Inositol hexakisphosphate in urine: the relationship between oral intake and urinary excretion

To study the relationship between the oral intake of inositol hexakisphosphate (InsP6, phytic acid, an inhibitor of urinary crystallization) and its urinary excretion, to establish their possible mutual influence.

Two groups of male Wistar rats (six animals each) received either; tap water and normal rat food pellets (controls); or a liquid diet in which InsP6 was absent and which then received gradually increasing amounts of InsP6. The urinary levels of InsP6 were then assessed regularly in both groups.

When InsP6 was absent from the diet, urinary excretion declined to undetectable levels after 22 days. The addition of increasing amounts of InsP6 to the liquid diet caused an increase in its urinary excretion after about 10 days. Adding InsP6 in amounts > 425 mg/L caused no further increases in urinary excretion. Adding inositol (with no InsP6) to the liquid diet caused only a slight increase in the urinary excretion of InsP6.

These results showed that InsP6 urinary levels were related to its oral intake; consequently, a low consumption of InsP6 would cause a urinary deficit of this crystallization inhibitor and thus an increase in the risk of developing urinary calcium stones. Although urinary excretion was dose-dependent, there was an ingested amount (20.9 mg/kg) above which there was no increase in the amount excreted. This intake is easily obtained by consuming a normal diet (rich in InsP6) indicating that to maintain appropriate urinary levels of InsP6, the consumption of InsP6 supplements is only necessary when the diet is particularly poor in InsP6.