Measurement of calcium and phosphorus fluxes during lactation in the rat.

Novel, two-compartment metabolic cages for rats were constructed to examine Ca and P fluxes during lactation. Four consecutive balance studies of 4 d each were performed in lactating rats and nonlactating controls. Lactating rats had dramatically higher intestinal absorptions of Ca and P resulting from both increased food consumption and elevated fractional absorptions of these elements. Urinary Ca excretion was reduced during lactation whereas urinary P excretion was unchanged. Endogenous fecal Ca excretion (determined by infusing 45Ca and dividing fecal 45Ca content by urinary 45Ca specific activity) was similar in lactating and nonlactating rats. In a separate study, pup retention of ingested Ca averaged 96%. Therefore, the difference in pup carcass Ca content between d 1 and 17 of age is assumed to equal 96% of the Ca transfer rate into milk. Similar calculations were applied to P. Milk transfer rates were determined to be 126 and 99 mg/d for Ca and P, respectively. These calculations indicate that 19% of the Ca transferred to milk was derived from the maternal skeleton with the maternal diet supplying the remaining Ca. This study provides the first comprehensive analysis of the alterations in Ca and P fluxes that occur during lactation in the rat.