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A Possible Synergism of Dietary Phosphate and Urine-Acidifying Salts in Preventing Silica Urolithiasis in a Rat Model¹

Station Biochemistry Section, Chemistry Department, South Dakota State University, Brookings, SD 57007

Using an animal model involving rats fed tetraethylorthosilicate, the minimal effective dietary concentration of ammonium chloride for reduction of silica urolithiasis was determined to be 0.10 equivalents/kg diet. Ammonium sulfate appeared to be only slightly less effective. The lower incidences of urolithiasis were associated with urinary pH < 7. A subprophylactic concentration (0.067 equivalents/kg diet) of ammonium chloride was factored with three levels of supplemental phosphorus (0, 0.15 and 0.30%) from Na₂ HPO₄ to determine whether the antiurolithic effects of dietary phosphate and urinary-acidifying salts are synergistic. Phosphate had no effect on urinary pH. A 50% urolith incidence occurred in controls; the incidence was 25% (P = 0.08) with 0.15 and 10% (P < 0.01) with 0.30% phosphorus. Urinary pH was 7.5 in controls compared with 7.2 in rats given the subprophylactic level of ammonium chloride, but ammonium chloride alone had no effect on urolithiasis. However, complete protection from urolithiasis was provided by each of the two levels of phosphorus in combination with ammonium chloride. It is concluded that supplemental dietary phosphorus is most effective for protection against silica urolithiasis under conditions contributing toward urinary acidification due to a possible synergism between dietary phosphorus and urinary acidifying salts.

KEY WORDS: • silica • uroliths • phosphorus • ammonium chloride • ammonium sulfate • rats

¹ Published with approval of the Director of South Dakota Agricultural Experiment Station as publication number 2214 of the Journal Series.

² To whom reprint requests should be sent.

Manuscript received 27 January 1987. Revision accepted 26 May 1987.