Soy oxalate and phytate and risk of kidney stones

Consumption of foods made from soybeans is increasing because of the desirable nutritional value of soybeans. However, some soy foods contain large concentrations of oxalate and/or phytate. Oxalate is a component of calcium oxalate kidney stones, whereas phytate is an inhibitor of calcium kidney stone formation. Information on oxalate and phytate concentrations and bioavailability from soy foods is important for kidney stone patients and individuals at risk for kidney stones. Thirty commercial soy foods contained from 2 to 58 mg of total oxalate and 76 to 528 mg phytate per serving. The correlation between oxalate and phytate in the soy foods was significant (r = 0.71, P < 0.001) indicating that oxalate-rich soy foods also contain large concentrations of phytate. There also was a significant correlation between the molar divalent ion binding potential of oxalate plus phytate and the molar concentration of calcium plus magnesium (r = 0.90, P < 0.001) in soy foods. Oxalate and phytate absorption in 11 healthy non-stoneformers and 11 stoneformers was determined by measuring changes in urinary excretions after ingestion of six oxalate-phytate loads including three standards and three tofus containing selected combinations of oxalate and phytate concentrations. After correction for endogenous synthesis, only the oxalate standard and the oxalate plus phytate standard produced significant increases in oxalate excretion in both groups of subjects, with the changes in oxalate excretion being greater after ingestion of the oxalate plus phytate standard. Phytate forms insoluble complexes with divalent cations in the gastrointestinal tract and may consequently increase free oxalate available for absorption and urinary excretion. However, urinary phytate excretion did not significantly change after consumption of the three standards in the two groups of subjects. In general, ingestion of the three tofus did not affect oxalate or phytate excretion, nor the Tiselius risk indices. Although soy foods containing small concentrations of oxalate and large concentrations of phytate may be advantageous for kidney stone patients or persons with high risk of kidney stones, consumption of tofu is not likely to induce clinically significant increases in urinary oxalate or phytate excretion in stoneformers or non-stoneformers.