INORGANIC PHOSPHORUS REDUCES HYPERCALCIURIA DURING TOTAL PARENTERAL-NUTRITION BY ENHANCING RENAL TUBULAR CALCIUM-ABSORPTION

Background: Increasing the inorganic phosphorus content of total paren teral nutrition (TPN) formulas has been shown to decrease TPN-induced hypercalciuria in experimental animals and humans. The mechanism of th is 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 phosp horus. Urinary calcium excretion, serum ultrafilterable calcium, filte red calcium load, fractional calcium excretion, urinary cyclic adenosi ne 5'-monophosphate (cAMP), and serum levels of ionized calcium, parat hyroid hormone (PTH), and vitamin D metabolites were determined at the end of each study period. Results: Urinary calcium excretion was sign ificantly lower when the patients received the higher inorganic phosph orus formula. Increasing the inorganic phosphorus in the TPN formula d id not change ultrafilterable calcium or filtered calcium load, but si gnificantly reduced fractional calcium excretion. No differences in se rum levels of ionized calcium, PTH, 25-hydroxyvitamin D, 1,25-dihydrox yvitamin D, or urinary cAMP were observed between treatments. Conclusi ons: These results demonstrate that increasing the inorganic phosphoru s content of the TPN formula decreases urinary calcium excretion by in creasing renal tubular calcium resorption. This effect is not due to a lterations in the PTH-1,25-dihydroxyvitamin D axis, but likely reflect s a direct action of inorganic phosphorus on the renal tubules.