Inorganic phosphorus reduces hypercalciuria during total parenteral nutrition by enhancing renal tubular calcium absorption.

BACKGROUND

Increasing the inorganic phosphorus content of total parenteral nutrition (TPN) formulas has been shown to decrease TPN-induced hypercalciuria in experimental animals and humans. The mechanism of this effect, however, has been uncertain.

METHODS

By using a randomized cross-over design, seven patients on cyclic TPN were given otherwise identical formulas providing either 15 or 45 mmol/d of inorganic phosphorus. Urinary calcium excretion, serum ultrafilterable calcium, filtered calcium load, fractional calcium excretion, urinary cyclic adenosine 5'-monophosphate (cAMP), and serum levels of ionized calcium, parathyroid hormone (PTH), and vitamin D metabolites were determined at the end of each study period.

RESULTS

Urinary calcium excretion was significantly lower when the patients received the higher inorganic phosphorus formula. Increasing the inorganic phosphorus in the TPN formula did not change ultrafilterable calcium or filtered calcium load, but significantly reduced fractional calcium excretion. No differences in serum levels of ionized calcium, PTH, 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, or urinary cAMP were observed between treatments.

CONCLUSIONS

These results demonstrate that increasing the inorganic phosphorus content of the TPN formula decreases urinary calcium excretion by increasing renal tubular calcium resorption. This effect is not due to alterations in the PTH-1,25-dihydroxyvitamin D axis, but likely reflects a direct action of inorganic phosphorus on the renal tubules.