder involving the transport of certain amino acids (for example, tryptophan and histidine) in the small intestine and kidneys.

Hartnup disorder is possibly the most common metabolic condition involving amino acids. It is a genetic disorder inherited in an autosomal recessive manner, which means that the child must inherit the defective gene from both parents in order to be seriously affected.

Although most people show no symptoms, photosensitivity (sensitivity to light) is the major symptom, and uncoordinated movements and mood changes appear together in some cases. Symptoms usually first appear in childhood.

• Skin rash, usually on exposure to sunlight
• Widely spaced episodes of uncoordinated movement (rare)
• Mood changes and psychiatric disease (rare)
• Neurologic problems, such as abnormal muscle tone

A urinalysis that checks for elevated levels of "neutral" amino acids and normal levels of other amino acids may be done for this disorder. A genetic test may become available soon.

• Supplements containing nicotinamide are often the main treatment. This can effectively treat the rash, and can sometimes treat the uncoordination and psychiatric symptoms.
• A high protein diet is recommended.
• Avoiding sun exposure by wearing protective clothing and using a sunscreen of 15 factor or higher is recommended.
• Psychiatric treatment, such as the use of antidepressants or mood stabilizers, may be needed if mood swings or psychiatric problems occur.

Most people with this disorder can expect a normal lifespan with no disability. Rarely, there have been reports of severe bouts of neurologic disease and even deaths in families with this disorder.

There are usually no complications. The minority of people who develop symptoms can experience rash, uncoordinated movements, and, less frequently, psychiatric symptoms.

Call for an appointment with your health care provider if you have symptoms suggestive of this condition, particularly if you have a family history of Hartnup disorder. Genetic counseling is recommended if you have a family history of this condition and are planning a pregnancy.

Genetic counseling may help prevent some cases. A high protein diet may prevent amino acid deficiencies that cause symptoms.