Exploring the Potential Relationship Between Phytate Ingestion, Urinary Phytate Excretion, and Renal Stone Risk in a Unique Human Model: No Hard Evidence in Support of Phytate as a Stone Inhibitor.

OBJECTIVE: Dietary phytate (IP6) enjoys a reputation as an inhibitor of calcium renal stone formation, although there are very few human studies to support this notion. In South Africa, urolithiasis occurs in the white (W) but is rare in the black (B) population. We undertook this unique human model to further investigate the IP6 theory. METHODS: Healthy W and B males completed baseline food-frequency recall questionnaires. Dietary intake of IP6 was restricted for 18 days. An IP6 dietary supplement was ingested on days 15-18. Twenty-four-hour urinary phytate and other urinary components were determined. Relative supersaturations of calcium salts were calculated. The urinary metastable limit (MSL) of calcium oxalate (CaOx) and its crystallisation kinetics were determined experimentally. RESULTS: Habitual dietary intake of IP6 and its urinary excretion were significantly higher in B than in W (1650 ± 202 vs. 640 ± 134 mg/d, P = .0002 and 1.13 ± 0.12 vs. 0.75 ± 0.13 µM, P <.05, respectively). In B, urinary phytate decreased significantly after 15 days of IP6 restriction, but in W, its excretion remained constant. After supplementation, urinary IP6 increased significantly in both groups reaching levels commensurate with the baseline value in B. No significant differences occurred in B in any of the routine urinary risk factors throughout the trial. However, in W, urinary citrate excretion increased on day 18 relative to day 0. There were no significant intragroup or intergroup changes in relative supersaturation, metastable limit, or crystallization kinetics. CONCLUSIONS: Despite notable differences in the renal handling of ingested IP6, there were no changes in any of the well-established urinary risk factors for calcium renal stone formation in either of our uniquely different test groups. We conclude that, in the absence of hard evidence, claims that IP6 is a stone inhibitor remain unproven.

Potential thermodynamic and kinetic roles of phytate as an inhibitor of kidney stone formation: theoretical modelling and crystallization experiments.

Kidney stone formation is governed by thermodynamic (supersaturation) and kinetic (crystal nucleation, growth, aggregation) mechanisms. We adopted a dual theoretical and experimental approach to investigate the potential role of urinary phytate in this regard. Thermodynamic constants for eight protonated phytate species and seven calcium-phytate complexes were determined by potentiometry and incorporated into the speciation program JESS. Urine was collected from 16 heathy males and their urine compositions were used as input for JESS. Phytate concentration was varied during modelling. No statistically significant decreases in Ca2+ concentrations or in supersaturation values were predicted by JESS. Crystallization experiments were then performed in pooled urine. Endogenous phytate concentration was determined using a metal-dye assay. The pool was dosed with various concentrations of phytate to achieve final concentrations equivalent to those used for modelling. Experiments showed that phytate had no effects on Ca2+ concentrations (as predicted by our theoretical modelling), metastable limits or crystal nucleation and growth kinetics. However, crystal aggregation kinetics was inhibited. We speculate that HPhy-11, small amounts of which were revealed by modelling, may bind to crystal surfaces and inhibit aggregation. We conclude that phytate exerts a kinetic, but not a thermodynamic inhibitory effect on crystallization in urine.

Urinary phytate (Myo-inositol hexaphosphate) in healthy school children and risk of nephrolithiasis.

OBJECTIVE: Although the incidence of urolithiasis is lower in children than in adults, the number of children with urolithiasis is increasing. Phytate, a naturally occurring compound present in legumes, nuts, and whole meals, has antilithiasic activity. The aim of this study was to assess, for the first time, the urinary levels of phytate in children and to correlate these levels with other urinary parameters related to crystallization risk and to general dietary habits. DESIGN AND METHODS: This was a cohort study conducted from April 2012 to March 2013 in the Laboratory of Investigation in Renal Lithiasis and at Son Espases Universitary Hospital in Palma de Majorca, Spain. SUBJECTS: Subjects included 165 healthy schoolchildren aged 5 to 12 years. INTERVENTION: All subjects followed their habitual diet. Information on the main dietary habits of the study subjects was obtained by asking each child's parents to fill out a dietary questionnaire. MAIN OUTCOME MEASURE: Phytate and citrate concentration and excretion were measured in 2 urine samples (a spot sample and a 12-hour overnight sample) for each child. Furthermore, common urinary biochemical indicators of stone risk were measured in each sample. RESULTS: The urinary phytate concentrations were low in this child population because of low consumption of dietary phytate. The urinary concentrations of phytate and citrate were low in 27.5% of these children. CONCLUSION: Because both substances are important inhibitors of crystallization, these finding suggests that these children are at risk of crystallization. Moreover, their diets consisted of foods rich in animal protein, with insufficient consumption of vegetables, legumes, and fruits.