Diurnal variation in urinary excretion of calcium and strontium

A marked variation in the 4-hourly urinary excretion of calcium and strontium was demonstrated in two male subjects on normal diet and activity. The general pattern of excretion was not, however, the same in the two who differed also in their pattern of response to a regular dietary intake of milk 4-hourly. Further experiment on one subject showed that on a regular 4-hourly diet of his normal calcium content a regular rhythm for calcium excretion was manifest only when activity was also controlled by his confinement to bed. Excretion of calcium was correlated, but not undissociably, with that of sodium. A similar daily rhythm was elicited by a diet low in calcium ( < 100 mg/day). Renal clearance of calcium and strontium ran parallel with a variation of up to sevenfold within a 24 h period. The results indicate a natural nycthemeral* variation in renal excretion of calcium and strontium, but this natural rhythm may be obscured by extrinsic factors, such as muscular exercise and loading of an internal reservoir such as the ‘exchangeable pool’.