Phosphorus is an essential mineral that exists mainly as hydroxyapatite (85 percent) or as an intracellular constituent (10 to 15 percent). Only about 1 percent is in the ECF, and therefore serum measurements may not accurately reflect total body stores. Serum phosphate levels decrease with age from a high of 4.0 to 7.0 mg/dL in newborns to 3.0 to 5.0 mg/dL in adults. The total body phosphorus in a normal man is approximately 700 g (10 to 15 g/kg). This is predominantly in bone (80 percent), where phosphorus plays a major role in structural integrity. Serum Ca2+ and phosphate are inversely proportional, and the product of their two concentrations is approximately 30 to 40 mg/dL. When phosphorus is not in the form of hydroxyapatite, it may be extracellular where it is in the form of inorganic ions. Intracellular phosphorus is bound to protein or exists as organic esters. Levels of inorganic phosphates are normally between 2.3 and 4.5 mg/dL.
Phosphorus, unlike the other elements, is 85 percent free and only 15 percent bound to proteins. Phosphate may be present in different forms such as H 2PO4- or HPO42-, and levels change with pH. The 85 percent of phosphorus that is free can be bound to Na +, Ca2+, or Mg2+. Plasma phosphorus levels, unlike Ca2+ or Mg2+, demonstrate diurnal variation with a morning nadir and are affected by age, hormones (insulin and growth hormone), and the amount of carbohydrate ingestion. Therefore, fasting levels should be measured, since glucose infusion as well as carbohydrate and phosphorus ingestion lower serum levels.
Normal daily intake is between 10 and 12 mmol. Phosphorus absorption is proportional to dietary intake. Approximately 70 percent is absorbed via passive transport, with the remainder via active transport, which is dependent on 1,25-(OH) 2-vitamin D. phosphate deficiency is rarely caused by a decrease in phosphorus oral intake unless absorption is affected. Phosphate excretion is proportional to oral intake. Excretion is predominantly in the urine by the glomerulus, with the majority resorbed in the proximal tubules. This is regulated by PTH, which acts to lower serum phosphate by increasing renal excretion. Proximal tubule absorption increases when serum phosphate levels drop, as well as with hypoparathyroidism, volume depletion, hypocalcemia, or the presence of growth hormone. Excretion increases in the presence of volume expansion, hypercalcemia, acidosis, hypomagnesemia, hypokalemia, glucocorticoids, diuretics, calcitonin, or PTH. Phosphate is essential to a wide variety of biochemical reactions, especially energy metabolism in the form of high-energy phosphate and phosphocreatine.
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