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 is the major symptom, and uncoordinated movements and mood changes often appear together in some cases. The condition usually begins 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.