Phosphate & Kidney Stones

Patients who form kidney stones often have lower serum phosphate and higher kidney phosphate losses, largely due to reduced tubular reabsorption. Earlier trials of high-dose phosphate supplementation in patients with idiopathic hypercalciuria reduced urine calcium, and several showed fewer stones. Use of phosphate for stone prevention later declined after a randomized trial of acid phosphate found no difference in stone formation despite lower urine calcium. Studies of neutral or alkaline phosphate, in contrast, did show benefit. Higher doses can also cause gastrointestinal side effects. This pilot tests whether a lower dose can reduce urine calcium while avoiding those negative effects. It is exploratory and will provide preliminary data for a larger grant-funded trial. This is a single-group, open-label pilot in which all participants receive the same intervention. Before each collection, participants hold certain medications and supplements that could affect urine chemistry, in discussion with their treating physician. Participants complete a baseline 24-hour urine collection at home, followed by a fasting baseline visit at the University of Chicago Medicine Clinical Research Center with vital signs and a blood draw. A coordinator calls at one week to check for side effects and adherence. Participants then repeat the 24-hour urine collection and a fasting visit with vital signs, a blood draw, and a side effect and adherence assessment at 2 weeks. Urine collections are analyzed by Litholink lab for a full set of stone risk chemistries, and supersaturation values are calculated. Serum is analyzed in the University of Chicago General Chemistry laboratory. Urine and serum samples are frozen and stored for possible later assays. Basic clinical variables are taken from the electronic health record and confirmed by interview. Analysis is descriptive, looking at the change in each participant from baseline to 2 weeks, and no formal hypothesis testing is planned.