Phosphorus makes up about 1% of total body weight. It is closely associated with calcium in human nutrition and has been called its metabolic twin. However, it has some unique characteristics and functions of its own.
The same factors that control calcium absorption also regulate phosphorus. Free phosphate is absorbed in the jejunum of the small intestine in relation to calcium and is also regulated by active vitamin D hormone, although the hormonal effect is greater on calcium absorprion than on phosphorus. Equal amounts of calcium and phosphorus should exist in the diet for an optimal ratio. Since phosphate occurs in food as a phosphate compound, mainly with calcium, the first step for its absorption is its splitting off as the free mineral. Factors similar to those that influence calcium absorption also affect phosphorus absorption. For example, an excess of calcium or other binding material, such as aluminum or iron, inhibits phosphorus absorption.
The kidneys provide the main excretion route for regularion of the serum phosphorus level. Usually, 85% to 95% of plasma phosphate is filtered at the renal glomeruli and largely reabsorbed at the tubules, along with calcium, under the influence of vitamin D hormone. But when increased phosphate excretion is needed to maintain the normal serum calcium-phosphate balance, parathyroid hormone acts to override the effect of vitamin D hormone. The amount of phosphorus excreted in the urine of a person ingesting an average diet is 0.6 to 1.8 g/day.