[3] Hartnup disease manifests during infancy with variable clinical presentation: failure to thrive, photosensitivity, intermittent ataxia, nystagmus, and tremor.

Pellagra, a similar condition, is also caused by low nicotinamide; this disorder results in dermatitis, diarrhea, and dementia.

Mental retardation, short stature, headaches, unsteady gait, and collapsing or fainting are common.

SLC6A19 is a sodium-dependent and chloride-independent neutral amino acid transporter, expressed predominantly in the kidneys and intestine.

[6] In Hartnup disease, urinary excretion of proline, hydroxyproline, and arginine remains unchanged, differentiating it from other causes of generalized aminoaciduria, such as Fanconi syndrome.

[citation needed] A high-protein diet can overcome the deficient transport of neutral amino acids in most patients.

In patients with niacin deficiency and symptomatic disease, daily supplementation with nicotinic acid or nicotinamide reduces both the number and severity of attacks.

Neurologic and psychiatric treatment is needed in patients with severe central nervous system involvement.