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Alkaline Urine During Pregnancy is Associated with Increased Gastrointestinal Alkali Absorption
Eileen Brandes, MD1, John Asplin, MD2, Vernon Pais, MD1.
1Dartmouth-Hitchcock Medical Center, Lebanon, NH, USA, 2Litholink, Itasca, IL, USA.

Introduction: During pregnancy, there appears to be an increased risk of nephrolithiasis, in particular stones composed of calcium phosphate. The pathophysiology has been attributed to both hypercalciuria as well as alkaline urine during pregnancy, which predisposes to calcium phosphate precipitation. It has been proposed and widely accepted that alkaline urine during pregnancy is secondary to renal compensation of respiratory alkalosis. This is induced by increased minute ventilation related to the gravid uterus and increased maternal and fetal oxygen demands. However, this dogma has been challenged given that alkaline urine should be temporary and resolve once initial compensation has occurred. In this prospective study including pregnant women on a standardized diet, we sought to assess whether alkaline urine during pregnancy may alternatively be due to increased gastrointestinal alkali absorption (GIAA).
Materials & Methods: IRB approval was obtained for this prospective study. Women with singleton pregnancy presenting to the obstetrical department were offered participation. Subjects were all placed on a controlled, standardized diet after assessment of metabolic needs by an obstetrical. The standardized diet consisted of boost plus, boost breeze and unlimited water. Collections were obtained during the third trimester and while the patient was lactating. Subjects remained on the controlled diet for 48 hours: the 24 hours preceding urine collection and the 24 hours of the urine collection. GIAA was estimated from urine using the following equation: [(Na + K + Ca + Mg (Cl + 1.8P)]. Statistical analysis were performed in STATA using T-test.
Results: Of the twenty participants currently enrolled in this study, after ensuring quality of collections by comparing 24 hour creatinine samples between individual patients, there were eleven patients who completed both pre- and post-partum 24 hour urine collections. There was no significant difference in known dietary-related factors including urinary volume, sodium, sulfate and urea nitrogen. Urinary pH was 6.6 pre-partum versus 5.95 post-partum (p<0.00001). GIAA was 28.8 mEq pre-partum versus 11.3 mEq post-partum (p=0.00097).
Conclusions: We have confirmed that, on a standardized formula diet, alkaline urine is observed acutely during pregnancy. Moreover, for the first time, we have demonstrated that this coincides with significantly augmented GI alkali absorption during pregnancy compared with early postpartum. This will allow improved understanding of the pathophysiologic underpinnings of stones in pregnancy.


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