A recently published study showed no correlation between urinary pH and citrate concentration
BY DEBRA BLAIR, MPH, RD, CSR
The established practice for preventing urinary calculi is being challenged by results of a recent study (Whitson et al, Urology. 2007;70:634-637), which showed no correlation between urinary citrate levels and urinary pH. The study authors found that calcium citrate has been commonly prescribed as a stone inhibitor with benefit attributed to the prevention of calcium-oxalate deposits, diminished growth of calcium-oxalate crystals, and urine alkalinization.
Despite the prevalence of kidney stones, an understanding of etiologic factors and best preventive treatments remain subjects of investigation. Acknowledging the need for quality evidence regarding use of potassium citrate (Urocrit-K) to diminish calcium oxalate stone risk, the American Academy of Family Physicians (Am Fam Physician. 2006;74:86-94, 99-100) has given it an evidence rating of “B” (“inconsistent or limited-quality patient-oriented evidence”). This is based on the “findings from one randomized trial and usual clinical practice.”
To clarify inconsistencies in previous studies regarding the possible relationship of citrate supplementation to urinary pH in stone prevention, Whitson et al examined 24-hour urine collections from 572 outpatients who had been referred for stones during the four-year period from 2001-2005. Along with comparing urinary citrate to urinary pH, the study also looked at its potential relationship to age, gender, urine volume, and prescribed medications (allopurinol, alkalinization, hydrochlorothiazide). Subgroup analysis also included assessing associations between self-reported compliance and potassium citrate use as verified by urinary potassium level.
Significant study results reported by Whitson et al include:
• No correlation between urinary pH and citrate concentration.
• Lack of citrate, pH correlation when stratified by age, gender, or urine volume.
• No correlation with urinary pH and urinary citrate in self-reported supplemental potassium citrate users (urinary potassium greater than 100 mEq).
The authors hypothesize that global acid-base balance drives urinary pH, and based on study results conclude that “urinary citrate is not a primary determinant of urinary pH, regardless of supplementation status.” Recognizing the limitations of this retrospective study, the authors plan “a prospective study of urine collections obtained from patients with nephrolithiasis both before the first initiation of citrate therapy and at defined intervals during therapy, which will allow us to characterize in more detail the relationship between urinary citrate and urinary pH.”
With study results questioning the relationship between citrate supplementation and urinary stone formation, are there dietary interventions that may be useful? A recent literature review of dietary and holistic treatments of interest in this regard is provided by Laura Flagg, MSN, RN, CNP in Urology Nursing (2007;27:113-122). While there is currently a lack of evidence-based guidelines ad-dressing urinary stone prevention, the author presents some diet-related “recommendations to guide clinicians in patient education,” for which evidence of potential benefit have been accumulating. These include:
• Sufficient fluid intake. Results of a five-year prospective, randomized, controlled trial involving 199 patients by Borghi et al (J Urol. 1996;155:839-843) demonstrated that fluid intake sufficient to produce approximately 2-2.5 liters of urine daily is associated with reduced risk of stone recurrence.
• Adequate dietary calcium. Intake of 1000-1200 mg/day (Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes) from dietary sources should be encouraged. Studies including Siener et al (Kidney Int. 2003;63:1037-1043) have supported the benefit of higher dietary calcium intake in prevention of kidney stones. Regarding calcium from supplements, a 20% increased risk for kidney stones has been observed with use in older women versus younger (Curhan et al, Ann Intern Med. 1997;126:497-504), though the etiology of this association is unclear.
• Maintaining healthy body weight. Obesity and has been recognized as a risk factor for kidney stones, with BMI greater than 30 kg/m2 and waist circumference greater than 40 inches almost doubling the chance of occurrence (Taylor et al, JAMA. 2005;293:455-462).
• Moderate protein intake and reduced dietary sodium. These are areas of continued research with regard to lowering risk of urinary calculi. Current prudent advice would include
the RDA for protein intake (0.8 g/kg/day) and limiting sodium to 2.5 g or less.