Calcium requirement is a sliding scale

It must be a source of some surprise to rational scientists that the human requirement for calcium, an apparently inoffensive nutrient that contributes so much to our physical stability, arouses strong emotions in many breasts. Calcium requirements and allowances seem to attract more controversy and generate more heat than do the requirements and allowances for any other nutrient, the latest example of this being a recent controversy in the columns of the New York Times (1). The problem may be that calcium turnover is too slow and the effects of deprivation and replenishment too gradual to be easily demonstrated in humans; perhaps it is the very efficacy of the calcium homeostatic system that makes this system difficult to study. Whereas plasma concentrations of other nutrients (eg, sodium, potassium, phosphorus, and magnesium) can be lowered relatively easily and quickly by experimental deprivation (2), plasma (ionized) calcium is so well protected through access to the reserve stores in the skeleton that it cannot be used as a marker of calcium nutrition. Although there is overwhelming evidence that calcium deprivation causes osteoporosis in experimental animals (3), it would be both immoral and impractical to try to reproduce such experiments in humans. The calcium requirement therefore must be estimated by indirect means that, even if they satisfy many of the experts in the field, are open to criticism by others. Nonetheless, there is no smoke without fire and it may be that this controversy does reflect a deeper reality, although not perhaps the one that the critics of the calcium story envisage.BE Christopher Nordi