PHOSPHORUS
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. |