Inorganic phosphate anions are efficiently absorbed across the small intestine, primarily in cotransport with cations, in order to maintain the electrical neutrality of cells. The efficiency of absorption (net) of Pi ranges between 60 and 70 percent in adults, almost twice the efficiency (net) of calcium from the diet (that is, 28 to 30 percent) (Anderson 1991). For example, for every 1,000 moles of Pj, approximately 700 are absorbed (net), compared to only 300 (net) from 1,000 of Ca in the diet. Therefore, the excretory mechanisms have to work more efficiently to eliminate the extra Pj absorbed following meals. In children, the net absorption efficiencies may be as high as 90 percent for Pi and 50 percent for Ca. Several factors have adverse effects on Pi absorption, but these typically have little overall influence on Pi utilization, homeostasis, or metabolism. A few factors may enhance Pi absorption, but these, too, have little significance for the overall economy of Pi in the body (Allen and Wood 1994). A high P intake has been reported to have little effect on calcium absorption (Spencer et al. 1978).
The absorption efficiency of Pi declines later in life so that the net absorption of phosphorus from foods is somewhat reduced; probably this occurs in a similar fashion as calcium absorptive efficiency is lowered with age, especially after age 65 in females.
Pi ions are absorbed across all three segments of the small intestine, but the rapid entry into the bloodstream of radioactively labeled phosphates suggests that duodenal absorption occurs both very efficiently and at a high transfer rate. Therefore, the bulk of the absorbed Pi ions are transported across this segment, lesser amounts across the jejunum, and still lesser amounts across the ileum. If the hormonal form of vitamin D - calcitriol or 1,25-dihydroxyvitamin D - is elevated, Pi absorption can be even further enhanced in all segments of the small intestine.
World History
