P balance means that intake of P from foods equals losses in urine and feces (and other sources, such as sweat and skin, which are seldom measured). In effect, Pi ions that are absorbed are accounted for by losses from the body. Under balance conditions, no net gain or loss of Pi ions occurs. This zero-balance state probably only exists during the adult years from roughly 20 to 60. During growth, and during pregnancy and lactation, positive balance states tend to predominate, whereas in late life, phosphate retention may increase and become a major health problem for individuals with declining renal function. Phosphate retention (positive P balance) results from the declining effectiveness of PTH in enhancing renal excretion of Pi ions with decreasing renal function.
A schematic diagram of the P balance of an adult male is shown in Figure fVB.5.3.An adult male would typically consume 1,200 to 1,400 mg of P a day, whereas an adult female would consume 900 to 1,000 mg per day (USDA 1994; Calvo and Park 1996).
These estimated intakes of P by gender, however, do not include phosphate additives in foods.
Positive P balance (both Po and Pj), or the net gain of this element by the body, is difficult to measure, but numerous balance studies suggest that P homeostasis (that is, zero balance) is typically maintained even when Ca balance may be significantly negative. Radioisotopic and stable nuclide studies have greatly advanced our knowledge about the fluxes of Pi and Ca ions across the gut and renal tubules in animal models and, to a lesser extent, in human subjects. Balance studies without the use of stable or radioactive nuclides are notoriously fraught with potential errors of collection and measurement, and these difficulties make such studies generally unreliable in the precise quantitative sense.
The uptake of Pi ions by cells requires carrier mechanisms or cotransport systems because of the electrical charge and water-solubility properties of these anions. Pi ions typically cotransport with glucose in postprandial periods, but their charge must be neutralized by cations, typically not calcium ions. Also, after meals, Pi ions enter the bone fluid compartment, but in this case, typically with calcium ions. Calcitonin has been considered primarily responsible for the uptake by bone tissue of these two ionic species following food ingestion and the intestinal absorption of the ions (Talmage, Cooper, and Toverud 1983).After entry into cells, the Pi ions in the cytosol are almost immediately used to phosphorylate glucose or other molecules, and a small fraction of the ions are stored as organic molecules or inorganic salts within cellular organelles.
Some Pi ions that enter bone may enter bone cells, especially osteoblasts or lining cells, whereas other ions bypass the cells and go directly to the BFC, an extension of the blood/extracellular-fluid continuum. In the BFC, Pi ions in solution increase the Pi concentration (activity) that permits these ions to combine with Ca ions in excess of their solubility product constant (Ksp) and form mineral salts (precipitate) in bone extracellular tissue. The formation of hydroxyapatite crystals (mineralization) is essential for structural support and protection of internal organs from environmental trauma. Pi ions are, therefore, essential for the formation of the endoskeletons typical of most vertebrates except cartilaginous fish.
Approximately 60 to 70 percent of Pi ions are cleared by the kidneys in healthy individuals. If PTH is elevated, Pi excretion is enhanced even more, so that Pi losses are further increased. Under the same conditions of elevated PTH, the secretion of Pi by the gut is also increased. The endogenous fecal secretion of phosphates is the second major route of loss that the body uses to maintain Pi ion homeostasis.
World History
