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# Hartnup disease

In Hartnup disease the gut and [kidney](https://durgical.com/en/anatomy/kidney/) do not recover several neutral amino acids well, including tryptophan; pellagra-like lesions may appear.

Updated 2026-11-01

## Structures involved

Tap one to see what it is and open it in 3D.

- [Jejunum](https://durgical.com/en/anatomy/jejunum/)
- [Kidneys](https://durgical.com/en/anatomy/kidney/)
- [Ileum](https://durgical.com/en/anatomy/ileum/)
- [Skin](https://durgical.com/en/anatomy/skin/)
- [Cerebellum](https://durgical.com/en/anatomy/anterior-quadrangular-lobule/)

## In biochemistry

- [Tryptophan](https://durgical.com/en/biochemistry/molecules/tryptophan/)
- [Molecules derived from amino acids](https://durgical.com/en/biochemistry/topics/molecules-derived-from-amino-acids/)
- [Membrane structure](https://durgical.com/en/biochemistry/topics/membrane-structure/)
- [Membrane transport](https://durgical.com/en/biochemistry/topics/membrane-transport/)
- [Amino acids: structure, charge and pKa](https://durgical.com/en/biochemistry/topics/amino-acids-structure-charge-and-pka/)

## In physiology

- [Protein digestion and absorption](https://durgical.com/en/physiology/protein-digestion-and-absorption/)
- [Proximal tubule transport](https://durgical.com/en/physiology/proximal-tubule-transport/)

## Why it happens

- [Vitamin deficiencies: what each one does to tissue](https://durgical.com/en/general-pathology/vitamin-deficiencies-what-each-one-does-to-tissue/)

## What it is

An inherited disorder of the neutral amino acid transporter B⁰AT1 in the gut and renal tubule. Neutral amino acids such as tryptophan are lost in urine.

## Who gets it

It is estimated at 1 in 30,000 people; most affected people have no symptoms.

## Causes

- Variants in both alleles of the SLC6A19 gene (autosomal recessive)

## Risk factors

- Diet low in protein or niacin
- Strong sun exposure
- Fever, stress or drugs such as sulfonamides

## Pathophysiology

Without B⁰AT1, neutral amino acids are poorly absorbed and poorly reabsorbed. Available tryptophan falls and with it the synthesis of niacin and NAD⁺. When diet does not make up for it, photosensitive [skin](https://durgical.com/en/anatomy/skin/), ataxia and pellagra-like psychiatric symptoms appear.

## Symptoms and signs

- Red, scaly rash on sun-exposed skin
- Intermittent ataxia
- Mood changes, anxiety or confusion
- Many people have no symptoms

## Red flags

- Ataxia or altered consciousness with a photosensitive rash
- Associated malnutrition

## Diagnosis

**Clinical**
Episodes of photosensitive rash and ataxia, especially in children with a poor diet.

**Tests**
Urine amino acids: excess neutral amino acids with normal levels of the others; SLC6A19 gene testing.

**Imaging**
Usually not needed; brain MRI is used to rule out other causes of ataxia.

## Treatment

**Lifestyle**
Protein-rich diet and sun protection

**Medical**
Nicotinamide (a form of niacin) when there are pellagra-like symptoms

## Complications

- Pellagra-like skin lesions
- Recurrent neurological episodes

## Prognosis

Good: symptoms usually improve with age and an adequate diet.

## Test yourself

Think of the answer, then tap the question to see it.

## Which transporter fails in Hartnup disease?

B⁰AT1 (SLC6A19 gene), which absorbs and reabsorbs neutral amino acids in gut and kidney.

## Why do pellagra-like lesions appear?

Tryptophan is lost and less is left to make niacin and NAD⁺.

## What does the urine amino acid test show?

A selective excess of neutral amino acids, such as tryptophan, with normal or low plasma amino acids.

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## Related conditions

- [Cystinuria](https://durgical.com/en/conditions/cystinuria/)
- [Carcinoid syndrome](https://durgical.com/en/conditions/carcinoid-syndrome/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Hartnup disease: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/hartnup-disease/) (MedlinePlus, US National Library of Medicine, 2026); [SLC6A19 gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/slc6a19/) (MedlinePlus, US National Library of Medicine, 2026); [Amino acid transport across the plasma membrane](https://reactome.org/PathwayBrowser/#/R-HSA-352230) (Reactome, 2026).

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Source: https://durgical.com/en/conditions/hartnup-disease/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/hartnup-disease/