PHOSPHOROUS

Function and essentiality About 80 per cent of the phosphorus (P) in the human body ie 600-900 g (19-29 mol) is present in bones as a calcium salt that gives rigidity to the skeleton. The rest is in soft tissues as inorganic phosphate and as a constituent of all the major classes of biochemical compounds. The energy that drives most metabolic processes is derived from the phosphate bonds of Adenosine triphosphate (ATP) and other high energy phosphate compounds. Phosphorylation-dephosphorylation reactions are crucial to many aspects of metabolic control.


Metabolism Absorption of P is about 60 per cent of intake (1). Plasma concentrations fall within the range 24.7-43.3 mg/L (0.8-1.4 mmol/L) and are regulated by excretion. At low P intakes, plasma P falls and urinary excretion is reduced. To maintain a plasma phosphate concentration of 23.9 mg (0.8 mmol)/L an intake of 400 mg (12.9 mmol)/d with a bioavailability of 60 per cent is needed (1). This is well below usual intakes.


Requirements Phosphorus requirements are conventionally set as equal to calcium in mass terms, ie 1 mg P: 1 mg Ca. However these elements are present in the body in nearly equimolar amounts and the Panel took the view that it was more rational to set the ratio in the diet as 1 mmol P: 1 mmol Ca and recommended that as a matter of prudence the RNI for P be set equal to the calcium RNI in mmol* (see paras 1. 3.16 and 22.1 1. 1 - not yet available). For Ca intakes different from the RNI, P intake should be equivalent to actual Ca intakes.


Intakes As phosphate is a major constituent of all plant and animal cells, P is present in all natural foods. It is also present in many food additives. Household food purchases in Britain provide 1.2-1.3 g (39-42 mmol)/person/d (2) of which about 10 per cent is added in manufacture (3).


Guidance on high intakes As discussed in para 22.8 (not yet available) a varying Ca:P ratio can be tolerated by adult populations with no effect on Ca balance (4), although for infants the ratio remains extremely important and should not be outside the range of 1.2:1 to 2.2:1 mg/mg or 0.9:1 to 1.7:1 mmol/mmol (5). Nevertheless the maximum tolerable daily intake is set at 70 mg/kg body weight (6) which is about 4.5 g/d for a 65 kg man and well above any level likely to be obtained from dietary sources.

*1 mmol Ca= 40 mg, I mmol P 30.9 mg


References

1 Marshall D H, Nordin B E C, Speed R. Calcium, phosphorus and magnesium requirement. Proc Nutr Soc 1976; 35, 163-173.

2 Gilbert L, Wenlock R W, Buss D H. Phosphorus in the British household food supply. Hum Nutr.- Appi Nutr 1985; 39A: 208-212.

3 Davidson L S P, Passmore R. Human Nutrition and Dietetics. 7th ed. London: Churchill Livingstone, 1980.

4 Spencer M, Cramer L, Osis D, Worris C. Effect of phosphorus on the absorption of calcium and on the calcium balance in man. J Nutr 1978; 108: 447-457.

5 Department of Health and Social Security. Artificial feeds for the Young Infant. London: HMSO, 1980. (Reports on health and social subjects; 18).

6 World Health Organization. Evaluation of Certain Food Additives and Contaminants Geneva: World Health Organization, 1982. (WHO Technical Report Series; 683).