Medical Nutrition Therapy for Pediatric Kidney Stone Prevention, Part 3: Cystinuria.
Audience: Pediatrics; Nephrology Outpatient Clinic CYSTINURIA IS AN inherited, genetic condition that causes hyper-excretion of cystine and other dibasic amino acids (lysine, arginine, and ornithine) in the urine due to impaired transport in the proximal renal tubules. The low solubility of cystine leads to precipitation and the formation of renal stones. While rare, this condition accounts for a relatively significant proportion of pediatric nephrolithiasis, up to 10% in children compared to 1% in adults. Furthermore, as compared to other causes of pediatric stone formation, cystinuria is more likely to result in frequent recurrence, morbidity, and need for medical interventions. Careful attention to disease management is required to prevent disease-related complications, specifically chronic kidney disease. The dietary management of cystinuria is a two-pronged approach involving (1) a reduction in the overall urinary excretion of cystine and (2) an increase in urinary cystine solubility. The first objective may be obtained by restricting dietary intake of animal proteins, which, in comparison to plant-based proteins, are generally higher in cystine and its metabolic precursor methionine. Avoiding excessive dietary intake of sodium will also reduce urinary excretion of cystine. The second objective, increased urinary cystine solubility, can be improved with increased fluid intake; in pediatric patients, this is commonly estimated using body surface area and approximates 2 L/m 3 body surface area. Cystine solubility also increases with urinary alkalinity, so once again it is important for patients to avoid excessive animal protein, whose sulfur containing amino acid profile acidifies the urine.