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Alkali absorption and citrate excretion in calcium nephrolithiasis
K Sakhaee1, R H Williams, M S Oh
1Center for Mineral Metabolism and Clinical Research, University of Texas Southwestern Medical Center, Dallas.
Summary
Net gastrointestinal alkali absorption significantly influences urinary citrate levels. Reduced alkali absorption, often dietary, contributes to hypocitraturia (low urinary citrate), particularly in stone formers without renal tubular acidosis.
Area of Science:
- Nephrology
- Gastroenterology
- Metabolic Disorders
Background:
- Hypocitraturia, a condition of low urinary citrate, is a risk factor for calcium nephrolithiasis.
- The gastrointestinal tract's role in alkali absorption and its impact on urinary citrate homeostasis is not fully understood.
Purpose of the Study:
- To investigate the relationship between net gastrointestinal (GI) alkali absorption and urinary citrate levels.
- To determine the origin of hypocitraturia in various patient populations, including those with renal tubular acidosis (RTA), chronic diarrheal syndrome (CDS), and idiopathic hypocitraturia.
Main Methods:
- Estimated net GI alkali absorption by calculating the difference between urinary cations (Ca, Mg, Na, K) and anions (Cl, P).
- Correlated 24-hour urinary citrate with net GI alkali absorption in 131 normal subjects.
- Analyzed urinary citrate and alkali absorption in patients with RTA, CDS, and idiopathic hypocitraturia, including a subset treated with potassium citrate.
Main Results:
- A significant positive correlation was observed between urinary citrate and net GI alkali absorption in normal subjects (r = 0.49, p < 0.001).
- This correlation was less pronounced in RTA patients, indicating a renal origin of hypocitraturia in this group.
- Patients with CDS and idiopathic hypocitraturia maintained a normal relationship between urinary citrate and alkali absorption, suggesting a dietary contribution to hypocitraturia.
Conclusions:
- Urinary citrate excretion is directly dependent on net GI alkali absorption.
- Hypocitraturia in CDS and idiopathic stone formers is likely linked to reduced dietary alkali intake, characterized by low vegetable/fruit consumption or high animal protein intake.
- Potassium citrate supplementation normalized the relationship between urinary citrate and alkali absorption in treated stone-forming patients.