Calcium, phosphorus and magnesium requirement

The nutrient requirement of an adult can be defined in various ways, some of which are unfortunately not amenable to direct measurement.Thus if we define requirement as the amount required to preserve normal health we come up against the impossible task of measuring normal health.Similarly, if we define it as the amount required to preserve normal weight, we encounter the difficulty of defining normal weight.These difficulties can be avoided with mineral nutrients, which pass through the body unchanged, by defining the requirement as the amount needed to preserve mineral balance.It can hardly be disputed that prolonged negative mineral balance (be it of sodium, potassium, magnesium, phosphorus or calcium) must ultimately deplete the body stores and be detrimental to health.This definition is adequate as far as it goes but is probably not quite sufficient.Adaptation to low intakes of Mg and P is achieved by lowering the plasma concentration and so lowering the urinary excretion.It would clearly be misleading to define requirement purely on the preservation of balance if this preservation of balance involved such a lowering of the plasma concentration of the nutrient concerned that it significantly impaired normal health.Thus there is abundant experimental evidence on animals and confirmatory data in man that hypophosphataemia can lead to osteomalacia (Lotz, Zisman & Bartter, 1968).Our definition of mineral requirement, therefore, has to take into account the preservation of a normal, or at least a safe, plasma concentration of the nutrient concerned.The purpose of the present communication is to look briefly at the Mg and P requirements of adults, which appear to be relatively low, and contrast them with the requirement for Ca, which is relatively high.We suggest that there are at least three reasons for this difference between Mg and P on the one hand and Ca on the other.First, there are differences in the absorptive meckmisms for the three elements.Secondly, there are differences in the plasma honioeostatic mechanisms.And thirdly, there are differences in the regulation of urinary excretion.In addition, there seem to be age and sex effects on Ca requirement but no comparable age and sex effects on Mg requirement. MethodologyOur balances were performed by the standard procedure previously described