The effect of diurnal variation on total plasma calcium concentration in normal subjects.

In 12 normal subjects plasma calcium and phosphorus concentrations were estimated during a normal working day. In the individual subjects fluctuations from the fasting values occurred during the period of study, but at no time during that period did the mean values differ significantly from the fasting plasma calcium or phosphorus concentrations. In three of the subjects the study was repeated while fasting during a normal working day. In these subjects the fluctuations in both plasma calcium and phosphorus concentrations were less than during a normal day while taking meals, and there was evidence of a diurnal variation in the plasma phosphorus concentration.