Effect of phosphorus on endogenous calcium losses during total parenteral nutrition.

Intravenous phosphorus can reduce urinary calcium losses in patients receiving total parenteral nutrition (TPN). We investigated the effect of intravenous P on urinary and fecal Ca loss in intravenously fed normal and thyroparathyroidectomized (TPTX) rats to assess the role of parathyroid hormone (PTH) and endogenous fecal Ca losses. Doubling the intravenous P load during TPN decreased urinary Ca losses by 54% (0.299 vs 0.137 mmol/d) in intact rats and by 43% (0.514 vs 0.294 mmol/d) in TPTX rats. Increased P load in normal rats had no effect on urinary cyclic AMP excretion, serum Ca, serum P, serum 1,25-dihydroxycholecalciferol, or endogenous fecal Ca losses. These observations suggest that the hypocalciuric effect of P during TPN is independent of PTH and is not caused by a repartitioning of obligatory Ca losses from the renal to the intestinal route.