The Effect of Dietary Phosphate Load on Urinary Supersaturation and Phosphate Metabolism in Non-Stone-Forming Asian Individuals.

Introduction: Phosphate ion is common in the core of urinary stones and may initiate stone formation. However, the precise role of phosphate in the initiation of stone formation remains obscure. We assessed the effects of dietary phosphate load on urinary stone risk and phosphate metabolism.

Methods: Ten non-stone-forming healthy volunteers completed this randomized, crossover study. Each subject was provided a high- or low-phosphate diet for 3 days. After a 2-day equilibration period with a moderate-phosphate diet, the participants received a low- or high-phosphate diet for another 3 days. Serum, fecal, and 24-h urine samples were collected at the end of each intervention.

Results: High dietary phosphate load increased urinary phosphate excretion, and low dietary phosphate decreased urinary phosphate excretion. In addition, urine pH was lower and supersaturation (SS) index of uric acid (UA) was higher after a high-phosphate diet. There was no significant difference in the SS index for calcium oxalate, brushite and hydroxyapatite, or 24-h urinary citrate between the high and the low dietary phosphate. Also, no significant change was observed in fecal phosphate excretion after a high- or low-phosphate diet. The serum phosphate level increased after a high-phosphate diet, but there was no difference in serum phosphate between low-phosphate and moderate-phosphate diets.

Conclusion: High dietary phosphate load led to higher urinary phosphate excretion, a higher SS index of UA, and lower urine pH. Stone formers should be advised to limit the intake of high-phosphate source diet, including high-protein diets and phosphate-based food additives.