Hartnup disease
Hartnup disease is a rare inherited condition that affects how the body absorbs certain amino acids, the building blocks of proteins. It mainly involves tryptophan, which the body uses to make niacin (vitamin B3) and other important molecules.
The problem comes from a faulty gene called SLC6A19 on chromosome 5. This gene makes a transporter that helps absorb neutral amino acids from the gut and reabsorb them in the kidneys. Because of this defect, people with Hartnup disease lose more amino acids in the urine and don’t have enough of them in the blood. The disease is inherited in an autosomal recessive pattern, meaning a child must inherit a faulty copy from both parents.
Symptoms can start in infancy or childhood, and sometimes later. Common signs include poor growth, sensitivity to sunlight with a sun-exposed rash, unsteady walking (ataxia), tremor, and sometimes headaches or mood changes. Attacks can be triggered by sun exposure, fever, illness, stress, or poor nutrition, and tend to improve with age. Because the body may have less niacinamide, some people develop pellagra-like skin symptoms, plus possible digestive or mental changes. Urine tests may show higher levels of neutral amino acids.
Treatment aims to reduce attacks and ensure enough niacin. A high-protein diet helps by providing more amino acids. Protecting the skin from the sun with clothing and sunscreen is important. If there is a niacin deficiency or frequent attacks, doctors may prescribe niacin or nicotinamide supplements, which can lessen how often or how severe the episodes are. For more severe brain or nerve problems, specialized medical care is needed.
This page was last edited on 2 February 2026, at 16:25 (CET).