Phosphorus

BACKGROUND

Phosphorus is the second most abundant inorganic element in the body and is a part of many important compounds, including deoxyribonucleic acid (DNA), ribonucleic acid (RNA), (S)-2-amino-3-[5-tertbutyl-3-(phosphonomethoxy)-4-isoxazolyl]propionic acid (ATPO), adenosine diphosphate (ADP), phospholipids and sugar phosphates. Phosphorus as phosphate is a major buffer of acid in urine by virtue of its monovalent, divalent and trivalent forms. Phosphate helps to protect blood systemic acid/ base balance, acts as a temporary store and transport mechanism for energy and helps in activating catalytic proteins. Eighty-five per cent of the body's phosphorus is in bone and the remainder is distributed through soft tissues (Diem 1970).

Inorganic phosphorus is only a tiny fraction of total body phosphorus but plays a critical role in blood and extracellular fluids. Phosphate enters the organic pool after absorption from the diet and resorption from bone. All urinary phosphorus and bone mineral phosphate are derived from the organic pool. Some phosphorus is absorbed with organic compounds such as peptides and lipids, but it is difficult to assess the relative amounts of inorganic and organic phosphorus consumed.

Phosphorus is widely distributed in natural foods and also found in food additives as phosphate salts, used in processing for retaining moisture, smoothness and binding. Most food sources are relatively bioavailable with the exception of plant seeds (beans, peas, cereals, nuts) that contain a special storage form of phosphate called phytic acid. Mammals are generally unable to hydrolyse and use phytate, although some foods also contain the enzyme phytase, as do colonic bacteria, which can release some phosphate from phytate. For adults, bioavailability estimates range from 55 to 70% (Lehmann 1996, Nordin 1989, Stanbury 1971).

Net phosphorus absorption is a linear function of phosphorus intake, indicating that diffusion is the main means of absorption. For infants, bioavailability is highest from human milk (85–90%), followed by cow's milk (72%) and soy formulas (about 59%). However, cow's milk and soy-based infant formulas generally contain substantially more phosphorus than human milk. As a result, phosphorus absorption for infants fed cow's milk and soy formulas appears to be almost twice that of infants fed human milk (Moya et al 1992).

Inadequate intakes or malabsorption of phosphorus as seen in vitamin D deficiency results in hypophosphataemia the symptoms of which include anorexia, anaemia, muscle weakness, bone pain, rickets, osteomalacia, general debility, increased susceptibility to infection, paresthesias, ataxia, confusion and possibly death (Lotz et al 1968). Phosphorus is so widespread in the food supply that dietary phosphorus deficiency is extremely rare, the exception being long-term, severe food restriction.

In the past, a great deal of emphasis was placed on the calcium:phosphorus ratio (Ca:P) of diets (Chinn 1981), particularly those of infants (Fomon & Nelson 1993). This is a useful concept during periods of rapid growth but has little relevance in adults when assessing requirements. Also, the ratio does not take into account differing bioavailabilities and adaptive responses of the two nutrients. In balance studies in human adults, Ca:P molar ratios ranging from 0.08 to 2.4 (a 30 fold range) had no effect on either calcium balance or absorption (Heaney & Recker 1982, Spencer et al 1965, 1978). For this reason, other indicators are now used to assess phosphorus requirements, including measurement of inorganic phosphorus in serum (serum P i ) or phosphorus balance.

As phosphorus intake directly affects serum P i and because both hypo-and hyperphosphataemia directly cause dysfunction, serum P i is seen as the best indicator of nutritional adequacy of phosphorus intake. Results of phosphorus balance studies can reflect changes occurring in the body in addition to dietary intake of phosphorus and, as such, are of limited use.