EFFECTS OF MODERATE DIETARY PHOSPHORUS RESTRICTION ON INTESTINAL-ABSORPTION AND EXTERNAL BALANCES OF PHOSPHORUS AND CALCIUM IN GROWING FEMALE RABBITS

The effects of moderate dietary P restriction on intestinal net fluxes and external balances of P and Ca were studied in growing female albino rabbits that were fed a P-deficient diet for 10 consecutive days while they were housed in metabolism cages. Intestinal P secretion occurred during the first 24 h of P restriction and thereafter changed to absorption. During recovery, when the rabbits were consuming a normal diet, P absorption was significantly greater than either prerestriction control values or than in a separate group of time control rabbits. Phosphorus balance was negative during the 1st day of P restriction but thereafter became positive. This pattern occurred because intestinal P secretion changed to absorption and because urinary losses of P were negligible. Intestinal Ca absorption increased within 24 h of P restriction, reached maximal values by 4 days, and remained elevated for each of the remaining 6 days that dietary P was low. It also was elevated compared to P-sufficient time controls for 8 days after replenishment of dietary P. Despite increased intestinal absorption, Ca balance was significantly reduced during P restriction because of substantial hypercalciuria. Thus, selective dietary P restriction reduced the positive balances of both P and Ca that are characteristic of growth. We conclude that in growing rabbits moderate dietary P restriction induces both intestinal and renal adaptations that conserve this mineral; concomitantly, positive Ca balance is reduced. With dietary P replenishment, adaptations persist to restore the positive mineral balances that were lost because of dietary P restriction during growth.