Dietary salt and protein intake and urinary cystine excretion in patients with cystinuria.

Background: Cystinuria is a rare autosomal recessive disorder characterized by impaired renal reabsorption of cystine and dibasic amino acids, leading to recurrent nephrolithiasis. While dietary salt and protein restriction are commonly recommended, evidence supporting their effectiveness in reducing urinary cystine excretion is limited. This study investigated whether intra-individual changes in dietary salt and protein intake are associated with changes in cystine excretion over time. Methods: We conducted a retrospective cohort study of 41 adult patients with recurrent cystine stones treated at a tertiary kidney stone clinic between 2004 and 2023. All patients underwent five 24-hour urine collections at intervals of 6-12 months. Urinary sodium and urea excretions were used as surrogates for salt and protein intake, respectively. Mixed-effects linear regression models assessed within-person associations between dietary intake and urinary cystine excretion, adjusting for age, sex, and time. Results: Cystine excretion showed considerable intra-individual variability (intraclass correlation coefficient: 0.457). An increase in urinary urea of 3.5 g/24 h (reflecting ∼10 g/day higher protein intake) was associated with a 164 µmol/24 h increase in cystine excretion (95% CI: 57 to 271, Conclusions: Protein intake is moderately associated with urinary cystine excretion, whereas salt intake has minimal effect. These findings suggest that, while protein moderation may contribute to cystine reduction, fluid intake and urine alkalinization remain the primary determinants of urinary cystine levels.