Phosphorus (P) is an essential mineral that is found in all cells within
the body. The body of the human adult contains about 400-500 g. The
greatest amount of body phosphorus can be found primarily in bone (85%)
and muscle (14%). Phosphorus is primarily found as phosphate (PO4
2-). The major building blocks of biology
are covalent molecules comprising proteins, polysaccharides, and nucleic
acids. The nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid
(RNA) are polymers based on phosphate ester monomers. The high-energy
phosphate bond of ATP is the major energy currency of living organisms.
Cell membranes are composed largely of phospholipids. The inorganic
constituents of bone are primarily a calcium phosphate salt called
hydroxyapatite. A variety of enzymatic activities are controlled by
alternate phosphorylation and dephosphorylation of proteins. The
metabolism of all major metabolic substrates depends on the functioning of
phosphorus as a cofactor in a variety of enzymes and as the principal
reservoir for metabolic energy.
Deficiencies: Dietary phosphorus absorption is about 60-70%; any excess absorbed is readily excreted in the urine. Since renal conservation of phosphorus occurs quite efficiently on low phosphorus diets, a deficiency rarely occurs in a healthy population. However, various disease states or conditions (e.g., gastrointestinal malabsorption, diabetes mellitus, renal tubular dysfunction, antacid abuse, and premature birth) can be associated with low blood phosphorus levels (i.e., hypophosphatemia) and depleted phosphorus stores.
Diet recommendations: New recommendations for dietary phosphorus include a value, the Recommended Dietary Allowance (RDA), which an individual should aim to meet, and a value, the Tolerable Upper Level (UL), which should not be exceeded. Intakes between the RDA and the UL ensure that serum phosphorus levels will be maintained in the normal range. Values for infants are called Adequate Intake (AI) levels, and are based on a significant portion of intake being from breast milk. It should also be noted that there are no additional requirements for P during pregnancy or lactation. The Recommended intake levels for P (mg per day) are set based on life stage groups. For infants 0 to 6, and 6 to 12 months, the RDA is 100 and 275 mg, respectively. No UL has been set for these ages as supplementation would be unlikely. For children 1 to 3 and 4 to 8 years, the RDA is 460 and 500, respectively, and the UL is 3000 mg. For youth 9 to 18 years, the RDA is 1250 mg, which indicates the higher need for phosphorus during the adolescent growth spurt; the UL for youth is 4000 mg. Adults 19 years and older have an RDA of 700 mg. The UL is 4000 mg up to age 70, then declines to 3000 mg after age 70 years.
Food Sources: Phosphorus is found widely distributed in foodstuffs. In the United States, the average daily intake is about 1600 mg for males and 1000 mg for females. In general, food sources rich in protein (milk, meat, eggs, legumes and grains) are also high in phosphorus. The relative contributions of food groups to phosphorus intake are: 60% from milk, meat, poultry, fish, and eggs; 20% from cereals and legumes; 10% from fruits and fruit juices; 4% from alcoholic beverages; and 3% from soft drinks and other beverages.
Toxicity: A diet containing a 2:1 dietary ratio of phosphorus to calcium can cause low blood calcium (hypocalcemia) and secondary hyperparathyroidism with excess bone resorption and bone loss in animals. Human breast milk, with a lower phosphorus content than cow milk, is considered better suited to the needs of the infant. For older ages, typical diets in the United States frequently exceed the recommended ratio; however, these diets are not believed to be harmful unless calcium intake is also very low. As intake of phosphorus rises, so does serum phosphorus. Elevated serum phosphorus levels (hyperphosphatemia) can occur in patients with renal failure due to a poor ability to excrete phosphorus in the urine. As indicated by UL values, intake of phosphorus exceeding 3 to 4 grams may be harmful in healthy individuals.