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Lysinuric protein intolerance
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

Lysinuric protein intolerance

Lysinuric protein intolerance is a metabolic disorder where the body cannot properly absorb specific amino acids, leading to digestive distress and systemic health complications.

Overview

Lysinuric protein intolerance (LPI) is a disorder caused by the body's inability to digest and utilize three specific amino acids: lysine, arginine, and ornithine. Because these amino acids are found in protein-rich foods, individuals with LPI typically experience nausea and vomiting after consuming protein. Symptoms usually appear after infants are weaned and begin eating solid foods.

Clinical Features

  • Digestive and Systemic: Enlarged liver and spleen (hepatosplenomegaly), short stature, and muscle weakness.
  • Bone and Lung Health: Brittle bones prone to fracture and a lung disorder called pulmonary alveolar proteinosis, where protein deposits interfere with lung function.
  • Renal and Metabolic: Accumulation of amino acids can lead to end-stage renal disease (ESRD). A lack of arginine and ornithine disrupts the urea cycle, causing elevated ammonia levels (hyperammonemia), which can lead to coma and intellectual disability.

Causes

LPI is caused by variants in the SLC7A7 gene. This gene provides instructions for producing the y+L amino acid transporter 1 (y+LAT-1) protein. This protein is responsible for transporting lysine, arginine, and ornithine between cells. When this transporter is disrupted, these amino acids are not absorbed in the small intestine and are lost in the urine rather than being reabsorbed by the kidneys.

Inheritance

This condition is inherited in an autosomal recessive pattern. Both copies of the SLC7A7 gene in each cell must have variants for the condition to manifest. Parents of an affected individual are typically carriers who do not exhibit symptoms of the disorder.

Frequently Asked Questions

Daily prevention relies on a carefully planned protein-restricted diet coupled with oral L-citrulline supplementation, which bypasses the defective intestinal and renal transporter to replenish the urea cycle and clear excess nitrogen. In an acute hyperammonemic crisis, protein intake is immediately stopped, intravenous glucose is administered to halt catabolic protein breakdown, and nitrogen-scavenging medications such as sodium benzoate or sodium phenylacetate are infused, with hemodialysis utilized if ammonia levels remain dangerously elevated.
While primary urea cycle enzyme defects typically show normal or elevated plasma lysine, LPI is characterized by low or low-normal plasma concentrations of lysine, arginine, and ornithine, paired with markedly increased urinary excretion of these same dibasic amino acids on quantitative amino acid chromatography. Patients with LPI also frequently present with elevated serum ferritin, high lactate dehydrogenase (LDH), subclinical pancytopenia, and
lthough patients with LPI suffer from low systemic lysine levels, oral lysine supplements are poorly absorbed by the impaired y+LAT-1 transporter in the intestinal epithelium. Unabsorbed lysine remains trapped in the digestive tract, drawing water into the bowel lumen and causing severe nausea, intractable vomiting, and osmotic diarrhea. As a result, treatment focuses on clearing ammonia with L-citrulline rather than force-replacing dietary lysine.
Patients with LPI require periodic high-resolution chest computed tomography (HRCT) and pulmonary function tests, including DLCO, to screen for subclinical pulmonary alveolar proteinosis (PAP), which can develop insidiously without cough or dyspnea. Renal function must also be checked regularly using urinalysis for proteinuria, serum creatinine, and estimated glomerular filtration rate (eGFR) assessments, as chronic renal amino acid loss and immune complex deposition can progress toward end-stage renal disease.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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